WEBVTT 00:00:00.000 --> 00:00:01.912 [SQUEAKING] 00:00:01.912 --> 00:00:04.302 [RUSTLING] 00:00:04.302 --> 00:00:10.040 [CLICKING] 00:00:10.040 --> 00:00:14.480 PROFESSOR: Today we're going to discuss bioenergetics, continue 00:00:14.480 --> 00:00:18.020 that discussion and use that as a transition 00:00:18.020 --> 00:00:23.540 into also beginning to see how that applies to understanding 00:00:23.540 --> 00:00:28.620 glycolysis, the pathway that allows glucose breakdown. 00:00:28.620 --> 00:00:31.980 So just as a reminder of a couple of the points 00:00:31.980 --> 00:00:35.100 that we discussed last time, so last time 00:00:35.100 --> 00:00:37.780 we discussed what makes reactions favorable. 00:00:37.780 --> 00:00:41.430 And so remember, you could have any reaction, A to B-- 00:00:41.430 --> 00:00:43.020 A to B could be a single step. 00:00:43.020 --> 00:00:46.050 A to B could be an entire pathway, all steps 00:00:46.050 --> 00:00:49.260 in the pathway, that whether or not that reaction 00:00:49.260 --> 00:00:52.380 or pathway happens is determined by thermodynamics. 00:00:52.380 --> 00:00:54.130 It's not determined by enzymes. 00:00:54.130 --> 00:00:55.810 Enzymes are important, of course, 00:00:55.810 --> 00:00:58.560 to control the rate of reactions so they 00:00:58.560 --> 00:01:01.440 may allow things to happen that otherwise wouldn't happen. 00:01:01.440 --> 00:01:03.630 But whether the reaction happens or not 00:01:03.630 --> 00:01:06.310 is determined by free energy. 00:01:06.310 --> 00:01:08.370 And so free energy, delta G, remember 00:01:08.370 --> 00:01:11.770 we discussed is the following relationship. 00:01:11.770 --> 00:01:14.460 So it's related to this constant delta G0 00:01:14.460 --> 00:01:16.740 prime, which is related to the equilibrium 00:01:16.740 --> 00:01:20.970 constant for any pair of products and reactants 00:01:20.970 --> 00:01:25.020 as well as this formula rt times the log of the products 00:01:25.020 --> 00:01:26.980 over the reactants. 00:01:26.980 --> 00:01:30.780 And so remember this means that whether a reaction happens 00:01:30.780 --> 00:01:33.030 depends on the equilibrium constant, 00:01:33.030 --> 00:01:36.510 but it also depends on the actual conditions present. 00:01:36.510 --> 00:01:40.390 That is how much substrate and how much product is there. 00:01:40.390 --> 00:01:44.400 And so what this means is that biology can come up with ways 00:01:44.400 --> 00:01:48.150 by keeping the product concentrations low to do things 00:01:48.150 --> 00:01:51.510 that may move in the opposite direction of what you would 00:01:51.510 --> 00:01:54.210 predict from equilibrium. 00:01:54.210 --> 00:01:58.410 And also by coupling these less favorable reactions 00:01:58.410 --> 00:02:02.700 to more favorable reactions, you can also allow biology 00:02:02.700 --> 00:02:04.410 to do unfavorable stuff. 00:02:04.410 --> 00:02:07.260 And this is, of course, where the role of ATP comes in. 00:02:07.260 --> 00:02:11.180 And we discussed this at the end of last time as well. 00:02:11.180 --> 00:02:15.860 Of course, what's important then becomes the ratio of ATP 00:02:15.860 --> 00:02:17.300 to ADP. 00:02:17.300 --> 00:02:20.340 And to illustrate that, we use this example. 00:02:20.340 --> 00:02:25.160 We use the-- remember ATP hydrolysis has a delta G0 prime 00:02:25.160 --> 00:02:28.460 of minus 7.5 kcals per mole. 00:02:28.460 --> 00:02:30.048 That, of course, tells you this is 00:02:30.048 --> 00:02:31.340 about the equilibrium constant. 00:02:31.340 --> 00:02:34.010 That tells us the equilibrium lies to the right. 00:02:34.010 --> 00:02:37.400 We can couple it to this first reaction in glycolysis, 00:02:37.400 --> 00:02:40.880 which traps glucose in the cell by adding a phosphate group. 00:02:40.880 --> 00:02:45.200 Delta G0 prime for this is positive 3.3 kcals per mole. 00:02:45.200 --> 00:02:48.650 This positive number tells you that the equilibrium 00:02:48.650 --> 00:02:50.730 lies towards the left. 00:02:50.730 --> 00:02:53.300 But when we couple these two reactions together, 00:02:53.300 --> 00:02:55.250 we add these two numbers together, 00:02:55.250 --> 00:02:59.880 and now we get a delta G0 prime of minus 4.2 kcals per mole, 00:02:59.880 --> 00:03:01.580 which are the coupled reaction. 00:03:01.580 --> 00:03:04.220 The equilibrium lies now again to the right. 00:03:04.220 --> 00:03:07.430 And that's how we can use ATP hydrolysis in order 00:03:07.430 --> 00:03:10.280 to carry out this unfavorable process, trapping 00:03:10.280 --> 00:03:12.060 the glucose in the cell. 00:03:12.060 --> 00:03:15.140 Now how much glucose will be trapped? 00:03:15.140 --> 00:03:19.945 Well, of course, that will come from this reaction up here. 00:03:19.945 --> 00:03:21.320 And that's because whether or not 00:03:21.320 --> 00:03:22.970 a reaction occurs or not of course 00:03:22.970 --> 00:03:25.070 depends not on the equilibrium constant 00:03:25.070 --> 00:03:30.120 but on the actual conditions and the delta G of that reaction. 00:03:30.120 --> 00:03:33.060 And so we can just plug that in for this system. 00:03:33.060 --> 00:03:37.430 And so we see delta G0 prime minus 4.2 kcals per mole. 00:03:37.430 --> 00:03:39.230 That says equilibrium lies to the right. 00:03:39.230 --> 00:03:42.110 But how far really depends on the conditions. 00:03:42.110 --> 00:03:46.500 And that's defined by this ratio of products over reactants. 00:03:46.500 --> 00:03:49.070 And you can imagine that this term here, as long 00:03:49.070 --> 00:03:54.200 as it's less than 4.2, you will favor glucose trapping 00:03:54.200 --> 00:03:56.760 inside the cell, whereas, of course, 00:03:56.760 --> 00:03:59.210 if it's greater than 4.2, then you'd favor-- 00:03:59.210 --> 00:04:03.080 the opposite direction would be favorable. 00:04:03.080 --> 00:04:07.520 Now this is really critical because this really 00:04:07.520 --> 00:04:11.480 explains how it is that ATP actually 00:04:11.480 --> 00:04:17.760 works to help cells do various reactions. 00:04:17.760 --> 00:04:22.070 And so if an ATP-ADP ratio exists such 00:04:22.070 --> 00:04:24.680 that it can make delta G less than 0 00:04:24.680 --> 00:04:26.750 for the reaction that it's coupled to, 00:04:26.750 --> 00:04:30.320 that is how it provides the energy that then allows 00:04:30.320 --> 00:04:33.000 unfavorable reactions to occur. 00:04:33.000 --> 00:04:35.690 And so please do not make the common mistake 00:04:35.690 --> 00:04:38.855 that many, many biologists make when they state, 00:04:38.855 --> 00:04:41.030 oh, ATP equals energy. 00:04:41.030 --> 00:04:42.350 There's a lot of ATP. 00:04:42.350 --> 00:04:44.960 The cell must have a lot of energy. 00:04:44.960 --> 00:04:48.020 Remember it's the ratio of ATP to ADP 00:04:48.020 --> 00:04:49.680 that provides the energy. 00:04:49.680 --> 00:04:52.400 And so that means ATP can be sky high, 00:04:52.400 --> 00:04:57.170 but if ADP is equally high, there's no energy there. 00:04:57.170 --> 00:05:00.060 Consequently, ATP can be very low. 00:05:00.060 --> 00:05:03.260 But if ADP is extremely low, now you still 00:05:03.260 --> 00:05:05.790 have a high ATP to ADP ratio. 00:05:05.790 --> 00:05:08.570 And that is what provides the energy. 00:05:08.570 --> 00:05:11.570 And that's because this occurs regardless of concentration. 00:05:11.570 --> 00:05:15.590 It's because it's this ratio of ATP to ADP 00:05:15.590 --> 00:05:20.120 that ultimately says how much energy is there for the ATP 00:05:20.120 --> 00:05:22.680 to drive a reaction. 00:05:22.680 --> 00:05:26.540 In other words, delta G, or whatever the reaction 00:05:26.540 --> 00:05:29.630 is coupled to, is going to be proportional 00:05:29.630 --> 00:05:34.280 to the ratio of ADP to ATP. 00:05:34.280 --> 00:05:35.690 Great. 00:05:35.690 --> 00:05:38.270 Now how much ATP do you need then? 00:05:38.270 --> 00:05:40.895 What ratio do you need to drive a reaction? 00:05:40.895 --> 00:05:42.770 Well, it's hard to say that in absolute terms 00:05:42.770 --> 00:05:46.460 because it's going to be defined by how that ratio is coupled 00:05:46.460 --> 00:05:50.940 to the ratio of whatever other reactants and products. 00:05:50.940 --> 00:05:54.410 It also tells you, remember, as we talked about before, we 00:05:54.410 --> 00:05:56.690 can switch the directions of these reactions 00:05:56.690 --> 00:05:58.190 and we just flip the signs. 00:05:58.190 --> 00:06:01.280 The directions here are arbitrary. 00:06:01.280 --> 00:06:03.170 And so that means it also tells us 00:06:03.170 --> 00:06:06.290 how much energy we need to put in if we're going 00:06:06.290 --> 00:06:10.590 to charge up an ATP-ADP ratio. 00:06:10.590 --> 00:06:12.470 In other words, if we want to synthesize 00:06:12.470 --> 00:06:17.930 ATP, that is take ADP plus phosphate 00:06:17.930 --> 00:06:20.300 and make an ATP, well, what's this? 00:06:20.300 --> 00:06:22.490 What we just-- the exact opposite sign 00:06:22.490 --> 00:06:24.830 of that delta G0 prime is going to now equal 00:06:24.830 --> 00:06:31.230 plus 7.5 kcals per mole. 00:06:31.230 --> 00:06:35.940 And so that means that the equilibrium of this reaction 00:06:35.940 --> 00:06:40.470 is going to lie to the left just as that one lied to the right. 00:06:40.470 --> 00:06:43.720 And it's going to lie to the left by that much. 00:06:43.720 --> 00:06:45.990 And if we want to know how much energy it then 00:06:45.990 --> 00:06:49.410 takes to make ATP, well, obviously, it's 00:06:49.410 --> 00:06:51.390 not going to happen because equilibrium 00:06:51.390 --> 00:06:52.560 favors this direction. 00:06:52.560 --> 00:06:54.720 It's not spontaneous in that direction. 00:06:54.720 --> 00:06:56.970 But at various conditions, we can 00:06:56.970 --> 00:07:00.030 know by plugging in our formula delta G equals delta 00:07:00.030 --> 00:07:09.305 G0 prime plus 7.5 kcals per mole plus RT times the log of ATP. 00:07:17.520 --> 00:07:20.420 So if I have my ADP-- 00:07:20.420 --> 00:07:24.740 I'm sorry-- my ATP concentration sufficiently low relative 00:07:24.740 --> 00:07:27.410 to my ADP concentration, I can still 00:07:27.410 --> 00:07:29.720 make this reaction happen. 00:07:29.720 --> 00:07:32.360 Now that's not going to give you an ATP-ADP ratio that 00:07:32.360 --> 00:07:35.360 is actually consistent with being able to do anything 00:07:35.360 --> 00:07:36.930 else into the cell. 00:07:36.930 --> 00:07:43.970 And so but what it says is that if I am able to couple energy 00:07:43.970 --> 00:07:48.320 from nutrient metabolism to this reaction 00:07:48.320 --> 00:07:53.750 and overcome this positive 7.5 kcals per mole, 00:07:53.750 --> 00:08:00.425 that basically will allow me to now synthesize ATP and maintain 00:08:00.425 --> 00:08:06.170 an ATP-ADP ratio in the cell that allows cells to carry out 00:08:06.170 --> 00:08:08.030 other functions by the same rules 00:08:08.030 --> 00:08:10.920 that we talked about that would be unfavorable. 00:08:10.920 --> 00:08:16.160 Now there's lots of implications here for metabolism, 00:08:16.160 --> 00:08:20.270 because it says how much input you need to synthesize ATP. 00:08:20.270 --> 00:08:23.210 How much energy you need to synthesize ATP 00:08:23.210 --> 00:08:26.420 is going to depend on how charged this ratio is. 00:08:26.420 --> 00:08:29.810 And how useful that ratio is to catalyze other reactions 00:08:29.810 --> 00:08:35.539 is also going to depend on how charged that ratio is. 00:08:35.539 --> 00:08:39.590 And in essence, this is why we cannot store energy as ATP. 00:08:39.590 --> 00:08:42.409 This is why we didn't eat ATP for breakfast 00:08:42.409 --> 00:08:46.280 and instead ate cereal that has a bunch of glucose in it, 00:08:46.280 --> 00:08:50.420 because we can use that glucose to get ATP but the ATP itself 00:08:50.420 --> 00:08:53.660 is not something that we really can burn to get energy 00:08:53.660 --> 00:08:55.160 in a sustained way. 00:08:55.160 --> 00:09:02.180 In other words, the delta G of ATP synthesis, 00:09:02.180 --> 00:09:05.840 making ATP is going to be proportional 00:09:05.840 --> 00:09:12.770 to the log of the ratio of ATP over ADP for all the reasons 00:09:12.770 --> 00:09:14.420 I described. 00:09:14.420 --> 00:09:19.280 The delta G of using ATP, ATP hydrolysis, 00:09:19.280 --> 00:09:22.940 using ATP to drive reactions is also 00:09:22.940 --> 00:09:28.010 proportional to RT times the log, in this case, of the ADP 00:09:28.010 --> 00:09:30.950 to ATP ratio. 00:09:30.950 --> 00:09:34.850 And so because you need to consume this ratio to get 00:09:34.850 --> 00:09:40.460 energy, and you need to put in energy to create this ratio, 00:09:40.460 --> 00:09:45.110 you are constant-- and how much is an exponential property-- 00:09:45.110 --> 00:09:48.200 you are constantly having to generate 00:09:48.200 --> 00:09:50.990 ATP in a way that allows you to then 00:09:50.990 --> 00:09:54.050 have a ratio that can then be consumed 00:09:54.050 --> 00:09:56.450 to fuel other reactions. 00:09:56.450 --> 00:09:58.380 And that's what's shown here on the slide. 00:09:58.380 --> 00:10:02.090 So I just graphed here what the ATP-ADP ratio 00:10:02.090 --> 00:10:04.940 is relative to a theoretical delta 00:10:04.940 --> 00:10:10.040 G that's shown here as a linear scale, here on a log scale, 00:10:10.040 --> 00:10:13.530 that basically there's this logarithmic relationship 00:10:13.530 --> 00:10:17.660 and so you need an exponential increase in energy input 00:10:17.660 --> 00:10:21.860 to drive this ratio up higher in order to, quote unquote, 00:10:21.860 --> 00:10:25.190 "store" more energy that can be used for later. 00:10:25.190 --> 00:10:29.720 And so this system is not one that you can use as an energy 00:10:29.720 --> 00:10:32.630 storage system and ultimately explains 00:10:32.630 --> 00:10:35.480 why cells have to do constant metabolism, 00:10:35.480 --> 00:10:38.630 have to constantly do some sort of metabolism 00:10:38.630 --> 00:10:41.750 to keep this ATP-ADP ratio charged 00:10:41.750 --> 00:10:46.070 for it to be in a useful range to have an ATP-ADP ratio that 00:10:46.070 --> 00:10:48.830 can be coupled to drive otherwise 00:10:48.830 --> 00:10:52.640 unfavorable reactions in our cells. 00:10:52.640 --> 00:10:54.440 So to say this another way, our cells 00:10:54.440 --> 00:10:58.400 need to constantly catabolize nutrients, burn glucose, 00:10:58.400 --> 00:10:59.930 burn some other fuel. 00:10:59.930 --> 00:11:01.760 They have to do this because they 00:11:01.760 --> 00:11:05.000 have to keep that ratio charged in a range 00:11:05.000 --> 00:11:08.120 where that can then support other cell functions, 00:11:08.120 --> 00:11:15.160 allow them to fight entropy and maintain order in biology. 00:11:15.160 --> 00:11:18.610 We alluded last time if you stop making ATP, well, 00:11:18.610 --> 00:11:20.020 what's going to happen? 00:11:20.020 --> 00:11:22.030 Well, that ATP-ADP ratio the cell 00:11:22.030 --> 00:11:24.960 has is going to fuel whatever reactions it 00:11:24.960 --> 00:11:28.480 can do until suddenly that ratio falls below the point where 00:11:28.480 --> 00:11:31.930 it's useful to drive those unfavorable reactions. 00:11:31.930 --> 00:11:34.060 In fact, if you stop ATP production, 00:11:34.060 --> 00:11:37.840 you will consume that ATP-ADP ratio in seconds. 00:11:37.840 --> 00:11:40.300 And this is why if you poison ATP production, 00:11:40.300 --> 00:11:42.340 cells die very quickly. 00:11:42.340 --> 00:11:45.430 Remember last time we mentioned you have a heart attack, 00:11:45.430 --> 00:11:47.200 blood flow stops going to a tissue. 00:11:47.200 --> 00:11:50.380 No more nutrients and oxygen. Cells die quickly. 00:11:50.380 --> 00:11:52.750 Cyanide poisoning does the same thing. 00:11:52.750 --> 00:11:55.690 Cells die quickly if you stop their ability 00:11:55.690 --> 00:12:00.920 to maintain this ATP-ADP ratio in the right range. 00:12:00.920 --> 00:12:03.910 And so remember the concentration, 00:12:03.910 --> 00:12:07.030 the absolute concentration of ATP for these reasons 00:12:07.030 --> 00:12:09.980 is not a good indicator of cell energy. 00:12:09.980 --> 00:12:12.430 The correct indicator of cell energy 00:12:12.430 --> 00:12:17.670 is the ratio of ATP to ADP. 00:12:17.670 --> 00:12:24.960 This term is sometimes referred to as the energy charge. 00:12:24.960 --> 00:12:30.390 And so that can be the ATP to ADP ratio, 00:12:30.390 --> 00:12:37.530 or oftentimes more correctly is the ATP to AMP ratio. 00:12:37.530 --> 00:12:40.260 Why the ATP to AMP ratio? 00:12:40.260 --> 00:12:43.030 Well, that's because once you understand this, 00:12:43.030 --> 00:12:46.770 you can now understand that cells have a failsafe mechanism 00:12:46.770 --> 00:12:50.790 to protect themselves if this ratio of ATP to ADP 00:12:50.790 --> 00:12:52.770 becomes too low. 00:12:52.770 --> 00:12:54.150 What is that mechanism? 00:12:54.150 --> 00:12:56.850 Well, you can carry out this reaction. 00:12:56.850 --> 00:13:00.120 Take two ATP molecules and turn them 00:13:00.120 --> 00:13:05.880 into transfer a phosphate to make an ATP plus an AMP. 00:13:05.880 --> 00:13:07.680 Why is that a failsafe mechanism? 00:13:07.680 --> 00:13:11.640 Well, if they care about this, if the ATP-ADP ratio is 00:13:11.640 --> 00:13:14.580 what maintains your ability to drive 00:13:14.580 --> 00:13:17.340 these thermodynamically unfavorable reactions, 00:13:17.340 --> 00:13:21.090 if I sacrifice the denominator by removing an ATP 00:13:21.090 --> 00:13:24.750 and get an ATP out of it, I've now at least temporarily 00:13:24.750 --> 00:13:26.610 fixed that ratio. 00:13:26.610 --> 00:13:30.090 Now this is something that obviously cannot go on forever. 00:13:30.090 --> 00:13:32.490 But in the short term, it does allow 00:13:32.490 --> 00:13:35.020 cells to protect that ratio. 00:13:35.020 --> 00:13:39.870 And in fact, what cells actually defend is this ATP-AMP ratio. 00:13:39.870 --> 00:13:44.820 And it's very, very difficult to change the ATP-AMP ratio 00:13:44.820 --> 00:13:48.040 in a cell and actually have it live. 00:13:48.040 --> 00:13:50.730 So it's really this ATP-AMP ratio 00:13:50.730 --> 00:13:53.880 that is most formally related to what 00:13:53.880 --> 00:13:56.740 would be called energy charge. 00:13:56.740 --> 00:14:00.840 But this is really why this is important is 00:14:00.840 --> 00:14:03.880 for all the reasons that I just described. 00:14:03.880 --> 00:14:04.930 All right. 00:14:04.930 --> 00:14:07.840 So hopefully, it's clear to you now 00:14:07.840 --> 00:14:14.110 why cells have to keep ATP-ADP ratio in the right range, why 00:14:14.110 --> 00:14:16.570 this is useful for them to now carry out 00:14:16.570 --> 00:14:19.570 all of these unfavorable reactions-- fight entropy, 00:14:19.570 --> 00:14:22.590 stave off death. 00:14:22.590 --> 00:14:26.430 And they can even have a failsafe here and sacrifice ADP 00:14:26.430 --> 00:14:31.280 as a way to try to maintain that as a very last resort. 00:14:31.280 --> 00:14:34.390 It also says why cells have to always do catabolism, 00:14:34.390 --> 00:14:37.090 because you always need the energy input to keep 00:14:37.090 --> 00:14:39.970 that ratio in the right range. 00:14:39.970 --> 00:14:43.330 However, you might imagine that, in real life, 00:14:43.330 --> 00:14:46.180 the messiness of biology, that there's 00:14:46.180 --> 00:14:48.850 lots of situations where the demand for ATP 00:14:48.850 --> 00:14:51.310 will suddenly increase. 00:14:51.310 --> 00:14:53.950 Well, of course, maybe you can just metabolize more sugar, 00:14:53.950 --> 00:14:57.760 do more nutrient catabolism to try to keep that ratio high. 00:14:57.760 --> 00:14:59.480 But that takes time. 00:14:59.480 --> 00:15:03.610 And so you need ways for cells to quickly respond 00:15:03.610 --> 00:15:07.540 to then allow them to mobilize all of this extra thing to-- 00:15:07.540 --> 00:15:09.760 extra fuel to do that. 00:15:09.760 --> 00:15:13.000 For example, we're walking around our ancestors 00:15:13.000 --> 00:15:16.120 on the plains somewhere and a lion appears. 00:15:16.120 --> 00:15:20.020 We don't have time to mobilize a bunch of new fuel. 00:15:20.020 --> 00:15:22.670 We have to be able to run away really quickly 00:15:22.670 --> 00:15:24.610 or the lion's going to eat us. 00:15:24.610 --> 00:15:27.670 And so cells have systems to suddenly respond 00:15:27.670 --> 00:15:33.050 to I need a lot more ATP demand to run away by having ways 00:15:33.050 --> 00:15:37.040 that one can buffer this ATP-ADP ratio, 00:15:37.040 --> 00:15:40.940 protect it in the short term, and allow cells time 00:15:40.940 --> 00:15:45.360 to ramp up nutrient metabolism in order to keep this high. 00:15:45.360 --> 00:15:45.860 All right. 00:15:45.860 --> 00:15:48.800 Well, if we wanted to create such a buffer system, 00:15:48.800 --> 00:15:51.050 how would we actually do it? 00:15:51.050 --> 00:15:52.820 Well, obviously, we have to follow 00:15:52.820 --> 00:15:56.360 all the rules of thermodynamics that we discussed. 00:15:56.360 --> 00:15:58.970 And we have to do so in a way that 00:15:58.970 --> 00:16:01.550 actually illustrates how it is that 00:16:01.550 --> 00:16:04.620 very well how metabolism works. 00:16:04.620 --> 00:16:09.290 In other words, if suddenly the ATP-ADP ratio 00:16:09.290 --> 00:16:14.750 drops, if we have a sudden drop in ATP-ADP ratio 00:16:14.750 --> 00:16:16.400 because we have increased demand, 00:16:16.400 --> 00:16:19.070 needing to run away from a lion, how 00:16:19.070 --> 00:16:21.350 could we actually create a buffer system 00:16:21.350 --> 00:16:23.210 that protects this? 00:16:23.210 --> 00:16:28.010 Well, to do it, we would need to couple ATP-ADP 00:16:28.010 --> 00:16:30.920 to some other product reactant pair, 00:16:30.920 --> 00:16:37.850 let's call them x and y, such that the ratio of y to x 00:16:37.850 --> 00:16:44.180 is coupled to the ratio of ATP to ADP. 00:16:44.180 --> 00:16:46.670 And you want it coupled in such a way 00:16:46.670 --> 00:16:51.560 that if that ratio goes down, this ratio can now protect ATP, 00:16:51.560 --> 00:16:55.730 basically favor ATP to ADP synthesis. 00:16:55.730 --> 00:16:57.920 However, when nutrients are plentiful 00:16:57.920 --> 00:17:00.830 and this ratio is really high, it then in turn 00:17:00.830 --> 00:17:05.180 can charge up this ratio so that it can buffer it later 00:17:05.180 --> 00:17:09.210 when the ATP-ADP ratio starts to fall. 00:17:09.210 --> 00:17:12.300 Now the most famous example of this 00:17:12.300 --> 00:17:15.400 actually comes from muscle physiology. 00:17:15.400 --> 00:17:18.599 And it's best described in terms of muscles 00:17:18.599 --> 00:17:21.270 and how they quickly respond to increased demand, 00:17:21.270 --> 00:17:23.460 like that running away from a lion. 00:17:23.460 --> 00:17:26.670 However, it in fact applies to many, many cells, 00:17:26.670 --> 00:17:31.410 including most non-muscle cells in animals. 00:17:31.410 --> 00:17:32.790 All right. 00:17:32.790 --> 00:17:35.040 And I should say that it turns out 00:17:35.040 --> 00:17:38.520 that if you turn off ATP synthesis in a muscle, 00:17:38.520 --> 00:17:41.970 people have estimated that your ATP-ADP ratio will fall 00:17:41.970 --> 00:17:46.230 into a range where it is no longer useful to do anything 00:17:46.230 --> 00:17:50.820 within a matter of seconds, some claim even less than a second. 00:17:50.820 --> 00:17:54.580 And so this system ends up being very important. 00:17:54.580 --> 00:17:57.300 Now the system that muscle uses, and many other cells 00:17:57.300 --> 00:18:06.170 do as well, is something called the creatine/creatine phosphate 00:18:06.170 --> 00:18:07.550 system. 00:18:07.550 --> 00:18:10.790 Now if you guys are weightlifters or bodybuilders 00:18:10.790 --> 00:18:13.460 out there, you probably have heard of creatinine 00:18:13.460 --> 00:18:15.710 because it's a very popular supplement that 00:18:15.710 --> 00:18:19.160 is taken for weightlifting, something I obviously 00:18:19.160 --> 00:18:20.720 don't do a lot of. 00:18:20.720 --> 00:18:26.120 But that creatine/creatine phosphate is out there. 00:18:26.120 --> 00:18:30.410 I'll describe to you what it actually does for muscle cells. 00:18:30.410 --> 00:18:33.470 And you can decide for yourself if this is actually 00:18:33.470 --> 00:18:39.050 useful as a energy or weightlifting supplement. 00:18:39.050 --> 00:18:41.300 So what is creatine/creatine phosphate? 00:18:41.300 --> 00:18:42.600 What does it look like? 00:18:42.600 --> 00:18:45.410 Well, it's basically looks like this. 00:18:50.880 --> 00:18:54.090 By the way, I will use this abbreviation often 00:18:54.090 --> 00:18:56.310 throughout this course, just a shorthand 00:18:56.310 --> 00:18:58.360 to draw a lot of these structures. 00:18:58.360 --> 00:19:02.850 So this here would be a carboxylic acid group 00:19:02.850 --> 00:19:08.550 drawn like this or like that, drawn to the carboxylic acid 00:19:08.550 --> 00:19:12.510 group just so you understand my shorthand. 00:19:30.230 --> 00:19:30.920 OK. 00:19:30.920 --> 00:19:34.610 So this here is the molecule creatine. 00:19:34.610 --> 00:19:42.410 If I put a phosphate here, this now becomes creatine phosphate. 00:19:42.410 --> 00:19:45.350 Again, just to remind you, this P with a circle over it 00:19:45.350 --> 00:19:46.860 is a phosphate group. 00:19:46.860 --> 00:19:49.160 I'll draw this particular phosphate group 00:19:49.160 --> 00:19:51.770 because it's a nitrogen-phosphate bond, 00:19:51.770 --> 00:19:55.560 a phosphoramidate bond. 00:19:55.560 --> 00:19:59.160 This is actually a bond that is more labile than the PO bonds 00:19:59.160 --> 00:20:02.250 that we're used to seeing in the phosphodiesters like ATP, 00:20:02.250 --> 00:20:04.170 et cetera. 00:20:04.170 --> 00:20:04.800 All right. 00:20:04.800 --> 00:20:07.920 So that's creatine and creatine phosphate. 00:20:07.920 --> 00:20:18.910 And if we draw out the reaction, creatine phosphate 00:20:18.910 --> 00:20:26.680 goes to creatine plus inorganic phosphate, so hydrolysis 00:20:26.680 --> 00:20:28.330 of creatine phosphate. 00:20:28.330 --> 00:20:36.427 This has a delta G0 prime of minus 10 kcals per mole. 00:20:36.427 --> 00:20:37.260 What does that mean? 00:20:37.260 --> 00:20:38.983 It's negative. 00:20:38.983 --> 00:20:40.650 Tells us about the equilibrium constant. 00:20:40.650 --> 00:20:43.020 Equilibrium lies to the right. 00:20:43.020 --> 00:20:48.591 Remember ATP to ADP hydrolysis shown over there, 00:20:48.591 --> 00:20:51.090 that hydrolysis has a delta G0 prime 00:20:51.090 --> 00:20:53.760 of minus 7.5 kcals per mole. 00:20:53.760 --> 00:20:55.980 Equilibrium also lies to the right. 00:20:55.980 --> 00:20:57.570 This number is more negative. 00:20:57.570 --> 00:20:59.970 This equilibrium lies further to the right 00:20:59.970 --> 00:21:03.120 than ATP hydrolysis does. 00:21:03.120 --> 00:21:06.810 So now what happens if we couple this reaction 00:21:06.810 --> 00:21:09.040 to the ATP hydrolysis reaction? 00:21:09.040 --> 00:21:16.260 So now we can do ATP plus creatine goes 00:21:16.260 --> 00:21:23.510 to creatine phosphate plus ADP. 00:21:23.510 --> 00:21:26.640 So we've coupled those two reactions together. 00:21:26.640 --> 00:21:32.240 So delta G0 prime, ATP to ADP is minus 7.5. 00:21:32.240 --> 00:21:34.580 Creatine phosphate to creatine in 00:21:34.580 --> 00:21:37.730 that direction is negative 10. 00:21:37.730 --> 00:21:41.480 In this direction, it would be positive 10. 00:21:41.480 --> 00:21:43.640 And so as I've drawn the reaction, 00:21:43.640 --> 00:21:49.700 this delta G0 prime will be plus 2.5 kcals per mole. 00:21:49.700 --> 00:21:56.590 10 minus 7.5 plus 2.5, which tells us then 00:21:56.590 --> 00:22:01.960 that this equilibrium now lies to the left. 00:22:01.960 --> 00:22:06.880 However, which direction this reaction actually goes 00:22:06.880 --> 00:22:10.120 is not determined only by the equilibrium. 00:22:10.120 --> 00:22:14.210 It's really determined by delta G. So we can calculate that. 00:22:14.210 --> 00:22:20.800 So delta G equals delta G0 prime plus RT 00:22:20.800 --> 00:22:37.340 log of, as we've drawn it, creatine phosphate 00:22:37.340 --> 00:22:41.690 over creatine and ADP over ATP. 00:22:41.690 --> 00:22:45.920 So you can now imagine that this term here 00:22:45.920 --> 00:22:47.460 is the sum of those two. 00:22:47.460 --> 00:22:50.450 So it's plus 2.5. 00:22:50.450 --> 00:22:59.370 And so if this term is less than 2.5, what does that mean? 00:22:59.370 --> 00:23:00.680 Well, if it's-- 00:23:00.680 --> 00:23:01.310 I'm sorry. 00:23:01.310 --> 00:23:06.290 Less than negative 2.5, then delta G will be negative 00:23:06.290 --> 00:23:09.890 and you will favor creatine phosphate production. 00:23:09.890 --> 00:23:14.420 If this term is greater than negative 2.5, 00:23:14.420 --> 00:23:17.030 then delta G will be positive and you 00:23:17.030 --> 00:23:20.360 will favor ATP production. 00:23:20.360 --> 00:23:22.020 Why is that important? 00:23:22.020 --> 00:23:24.950 Well, if you think about the conditions, 00:23:24.950 --> 00:23:28.190 how you could make this term less than 2.5 00:23:28.190 --> 00:23:32.600 or less than negative 2.5 or greater than negative 2.5, 00:23:32.600 --> 00:23:34.130 basically, what's going to happen 00:23:34.130 --> 00:23:42.620 is that if the ATP-ADP ratio is high, 00:23:42.620 --> 00:23:45.710 lots of energy around, no problems charging up, 00:23:45.710 --> 00:23:49.520 lots of things to use, charge up this ATP/ADP to a very 00:23:49.520 --> 00:23:53.480 high ratio, well, now you will favor this direction, 00:23:53.480 --> 00:23:55.880 creatine phosphate synthesis. 00:23:55.880 --> 00:24:03.380 However, if ATP/ADP begins to fall and is lower, 00:24:03.380 --> 00:24:07.340 well, now this term is going to flip 00:24:07.340 --> 00:24:10.970 and you'll now favor ATP production. 00:24:10.970 --> 00:24:15.380 And so basically, by charging up this creatine/creatine 00:24:15.380 --> 00:24:18.410 phosphate ratio, it now creates a situation 00:24:18.410 --> 00:24:22.910 where it can buffer that ATP/ADP ratio so that when times are 00:24:22.910 --> 00:24:24.680 good, you charge up the buffer. 00:24:24.680 --> 00:24:27.560 When times are bad, you then consume the buffer 00:24:27.560 --> 00:24:31.520 in a way that allows you to protect your ATP/ADP 00:24:31.520 --> 00:24:33.330 ratio for a short time. 00:24:33.330 --> 00:24:35.210 And this is illustrated very nicely here 00:24:35.210 --> 00:24:38.330 in this slide that basically shows what 00:24:38.330 --> 00:24:40.020 happens in muscle physiology. 00:24:40.020 --> 00:24:42.500 And so this is just a theoretical curve 00:24:42.500 --> 00:24:45.830 that if you suddenly need to contract your muscle 00:24:45.830 --> 00:24:50.090 and you stop ATP production, well, ATP levels will fall. 00:24:50.090 --> 00:24:54.350 That ATP/ADP ratio more correctly will fall rapidly 00:24:54.350 --> 00:24:55.820 within the cell. 00:24:55.820 --> 00:24:59.030 Creatine phosphate will then now kick in 00:24:59.030 --> 00:25:03.620 to protect that ATP/ADP ratio as you can ramp up 00:25:03.620 --> 00:25:07.250 either anaerobic or aerobic metabolism to now buffer 00:25:07.250 --> 00:25:13.450 that ATP/ADP ratio and keep that muscle functioning. 00:25:13.450 --> 00:25:17.680 Now I want to make one point because oftentimes biochemistry 00:25:17.680 --> 00:25:21.280 textbooks will refer to things like ATP 00:25:21.280 --> 00:25:24.140 or creatine phosphate as, quote unquote, 00:25:24.140 --> 00:25:26.860 "high energy compounds." 00:25:26.860 --> 00:25:28.810 And of course, I guess they are high energy 00:25:28.810 --> 00:25:31.780 because they have these labile phosphate-nitrogen 00:25:31.780 --> 00:25:35.690 or phosphate-oxygen bonds. 00:25:35.690 --> 00:25:38.320 And because they're labile, hydrolysis is favorable. 00:25:38.320 --> 00:25:41.560 And to a certain extent, it's that hydrolysis 00:25:41.560 --> 00:25:44.710 that is providing energy. 00:25:44.710 --> 00:25:46.810 But remember it's not the metabolite 00:25:46.810 --> 00:25:50.650 or the bond in isolation that's really providing the energy. 00:25:50.650 --> 00:25:56.380 It's these ratios of ATP to ADP, creatine phosphate to creatine, 00:25:56.380 --> 00:25:59.200 that allow you to couple, for all the reasons 00:25:59.200 --> 00:26:01.750 we've spent now a lecture and a half discussing, 00:26:01.750 --> 00:26:08.380 two other reactions that really provides biological energy. 00:26:08.380 --> 00:26:11.120 All right. 00:26:11.120 --> 00:26:13.490 Now we're ready to discuss how it 00:26:13.490 --> 00:26:19.130 is that we can use glucose oxidation, nutrient catabolism, 00:26:19.130 --> 00:26:24.890 to keep ATP/ADP high, that is in a useful range, where it can 00:26:24.890 --> 00:26:27.890 allow cells to do otherwise thermodynamically 00:26:27.890 --> 00:26:29.570 unfavorable things. 00:26:29.570 --> 00:26:32.550 So how could we make this happen? 00:26:32.550 --> 00:26:35.960 Well, just like this example with creatine and creatine 00:26:35.960 --> 00:26:39.350 phosphate, if we want to use nutrient catabolism 00:26:39.350 --> 00:26:43.700 to do the same thing, we need to couple reactions 00:26:43.700 --> 00:26:46.940 of glucose oxidation, nutrient catabolism, that 00:26:46.940 --> 00:26:51.500 will allow us to synthesize ATP, ADP to ATP, 00:26:51.500 --> 00:26:53.810 and have that synthesis be favorable 00:26:53.810 --> 00:26:59.150 despite the high ATP/ADP ratio in cells. 00:26:59.150 --> 00:27:00.660 So let's think about it. 00:27:00.660 --> 00:27:01.730 So what is this? 00:27:01.730 --> 00:27:02.900 Nutrient catabolism. 00:27:02.900 --> 00:27:04.520 What did we talk about last time? 00:27:04.520 --> 00:27:08.480 Oxidizing glucose to carbon and CO2. 00:27:08.480 --> 00:27:11.580 So here's our reaction for that. 00:27:11.580 --> 00:27:19.880 So here's carbohydrate, glucose CH2O6 plus six O2 00:27:19.880 --> 00:27:25.850 goes to six CO2 plus six H2O. 00:27:25.850 --> 00:27:33.170 That's the chemical formula for burning wood. 00:27:33.170 --> 00:27:41.570 So delta G0 prime for this is minus 686 kcals per mole. 00:27:44.720 --> 00:27:46.460 Quite a negative number. 00:27:46.460 --> 00:27:52.850 Equilibrium greatly favors the completely burned state, 00:27:52.850 --> 00:27:56.880 oxidized state, of that carbohydrate. 00:27:56.880 --> 00:27:59.360 And so you can imagine that this equilibrium is 00:27:59.360 --> 00:28:04.160 so favorable to the right, if we couple this reaction to ATP 00:28:04.160 --> 00:28:08.240 synthesis, you're clearly going to have plenty of energy there 00:28:08.240 --> 00:28:10.580 in order to maintain that ATP/ADP 00:28:10.580 --> 00:28:15.350 ratio in the right range to be compatible with cells 00:28:15.350 --> 00:28:17.670 using it to do other things. 00:28:17.670 --> 00:28:18.170 All right. 00:28:18.170 --> 00:28:21.600 So how do we actually do that? 00:28:21.600 --> 00:28:23.720 Well, if we do it, we have to follow 00:28:23.720 --> 00:28:27.050 the same rules of thermodynamics and do the same things 00:28:27.050 --> 00:28:28.130 that we did before. 00:28:28.130 --> 00:28:34.160 Basically, we have to do this reaction in a way 00:28:34.160 --> 00:28:37.820 that we can draw out a similar type of equation 00:28:37.820 --> 00:28:41.360 as we did for creatine and creatine phosphate. 00:28:41.360 --> 00:28:45.650 And at the highest level, that is how nutrient catabolism 00:28:45.650 --> 00:28:49.620 will allow us to keep ATP/ADP ratio high 00:28:49.620 --> 00:28:52.910 even as ATP hydrolysis is constantly 00:28:52.910 --> 00:28:56.540 fueling all these other unfavorable processes 00:28:56.540 --> 00:28:58.810 in the cell. 00:28:58.810 --> 00:29:03.510 Now if we did this all in one step, our burning wood example, 00:29:03.510 --> 00:29:06.320 lots of heat is generated. 00:29:06.320 --> 00:29:07.760 That's too much energy. 00:29:07.760 --> 00:29:11.210 Not so good for biology. 00:29:11.210 --> 00:29:15.320 But rather what biology does, as we alluded to last time, 00:29:15.320 --> 00:29:17.780 is that because what we talked about in the beginning, 00:29:17.780 --> 00:29:20.150 that all of the things that we discussed 00:29:20.150 --> 00:29:22.610 are true for individual reactions 00:29:22.610 --> 00:29:28.370 or many, many reactions, basically, if we turn A into B 00:29:28.370 --> 00:29:31.400 and we do it in one step, like burning wood, 00:29:31.400 --> 00:29:37.400 or we do it in 500 steps, the thermodynamic consequences 00:29:37.400 --> 00:29:39.570 are exactly the same. 00:29:39.570 --> 00:29:45.530 And so what we need to do is do stepwise catabolism of glucose 00:29:45.530 --> 00:29:48.890 and take advantage of the fact that that delta G change is 00:29:48.890 --> 00:29:52.820 the same whether it's one step or many steps 00:29:52.820 --> 00:29:57.830 and in that stepwise fashion build in reactions where 00:29:57.830 --> 00:30:03.500 we create intermediate chemistries that allow us to do 00:30:03.500 --> 00:30:06.740 things like we did with creatine and creatine phosphate 00:30:06.740 --> 00:30:12.110 such that we can favor ATP synthesis when ATP/ADP ratio is 00:30:12.110 --> 00:30:14.130 in the physiological range. 00:30:14.130 --> 00:30:16.010 And in essence, this is the magic 00:30:16.010 --> 00:30:22.800 of how biology does this and basically couples 00:30:22.800 --> 00:30:26.770 nutrient catabolism in order to provide energy. 00:30:26.770 --> 00:30:30.330 And so the pathway that we're going to discuss today 00:30:30.330 --> 00:30:31.860 and in the next lecture is something 00:30:31.860 --> 00:30:41.130 called glycolysis, which simply put is glucose lysis. 00:30:41.130 --> 00:30:45.060 It's the stepwise chemical oxidation 00:30:45.060 --> 00:30:47.640 of glucose, which we know from our burning wood 00:30:47.640 --> 00:30:49.500 example is favorable. 00:30:49.500 --> 00:30:54.150 And we're going to use this pathway that basically will 00:30:54.150 --> 00:30:58.350 be a pathway to allow the stepwise lysis of glucose, 00:30:58.350 --> 00:31:03.180 a favorable reaction, to individual steps that set it up 00:31:03.180 --> 00:31:06.360 in a way such that ADP to ATP synthesis 00:31:06.360 --> 00:31:12.780 is favored despite a physiological ATP/ADP ratio. 00:31:12.780 --> 00:31:15.720 Now I want to point out that this really illustrates, 00:31:15.720 --> 00:31:18.360 back to that high energy compound thing, 00:31:18.360 --> 00:31:21.220 that ATP is not the only energy currency in the cell. 00:31:21.220 --> 00:31:26.370 In fact, glucose is much more energy for a cell than ATP. 00:31:26.370 --> 00:31:29.910 You get much more energy released from burning wood 00:31:29.910 --> 00:31:32.670 than you get from hydrolyzing ATP. 00:31:32.670 --> 00:31:35.040 And in essence, this is why we use 00:31:35.040 --> 00:31:38.400 glucose-- why nature has used glucose, 00:31:38.400 --> 00:31:41.940 and other molecules like it, as an energy storage 00:31:41.940 --> 00:31:44.880 device, because here is a relatively stable molecule, 00:31:44.880 --> 00:31:47.490 can sit around for a long time, and you 00:31:47.490 --> 00:31:52.670 can burn it when you need it to keep your ATP-ADP ratio high. 00:31:52.670 --> 00:31:54.080 All right. 00:31:54.080 --> 00:31:57.260 So, at the level of reactions and pathways 00:31:57.260 --> 00:32:01.520 now, how can we use glucose oxidation and couple 00:32:01.520 --> 00:32:03.140 it to drive ATP synthesis? 00:32:03.140 --> 00:32:05.540 Well, hopefully it's very clear that we 00:32:05.540 --> 00:32:07.400 have to basically set up a situation 00:32:07.400 --> 00:32:09.230 like this creatine-creatine-phosphate 00:32:09.230 --> 00:32:12.540 system that we already discussed. 00:32:12.540 --> 00:32:15.200 And so I want to start by discussing, 00:32:15.200 --> 00:32:19.220 what are the reactions in glycolysis that ultimately 00:32:19.220 --> 00:32:27.620 behave like this and allow this favorable ADP-to-ATP synthesis? 00:32:27.620 --> 00:32:30.350 And then we will go ahead and consider 00:32:30.350 --> 00:32:33.380 how we can make those reactions work 00:32:33.380 --> 00:32:36.560 in the context of an entire pathway that 00:32:36.560 --> 00:32:40.550 allows glucose oxidation. 00:32:40.550 --> 00:32:42.680 All right. 00:32:42.680 --> 00:32:49.050 Now, like creatine, phosphate, and creatine, 00:32:49.050 --> 00:32:52.080 both of the steps in glycolysis that 00:32:52.080 --> 00:32:56.190 favor ATP synthesis involve intermediates 00:32:56.190 --> 00:33:01.860 where you have phosphate on some molecule, 00:33:01.860 --> 00:33:09.210 plus ADP goes to ATP plus the phosphate 00:33:09.210 --> 00:33:11.170 coming off that molecule. 00:33:11.170 --> 00:33:14.040 Now, if we go to our creatine-creatine-phosphate 00:33:14.040 --> 00:33:17.370 system, obviously this equilibrium 00:33:17.370 --> 00:33:20.340 needs to lie to the right. 00:33:20.340 --> 00:33:25.650 And so the first one of these is as follows. 00:33:52.020 --> 00:33:56.980 It involves this molecule, phosphoenolpyruvate, also 00:33:56.980 --> 00:33:59.403 referred to as PEP. 00:33:59.403 --> 00:34:00.820 You will see throughout the course 00:34:00.820 --> 00:34:04.360 that we love to give these little acronyms 00:34:04.360 --> 00:34:07.450 to our metabolites because it's hard to constantly 00:34:07.450 --> 00:34:09.699 say and write out phosphoenolpyruvate. 00:34:09.699 --> 00:34:12.190 Much easier to say PEP. 00:34:12.190 --> 00:34:19.929 So phosphoenolpyruvate plus ADP goes to ATP. 00:34:19.929 --> 00:34:22.840 Plus, if I take the phosphate off of there, 00:34:22.840 --> 00:34:24.219 I get this molecule. 00:34:30.340 --> 00:34:33.875 This molecule is enolpyruvate. 00:34:38.230 --> 00:34:43.525 Enolpyruvate can be rearranged as follows. 00:35:04.200 --> 00:35:08.370 To make this molecule a ketone pyruvate. 00:35:08.370 --> 00:35:11.040 You'll note-- hopefully remember from your organic chemistry 00:35:11.040 --> 00:35:13.920 classes that ketones like pyruvate 00:35:13.920 --> 00:35:15.300 can exist in two forms. 00:35:15.300 --> 00:35:18.690 They can exist in the ketone form or the enol form, 00:35:18.690 --> 00:35:21.990 rearranging via the chemistry that I've shown. 00:35:21.990 --> 00:35:33.770 The keto form is greatly favored over the enol form. 00:35:33.770 --> 00:35:38.690 However, by having a phosphate trap pyruvate the enol form-- 00:35:38.690 --> 00:35:42.140 phosphoenolpyruvate, when you remove the phosphate 00:35:42.140 --> 00:35:45.290 and it's wanting to exist in the keto form, 00:35:45.290 --> 00:35:49.010 greatly enhances the energetics of this-- 00:35:49.010 --> 00:35:51.260 basically makes the reaction, the equilibrium 00:35:51.260 --> 00:35:54.260 want to lie very far to the right. 00:35:54.260 --> 00:35:56.750 And by the way of chemical tricks like this 00:35:56.750 --> 00:36:02.330 and others like it, you will see used over and over in biology. 00:36:02.330 --> 00:36:04.490 In fact, this is illustrated, if we 00:36:04.490 --> 00:36:07.730 asked what's delta G0 prime for this reaction, 00:36:07.730 --> 00:36:14.560 it's minus 15 kcals per mole. 00:36:14.560 --> 00:36:18.100 That tells us that the equilibrium of this reaction 00:36:18.100 --> 00:36:19.120 lies to the right. 00:36:19.120 --> 00:36:23.520 In fact, it lies far to the right, and in fact, this is-- 00:36:23.520 --> 00:36:29.360 you may remember ATP hydrolysis is negative 7.5 kcals per mole. 00:36:29.360 --> 00:36:35.380 So lies twice as far to the right as ATP hydrolysis does. 00:36:35.380 --> 00:36:41.830 And in fact, this minus 15 kcals per mole is the most favorable, 00:36:41.830 --> 00:36:44.950 at least in terms of individual reactions 00:36:44.950 --> 00:36:48.940 that at least I know of in metabolism. 00:36:48.940 --> 00:36:49.990 All right. 00:36:49.990 --> 00:37:02.930 Well, if this equilibrium for this is PEP to pyruvate, 00:37:02.930 --> 00:37:06.095 this here is delta G0 prime for ATP-ADP. 00:37:10.130 --> 00:37:12.200 So if we want to know what's delta G0 00:37:12.200 --> 00:37:17.130 prime for the entire reaction, well, if PEP to pyruvate 00:37:17.130 --> 00:37:21.990 is minus 15, ATP to ADP is minus 7.5. 00:37:21.990 --> 00:37:24.100 Well here, we're going in the opposite direction, 00:37:24.100 --> 00:37:25.890 so it'd be plus 7.5. 00:37:25.890 --> 00:37:31.230 So it's minus 15 plus 7.5, means that for the entire reaction, 00:37:31.230 --> 00:37:36.660 this is minus 7.5 kcals per mole. 00:37:36.660 --> 00:37:41.430 Means that this is favorable, equilibrium lies to the right. 00:37:41.430 --> 00:37:42.420 If we drew out-- 00:37:42.420 --> 00:37:45.340 I'm not going to do it again because I've done it a lot, 00:37:45.340 --> 00:37:47.400 but if we redrew out this equation, 00:37:47.400 --> 00:37:50.580 replacing pyruvate and PEP in the correct places, 00:37:50.580 --> 00:37:53.040 there for creatine, phosphate, and creatine, 00:37:53.040 --> 00:37:56.040 you would see that there's lots of conditions within cells 00:37:56.040 --> 00:37:59.250 that will favor ATP synthesis even 00:37:59.250 --> 00:38:03.820 when ATP-ADP ratio is high. 00:38:03.820 --> 00:38:04.320 All right. 00:38:04.320 --> 00:38:11.505 This reaction is catalyzed by an enzyme called pyruvate kinase. 00:38:17.570 --> 00:38:23.570 And pyruvate kinase is a classic example of a, quote-unquote, 00:38:23.570 --> 00:38:26.550 "irreversible reaction." 00:38:26.550 --> 00:38:29.600 However, I want to point out that the enzyme pyruvate 00:38:29.600 --> 00:38:33.830 kinase is actually named for the reverse reaction that's 00:38:33.830 --> 00:38:36.210 supposedly irreversible. 00:38:36.210 --> 00:38:37.170 Pyruvate kinase. 00:38:37.170 --> 00:38:39.680 If you take pyruvate and you act on the kinase 00:38:39.680 --> 00:38:41.530 as you learn from Professor Yaffe, 00:38:41.530 --> 00:38:42.780 that phosphorylates something. 00:38:42.780 --> 00:38:44.238 You phosphorylate pyruvate, you get 00:38:44.238 --> 00:38:47.220 phosphoenolpyruvate pyruvate kinase. 00:38:47.220 --> 00:38:54.590 And so remember that absolutely no reaction is reversible. 00:38:54.590 --> 00:38:57.440 It's only reversed-- what they mean by irreversible when they 00:38:57.440 --> 00:39:00.980 say pyruvate kinase step in glycolysis is irreversible, 00:39:00.980 --> 00:39:04.220 is that it is effectively irreversible 00:39:04.220 --> 00:39:07.190 under the conditions that are compatible with life 00:39:07.190 --> 00:39:09.110 that exist in cells. 00:39:09.110 --> 00:39:14.600 Of course, if you have very low ADP and very high ATP, 00:39:14.600 --> 00:39:19.400 you can come up with a way to net synthesize PEP. 00:39:19.400 --> 00:39:23.180 You just can't drive the reverse reaction 00:39:23.180 --> 00:39:27.080 under the ATP-ADP and the PEP pyruvate ratios that 00:39:27.080 --> 00:39:30.160 physiologically exist in cells. 00:39:30.160 --> 00:39:33.190 However, this is set up in a way such 00:39:33.190 --> 00:39:38.050 that you can favor ADP-to-ATP synthesis by converting 00:39:38.050 --> 00:39:41.050 PEP to pyruvate in a pathway, as we 00:39:41.050 --> 00:39:44.440 will see happens in glycolysis. 00:39:44.440 --> 00:39:45.640 All right. 00:39:45.640 --> 00:39:50.780 Now, the second reaction that I want to-- 00:39:50.780 --> 00:39:56.360 that allows ATP synthesis in glycolysis 00:39:56.360 --> 00:40:10.050 is catalyzed by an enzyme called phosphoglycerate kinase. 00:40:10.050 --> 00:40:13.630 And this carries out the following reaction. 00:40:27.660 --> 00:40:28.160 OK. 00:40:28.160 --> 00:40:41.090 So this molecule is 1,3-Bisphosphoglycerate, 00:40:41.090 --> 00:40:42.830 abbreviated 1,3-BPG. 00:41:08.120 --> 00:41:13.120 And basically the phosphate is transferred from here 00:41:13.120 --> 00:41:24.050 to ADP, which gives you this molecule, 3-phosphoglycerate 00:41:24.050 --> 00:41:24.710 or 3-PG. 00:41:28.750 --> 00:41:32.830 So this reaction, the delta G0 prime 00:41:32.830 --> 00:41:36.790 for the entire coupled reaction, both steps of the reaction, 00:41:36.790 --> 00:41:42.222 is minus 4.5 kcals per mole. 00:41:42.222 --> 00:41:43.930 Tells you about the equilibrium constant. 00:41:43.930 --> 00:41:47.650 Equilibrium lies to the right, doesn't lie as far to the right 00:41:47.650 --> 00:41:52.660 as the PEP plus ADP goes to ATP plus pyruvate reaction, 00:41:52.660 --> 00:41:55.660 but nonetheless, is one where there's 00:41:55.660 --> 00:42:00.640 lots of conditions that are favorable in cells to maintain 00:42:00.640 --> 00:42:04.630 a high ATP-ADP ratio, although we will see later 00:42:04.630 --> 00:42:08.393 that this reaction actually runs quite close to equilibrium 00:42:08.393 --> 00:42:10.060 in cells, so there's actually conditions 00:42:10.060 --> 00:42:12.250 where it can go in the opposite direction as well, 00:42:12.250 --> 00:42:15.290 and we'll get to that later in the course. 00:42:15.290 --> 00:42:17.710 I want to point out one thing about this, one thing that 00:42:17.710 --> 00:42:20.290 makes this reaction favorable, as you notice here 00:42:20.290 --> 00:42:23.980 that this is a phosphate on an acid group, 00:42:23.980 --> 00:42:26.290 a so-called acid anhydride. 00:42:26.290 --> 00:42:29.180 And this is a pretty good phosphate donor. 00:42:29.180 --> 00:42:31.840 And so you want to lose that phosphate, which 00:42:31.840 --> 00:42:35.200 is one of the reasons why this reaction is 00:42:35.200 --> 00:42:42.160 favorable for ATP synthesis even at high ATP-ADP ratios. 00:42:42.160 --> 00:42:44.020 All right. 00:42:44.020 --> 00:42:46.930 So these are the two reactions in glycolysis 00:42:46.930 --> 00:42:50.560 that can favor ATP synthesis at a high ATP-ADP ratio 00:42:50.560 --> 00:42:53.210 just like creatine and creatine phosphate. 00:42:53.210 --> 00:42:58.330 And so hopefully now it's become apparent to you that if during 00:42:58.330 --> 00:43:02.230 our pathway of glucose catabolism we can build 00:43:02.230 --> 00:43:07.030 a pathway that couples what we already know is favorable, 00:43:07.030 --> 00:43:10.640 glucose oxidation, to generate these intermediates, 00:43:10.640 --> 00:43:15.970 PEP and 1,3-Bisphosphoglycerate, lacerate we can then build 00:43:15.970 --> 00:43:20.560 a pathway where those reactions can make synthesis of ATP 00:43:20.560 --> 00:43:25.640 favorable even though there's a high ATP-ADP ratio in cells, 00:43:25.640 --> 00:43:29.560 and in fact, maintain that high ATP-ADP ratio in cells. 00:43:29.560 --> 00:43:32.750 And in essence, this is the logic or the goal, 00:43:32.750 --> 00:43:36.490 if you will, of glycolysis and why 00:43:36.490 --> 00:43:40.975 it's important to provide energy to cells. 00:43:40.975 --> 00:43:41.475 OK. 00:44:14.170 --> 00:44:14.800 All right. 00:44:14.800 --> 00:44:22.100 Now, if we're going to make a pathway where glucose oxidation 00:44:22.100 --> 00:44:27.310 is coupled to making PEP and 1,3-Bisphosphoglycerate, 00:44:27.310 --> 00:44:32.730 it's obvious we need to add phosphate to the system. 00:44:32.730 --> 00:44:37.020 There's no phosphate molecule on glucose. 00:44:37.020 --> 00:44:39.510 Now already mentioned earlier-- 00:44:39.510 --> 00:44:41.520 and I just erased it-- 00:44:41.520 --> 00:44:46.770 that to trap glucose in the cell, the first step 00:44:46.770 --> 00:44:50.220 in glycolysis, you add phosphate. 00:44:50.220 --> 00:44:53.280 But that phosphate donor is ATP. 00:44:53.280 --> 00:44:57.330 And so ATP being the phosphate donor doesn't help us here. 00:44:57.330 --> 00:44:59.970 If we're going to use phosphate transfer here, 00:44:59.970 --> 00:45:03.540 we need to have phosphate added into the system 00:45:03.540 --> 00:45:04.830 if this is going to work. 00:45:04.830 --> 00:45:08.280 We can't have ATP as our phosphate donor. 00:45:08.280 --> 00:45:12.780 You also saw lots of reactions from Professor Yaffe 00:45:12.780 --> 00:45:15.990 that involved phosphate transfer reactions. 00:45:15.990 --> 00:45:21.820 But again, ATP-- or GTP is the phosphate donor. 00:45:21.820 --> 00:45:27.030 And so for this system to work, we need a way to get inorganic 00:45:27.030 --> 00:45:33.780 phosphate onto a carbohydrate or we can't make things like PEP 00:45:33.780 --> 00:45:38.970 or 1,3-Bisphosphoglycerate that will allow ATP synthesis. 00:45:38.970 --> 00:45:43.860 Now we already saw earlier that additional phosphate to glucose 00:45:43.860 --> 00:45:45.120 is not favored. 00:45:45.120 --> 00:45:49.170 Remember, delta G0 prime of phosphorylating glucose 00:45:49.170 --> 00:45:53.540 was plus 3.3 kcals per mole. 00:45:53.540 --> 00:45:55.480 And it turns out that says equilibrium 00:45:55.480 --> 00:45:58.780 lies to the wrong side, so we can't net do that 00:45:58.780 --> 00:46:00.910 without ATP or energy input. 00:46:00.910 --> 00:46:04.100 And it turns out this is true for other sugars as well. 00:46:04.100 --> 00:46:08.710 And so you need energy input to add phosphate to the sugars. 00:46:11.350 --> 00:46:15.820 Plenty of energy input from glucose oxidation, 00:46:15.820 --> 00:46:20.200 but we need a reaction where phosphate addition is coupled 00:46:20.200 --> 00:46:22.930 to another reaction that makes it favorable, 00:46:22.930 --> 00:46:26.970 and we have to do this without making ATP. 00:46:26.970 --> 00:46:28.680 And again, it illustrates, again, 00:46:28.680 --> 00:46:31.530 the biological energy is not just about ATP. 00:46:31.530 --> 00:46:34.760 At a deeper level It's something different. 00:46:34.760 --> 00:46:37.840 And so to really discuss this and understand it, 00:46:37.840 --> 00:46:40.420 I need to introduce two concepts that 00:46:40.420 --> 00:46:45.070 are going to come up again over and over again in this class 00:46:45.070 --> 00:46:47.440 and across metabolism. 00:46:47.440 --> 00:46:51.820 The first of these is a very high-level bioenergetic 00:46:51.820 --> 00:46:55.810 concept, although it's really an energetic concept, 00:46:55.810 --> 00:46:59.270 and it comes back to what we already talked about. 00:46:59.270 --> 00:47:02.980 So, we know burning wood is favorable. 00:47:06.150 --> 00:47:20.660 The CN H2O N plus oxygen going to CO2 plus water. 00:47:20.660 --> 00:47:26.060 That's burning wood, lots of energy released. 00:47:26.060 --> 00:47:28.500 Drew it up there for a single-glucose molecule 00:47:28.500 --> 00:47:31.130 and what the energy release the delta G0 00:47:31.130 --> 00:47:33.870 prime is for that reaction. 00:47:33.870 --> 00:47:35.520 Now I want to point out chemically 00:47:35.520 --> 00:47:39.480 what is going on in this reaction. 00:47:39.480 --> 00:47:42.000 Chemically what is happening for this 00:47:42.000 --> 00:47:47.190 to occur is that electrons from the carbon 00:47:47.190 --> 00:47:50.610 are being moved to the oxygen. You 00:47:50.610 --> 00:47:55.440 are changing the oxidation state of carbon and of oxygen 00:47:55.440 --> 00:47:59.640 in order to carry out this reaction. 00:47:59.640 --> 00:48:04.230 Oxygen is a better electron acceptor than carbon, 00:48:04.230 --> 00:48:07.110 and so transferring those electrons 00:48:07.110 --> 00:48:12.240 from the carbon to the oxygen is favorable. 00:48:12.240 --> 00:48:17.580 That is, in essence, what is going on in this very favorable 00:48:17.580 --> 00:48:21.570 reaction to burn wood. 00:48:21.570 --> 00:48:26.740 Let me illustrate that a bit more explicitly here. 00:48:26.740 --> 00:48:31.080 So the carbon-- most of the carbons in carbohydrates 00:48:31.080 --> 00:48:35.100 are alcohols. 00:48:35.100 --> 00:48:41.570 So here, you can call this our carbohydrate, 00:48:41.570 --> 00:48:45.630 with the exception of the ketone or the aldehyde 00:48:45.630 --> 00:48:48.000 if it's a ketose or an aldose. 00:48:48.000 --> 00:48:51.805 All the other carbons in the sugar are alcohols. 00:48:57.750 --> 00:48:59.800 All the other bonds there. 00:48:59.800 --> 00:49:09.430 And so if we oxidize that carbon-- 00:49:09.430 --> 00:49:13.090 that is, if we remove electrons from the carbon, 00:49:13.090 --> 00:49:13.990 how can we do that? 00:49:16.680 --> 00:49:26.250 So this here generates this H- hydride ion. 00:49:26.250 --> 00:49:29.180 This is, in essence, two electrons 00:49:29.180 --> 00:49:31.070 that we are removing from the carbon. 00:49:39.470 --> 00:49:41.030 That gives us a ketone. 00:49:44.950 --> 00:49:48.510 So if we oxidize the alcohol and we remove two electrons, 00:49:48.510 --> 00:49:51.930 we get the ketone. 00:49:51.930 --> 00:49:54.855 We could oxidize this ketone further. 00:49:59.110 --> 00:50:03.060 That will give us the carboxylic acid. 00:50:03.060 --> 00:50:08.945 If we oxidize it even further, now we get CO2. 00:50:14.130 --> 00:50:20.640 So, moving in this direction, this is oxidation. 00:50:24.390 --> 00:50:27.600 Transferring electrons from the carbon to the oxygen 00:50:27.600 --> 00:50:30.390 to go in this oxidation direction, that 00:50:30.390 --> 00:50:32.850 is glucose oxidation is what's happening 00:50:32.850 --> 00:50:35.130 when we're burning wood. 00:50:35.130 --> 00:50:37.320 It's what happens when we burn glucose. 00:50:37.320 --> 00:50:38.415 Energy is released. 00:50:48.200 --> 00:50:57.980 This is largely what catabolism is about in biological systems. 00:50:57.980 --> 00:51:00.650 Now then reasons, then, that if we're 00:51:00.650 --> 00:51:05.150 going to move in the opposite direction, so-called reduction, 00:51:05.150 --> 00:51:14.060 that is going in this direction. 00:51:14.060 --> 00:51:14.930 Reduction. 00:51:17.740 --> 00:51:29.240 That's largely energy storage or anabolism in metabolism. 00:51:29.240 --> 00:51:34.160 We are storing energy that we can burn later. 00:51:34.160 --> 00:51:38.540 And of course, what's the more extreme version of this? 00:51:38.540 --> 00:51:41.600 Well, what if we add two more electrons to the carbon going 00:51:41.600 --> 00:51:43.380 in this direction? 00:51:43.380 --> 00:51:45.090 Well, what do we end up with? 00:51:45.090 --> 00:51:53.450 Well, now we end up with the saturated carbon. 00:51:53.450 --> 00:51:57.530 more reduced than the alcohol is the fully saturated carbon. 00:51:57.530 --> 00:52:00.260 You will see later that this is what fat is. 00:52:00.260 --> 00:52:02.810 You saw that in some of the earlier lectures. 00:52:02.810 --> 00:52:05.330 It's also what gasoline is. 00:52:05.330 --> 00:52:06.200 What's gasoline? 00:52:06.200 --> 00:52:10.010 It's fully saturated hydrocarbons that we then 00:52:10.010 --> 00:52:14.060 burn to CO2 and derive energy. 00:52:14.060 --> 00:52:18.230 Just like burning wood, burning gasoline all in one step, 00:52:18.230 --> 00:52:21.620 oxidizing the gasoline gives you energy. 00:52:21.620 --> 00:52:23.420 There's more energy in gasoline. 00:52:23.420 --> 00:52:25.610 You can release more energy per weight 00:52:25.610 --> 00:52:28.310 from gasoline, from fully-saturated hydrocarbons 00:52:28.310 --> 00:52:30.170 than you can from wood. 00:52:30.170 --> 00:52:31.580 Same thing happening here. 00:52:31.580 --> 00:52:33.410 What has more calories in your food? 00:52:33.410 --> 00:52:38.660 Everyone knows fat has more calories per weight than sugar. 00:52:38.660 --> 00:52:42.570 It's because the more reduced our carbon is, 00:52:42.570 --> 00:52:47.810 the more energy it stores, and as we oxidize that carbon, that 00:52:47.810 --> 00:52:51.400 is what releases the energy. 00:52:51.400 --> 00:52:57.700 And so energy transduction in biological systems-- 00:52:57.700 --> 00:52:59.735 and actually, in non-biological systems, 00:52:59.735 --> 00:53:02.110 as I just illustrated with the wood and gasoline example, 00:53:02.110 --> 00:53:04.450 but definitely in biological systems, 00:53:04.450 --> 00:53:09.370 is largely about oxidation and reduction reactions, 00:53:09.370 --> 00:53:14.570 and you are going to see this coming up over and over again. 00:53:14.570 --> 00:53:19.790 Now, based on what I just said, the real magic, if you will, 00:53:19.790 --> 00:53:26.020 of glycolysis can be further refined to say that what 00:53:26.020 --> 00:53:31.420 glycolysis does is it couples the favorable oxidation 00:53:31.420 --> 00:53:38.050 of carbon with reactions that allow phosphate additions 00:53:38.050 --> 00:53:40.780 and production of intermediates, like PEP 00:53:40.780 --> 00:53:45.490 and 1,3-Bisphosphoglycerate, where synthesis of ATP is 00:53:45.490 --> 00:53:51.190 favored despite the high ATP-ADP ratio in cells. 00:53:51.190 --> 00:53:53.800 If you think about it, that process 00:53:53.800 --> 00:53:56.800 then allows you to keep that ATP-ADP ratio 00:53:56.800 --> 00:53:58.810 in the range where it can then be 00:53:58.810 --> 00:54:01.510 useful for all the reasons we've been talking about now for two 00:54:01.510 --> 00:54:02.740 lectures. 00:54:02.740 --> 00:54:06.850 Allows the cell to couple that ATP-ADP ratio, that-- in ATP 00:54:06.850 --> 00:54:10.210 hydrolysis to otherwise carry out 00:54:10.210 --> 00:54:13.240 otherwise unfavorable reactions. 00:54:13.240 --> 00:54:13.960 OK. 00:54:13.960 --> 00:54:20.720 So, based on that, arguably the key, 00:54:20.720 --> 00:54:24.530 or at least a very important reaction in glycolysis, 00:54:24.530 --> 00:54:30.170 is the oxidation step which happens to add the phosphate 00:54:30.170 --> 00:54:34.260 and also happens to make 1,3-Bisphosphoglycerate, 00:54:34.260 --> 00:54:38.360 which we saw earlier can be used to drive ATP synthesis. 00:54:38.360 --> 00:54:40.460 And so that reaction is the following. 00:54:51.760 --> 00:54:54.360 So this molecule hopefully looks familiar to you 00:54:54.360 --> 00:54:57.090 from the carbohydrate lectures. 00:54:57.090 --> 00:55:09.310 This is D-glyceraldehyde 3-phosphate. 00:55:09.310 --> 00:55:11.200 I will abbreviate it frequently is 00:55:11.200 --> 00:55:14.830 G-3P for glyceraldehyde 3-phosphate, 00:55:14.830 --> 00:55:17.380 but it's D-glyceraldehyde 3-phosphate. 00:55:17.380 --> 00:55:19.420 Remember, it's a D, sugar, because the OH 00:55:19.420 --> 00:55:23.930 group is pointing to the right as we drew it this way. 00:55:23.930 --> 00:55:27.670 It's also an aldose because it's a aldehyde sugar, 00:55:27.670 --> 00:55:29.950 and it's phosphorylated on the 3 position-- 00:55:29.950 --> 00:55:33.520 remember the way we numbered sugars was starting 00:55:33.520 --> 00:55:34.390 at the carbonyl-- 00:55:34.390 --> 00:55:36.740 1, 2, 3. 00:55:36.740 --> 00:55:38.920 This is carbon-3, phosphorylation on the 3 00:55:38.920 --> 00:55:40.210 position. 00:55:40.210 --> 00:55:43.370 Glyceraldehyde 3-phosphate. 00:55:43.370 --> 00:55:44.060 OK. 00:55:44.060 --> 00:56:09.850 So, this step generates 1,3-Bisphosphoglycerate. 00:56:09.850 --> 00:56:11.380 Well, to do this, there's actually 00:56:11.380 --> 00:56:14.720 two things that happened here. 00:56:14.720 --> 00:56:18.040 First we had to add the phosphate to this. 00:56:18.040 --> 00:56:20.320 It says two phosphates on it. bisphosphoglycerate, 00:56:20.320 --> 00:56:22.030 this has a single phosphate. 00:56:22.030 --> 00:56:25.630 So inorganic phosphate has to be added 00:56:25.630 --> 00:56:28.720 to generate that molecule. 00:56:28.720 --> 00:56:34.000 By the way, adding that phosphate delta G0 prime 00:56:34.000 --> 00:56:37.300 is positive plus 1.5 kcals per mole. 00:56:37.300 --> 00:56:41.440 That means even equilibrium, this reaction 00:56:41.440 --> 00:56:43.960 would lie to the left, of course, 00:56:43.960 --> 00:56:46.880 because delta G0 prime is positive. 00:56:46.880 --> 00:56:50.470 However, remember the next step downstream of this 00:56:50.470 --> 00:56:54.490 is the one shown over here where we do ATP synthesis. 00:56:54.490 --> 00:56:56.390 That step is very favorable. 00:56:56.390 --> 00:56:58.810 So this is an example where metabolism 00:56:58.810 --> 00:57:03.000 has coupled a very favorable step to a step that's 00:57:03.000 --> 00:57:03.870 less favorable. 00:57:03.870 --> 00:57:08.280 That can keep the 1,3-BPG levels low and pull the entire system 00:57:08.280 --> 00:57:12.700 forward, the metabolic trick we discussed in the last lecture. 00:57:12.700 --> 00:57:13.680 OK. 00:57:13.680 --> 00:57:15.810 You'll note the other thing that's happening here, 00:57:15.810 --> 00:57:18.810 other than adding that phosphate group, 00:57:18.810 --> 00:57:26.700 is that we've changed this aldehyde to a carboxylic acid. 00:57:26.700 --> 00:57:29.970 That is an oxidation reaction. 00:57:29.970 --> 00:57:32.970 Ketone to the acid is an oxidation reaction. 00:57:32.970 --> 00:57:37.540 I will illustrate over here this so you can see 00:57:37.540 --> 00:57:41.970 why it's an oxidation reaction. 00:57:41.970 --> 00:57:43.370 So here's our aldehyde. 00:57:43.370 --> 00:57:47.240 If we take electrons from the carbon 00:57:47.240 --> 00:57:55.160 and that generates this hydride ion to electrons, 00:57:55.160 --> 00:57:56.660 what are we left with? 00:57:56.660 --> 00:58:01.907 Well, we're left with this intermediate 00:58:01.907 --> 00:58:04.365 that will never exist with a positive charge on the carbon. 00:58:14.810 --> 00:58:18.380 Water, which, of course, is abundant in biological systems. 00:58:18.380 --> 00:58:19.250 And then come-- 00:58:22.510 --> 00:58:24.100 OK. 00:58:24.100 --> 00:58:25.960 And it basically illustrates that as I 00:58:25.960 --> 00:58:30.490 go from the aldehyde to the acid, of course, 00:58:30.490 --> 00:58:33.040 I make a proton and a hydride ion, 00:58:33.040 --> 00:58:36.280 but really, I have changed the oxidation state of the carbon 00:58:36.280 --> 00:58:40.330 from the ketone to the acid state. 00:58:40.330 --> 00:58:45.460 Two electrons are lost there, is shown as that hydride ion. 00:58:45.460 --> 00:58:47.110 OK. 00:58:47.110 --> 00:58:48.950 So, what does that mean? 00:58:48.950 --> 00:58:52.690 Well, that means that we have to maintain that balance 00:58:52.690 --> 00:58:55.640 in this reaction as well. 00:58:55.640 --> 00:58:59.230 So water is going to have to come in 00:58:59.230 --> 00:59:02.110 and a proton will be produced. 00:59:02.110 --> 00:59:05.080 But the key thing is we also generate 00:59:05.080 --> 00:59:12.300 two electrons, this hydride ion that have to go somewhere. 00:59:12.300 --> 00:59:19.110 Now, finding a place for those two electrons to go 00:59:19.110 --> 00:59:24.360 is what brings up the second high-level concept 00:59:24.360 --> 00:59:27.810 that I now need to spend some time discussing. 01:00:06.130 --> 01:00:08.260 And this second high-level concept 01:00:08.260 --> 01:00:15.190 that I need to introduce is the role of cofactors, 01:00:15.190 --> 01:00:18.730 and cofactors also is effectively 01:00:18.730 --> 01:00:26.230 the role of vitamins and what role 01:00:26.230 --> 01:00:31.130 that these things play in metabolism. 01:00:31.130 --> 01:00:36.075 Most vitamins are used to support metabolic reactions, 01:00:36.075 --> 01:00:37.700 and we're going to learn all about what 01:00:37.700 --> 01:00:40.430 many of the vitamins that you guys are familiar with reading 01:00:40.430 --> 01:00:42.800 the side of your cereal box actually 01:00:42.800 --> 01:00:46.190 do throughout the course. 01:00:46.190 --> 01:00:51.900 Now cofactors, as I said, often involve vitamins, 01:00:51.900 --> 01:01:02.200 are molecules that provide useful chemical groups that 01:01:02.200 --> 01:01:05.530 facilitate the reaction chemistries that 01:01:05.530 --> 01:01:07.390 are needed in metabolism. 01:01:10.110 --> 01:01:13.790 So they provide some useful chemical groups 01:01:13.790 --> 01:01:20.230 to facilitate some of the chemistries 01:01:20.230 --> 01:01:24.810 that we need to carry out metabolic reactions. 01:01:24.810 --> 01:01:44.530 They are, in general, in small non-stoichiometric quantities 01:01:44.530 --> 01:01:46.780 in order to carry out this function. 01:01:46.780 --> 01:01:47.830 What do I mean by that? 01:01:47.830 --> 01:01:51.700 Meaning that you can imagine that the cell will convert lots 01:01:51.700 --> 01:01:57.240 of glyceraldehyde 3-phosphate to 1,3-Bisphosphoglycerate, 01:01:57.240 --> 01:02:00.690 but the enzyme, you don't need one enzyme for every time you 01:02:00.690 --> 01:02:01.860 convert that reaction. 01:02:01.860 --> 01:02:04.770 The same enzyme can do it over and over and over again. 01:02:04.770 --> 01:02:06.660 Well, it turns out, cofactors are much more 01:02:06.660 --> 01:02:08.520 like enzymes in that respect. 01:02:08.520 --> 01:02:11.220 They need to be recycled in a way 01:02:11.220 --> 01:02:13.980 that they can catalyze the turnover of reaction 01:02:13.980 --> 01:02:15.960 many, many, many times, and it'll 01:02:15.960 --> 01:02:17.790 be much clearer what I mean by this when we 01:02:17.790 --> 01:02:20.070 get into some of the details. 01:02:20.070 --> 01:02:23.730 Now, the vitamin concept comes from the fact 01:02:23.730 --> 01:02:38.710 that often these cofactors or parts of the cofactors 01:02:38.710 --> 01:02:43.660 are not made by animals, not made by humans. 01:02:46.810 --> 01:02:49.600 If they're not made by us, but we 01:02:49.600 --> 01:02:52.270 need them to do our metabolism, well now we 01:02:52.270 --> 01:02:55.360 have to get them from the diet, and this is really 01:02:55.360 --> 01:02:56.830 the concept of vitamins. 01:02:56.830 --> 01:02:59.470 These are small things that we have 01:02:59.470 --> 01:03:02.140 to get in quantities from the diet, 01:03:02.140 --> 01:03:05.170 and it turns out they're very important to help us carry out 01:03:05.170 --> 01:03:07.630 the metabolism we do. 01:03:07.630 --> 01:03:09.550 As an aside, the vitamin industry 01:03:09.550 --> 01:03:11.380 is a very big business. 01:03:11.380 --> 01:03:14.980 Lots of unfounded claims out there. 01:03:14.980 --> 01:03:16.480 I'm not going to get into-- 01:03:16.480 --> 01:03:17.770 that can get very political. 01:03:17.770 --> 01:03:20.890 I'm just going to say that just know that more is not 01:03:20.890 --> 01:03:23.680 always better, and the goal for this class 01:03:23.680 --> 01:03:27.340 is really to understand what some of these vitamins 01:03:27.340 --> 01:03:30.130 actually do. 01:03:30.130 --> 01:03:31.300 OK. 01:03:31.300 --> 01:03:35.950 Now, the relevant cofactor for the reaction that we're talking 01:03:35.950 --> 01:03:41.080 about is a cofactor called nicotinamide adenine 01:03:41.080 --> 01:03:47.590 dinucleotide, which is always abbreviated NAD or NAD+. 01:03:47.590 --> 01:04:05.325 This stands for nicotinamide adenine dinucleotide. 01:04:05.325 --> 01:04:09.330 You see why we want to abbreviate that NAD. 01:04:09.330 --> 01:04:14.070 And basically what nicotinamide adenine dinucleotide is is it's 01:04:14.070 --> 01:04:17.640 a factor that can solve our problem of where 01:04:17.640 --> 01:04:21.810 to put these electrons in this oxidation reaction. 01:04:21.810 --> 01:04:26.140 That is, you can accept two electrons 01:04:26.140 --> 01:04:29.350 from a reaction like this one here 01:04:29.350 --> 01:04:36.370 and cycle to another form of the cofactor called NADH. 01:04:36.370 --> 01:04:37.240 All right. 01:04:37.240 --> 01:04:39.490 What is nicotinamide adenine dinucleotide? 01:04:39.490 --> 01:04:42.760 Well first, let's draw what it looks like. 01:05:48.232 --> 01:05:48.810 OK. 01:05:48.810 --> 01:05:51.660 Sorry it's squeeze down here a little bit. 01:05:51.660 --> 01:05:57.500 So this here is adenine. 01:05:57.500 --> 01:06:01.455 This group up here is nicotinamide. 01:06:06.460 --> 01:06:10.270 So if you look at this, this is a dinucleotide. 01:06:10.270 --> 01:06:14.710 And so down here, this, of course, is, from here down, 01:06:14.710 --> 01:06:16.660 is AMP. 01:06:16.660 --> 01:06:18.680 That phosphate is now over here. 01:06:18.680 --> 01:06:21.460 This is a separate nucleotide, where 01:06:21.460 --> 01:06:25.330 rather than having one of the AGCTU bases 01:06:25.330 --> 01:06:29.770 that you're used to from RNA and DNA, nicotinamide is the base. 01:06:29.770 --> 01:06:36.510 And so it's this AMP plus this nicotinamide 01:06:36.510 --> 01:06:40.410 base is a monophosphate, or a nicotinamide adenine 01:06:40.410 --> 01:06:42.930 dinucleotide. 01:06:42.930 --> 01:06:49.820 This, the nicotinamide group, is derived 01:06:49.820 --> 01:06:53.840 from a vitamin called niacin. 01:06:53.840 --> 01:06:55.670 And so from the side of your cereal box, 01:06:55.670 --> 01:06:57.170 this is what niacin does. 01:06:57.170 --> 01:07:01.310 It allows you to generate the nicotinamide group that's 01:07:01.310 --> 01:07:04.670 part of this cofactor, nicotinamide adenine 01:07:04.670 --> 01:07:06.770 dinucleotide. 01:07:06.770 --> 01:07:10.700 Now, nicotinamide adenine dinucleotide 01:07:10.700 --> 01:07:16.800 is useful because of what that nicotinamide group can do. 01:07:16.800 --> 01:07:19.130 And so I'm not going to draw out the whole molecule, 01:07:19.130 --> 01:07:19.950 but I'll show you. 01:07:19.950 --> 01:07:27.960 So NAD+, which is the oxidized form of nicotinamide adenine 01:07:27.960 --> 01:07:32.270 dinucleotide, is as I drew it up there. 01:07:35.050 --> 01:07:38.930 So this here would be attached to the rest 01:07:38.930 --> 01:07:40.880 of the dinucleotide. 01:07:45.490 --> 01:07:48.880 So this is the oxidized form. 01:07:48.880 --> 01:07:55.580 I can have-- here's my hydride ion, which, remember, 01:07:55.580 --> 01:07:57.410 is two electrons. 01:08:05.110 --> 01:08:08.070 If I carry out that chemistry to add those two electrons 01:08:08.070 --> 01:08:26.479 to the nicotinamide group, you now end up 01:08:26.479 --> 01:08:33.970 with the reduced form of nicotinamide adenine 01:08:33.970 --> 01:08:36.580 dinucleotide NADH. 01:08:36.580 --> 01:08:41.710 So the difference between NAD+ and NADH, it's the H, 01:08:41.710 --> 01:08:44.710 but it's really the two electrons that were added 01:08:44.710 --> 01:08:53.380 in adding that H. So it's oxidized form and reduced form. 01:08:53.380 --> 01:08:57.760 So remember, I want to stress this again, at least in biology 01:08:57.760 --> 01:08:59.920 we can't create or destroy matter. 01:08:59.920 --> 01:09:01.810 You can do that in nuclear physics, 01:09:01.810 --> 01:09:05.080 but we can't do that in biology. 01:09:05.080 --> 01:09:08.729 And so that means that if we do an electron transfer like we 01:09:08.729 --> 01:09:13.200 did in this reaction where we oxidized the aldehyde 01:09:13.200 --> 01:09:16.080 in glyceraldehyde 3-phosphate to the acid 01:09:16.080 --> 01:09:20.279 in 1,3-Bisphosphoglycerate, those electrons have to be 01:09:20.279 --> 01:09:21.790 accounted for. 01:09:21.790 --> 01:09:24.240 And so if one molecule is oxidized, 01:09:24.240 --> 01:09:27.279 another one has to be reduced. 01:09:27.279 --> 01:09:29.430 So redox reactions always-- that is, 01:09:29.430 --> 01:09:31.859 redox, oxidation-reduction reactions, 01:09:31.859 --> 01:09:34.470 always have to happen in pairs. 01:09:34.470 --> 01:09:39.000 And so over there we have the aldehyde in glyceraldehyde 01:09:39.000 --> 01:09:50.399 3-phosphate is oxidized to form the acid 01:09:50.399 --> 01:09:52.299 in 1,3-Bisphosphoglycerate. 01:09:55.150 --> 01:09:58.950 As part of that reaction, we can couple it such 01:09:58.950 --> 01:10:03.780 that the nicotinamide group, which is oxidized in NAD, 01:10:03.780 --> 01:10:10.580 can be reduced to the nicotinamide group that 01:10:10.580 --> 01:10:12.077 is found in NADH. 01:10:14.780 --> 01:10:20.540 So that means we can draw this in here, 01:10:20.540 --> 01:10:34.190 is that having that cofactor coupled to this reaction 01:10:34.190 --> 01:10:40.720 now allows us to fix the redox problem of this reaction. 01:10:40.720 --> 01:10:45.730 Now this reaction here is catalyzed 01:10:45.730 --> 01:10:56.925 by an enzyme called glycerol 3-phosphate dehydrogenase. 01:10:56.925 --> 01:10:58.300 I'll draw that out in the second. 01:11:35.930 --> 01:11:42.010 Often abbreviated GAPDH, the loading control 01:11:42.010 --> 01:11:45.190 in all of your western blots, is the enzyme GAPDH, 01:11:45.190 --> 01:11:48.630 glyceraldehyde phosphate dehydrogenase. 01:11:48.630 --> 01:11:53.890 That is the enzyme that carries out that reaction. 01:11:53.890 --> 01:11:55.710 Now notice, the name fits. 01:11:55.710 --> 01:12:00.120 It's a dehydrogenase, it's removing a hydrogen, 01:12:00.120 --> 01:12:03.690 removing two electrons as a hydride ion, 01:12:03.690 --> 01:12:05.550 but it's really-- what's important here 01:12:05.550 --> 01:12:08.960 is the electrons moving, not the hydrogen itself. 01:12:08.960 --> 01:12:11.700 So in that way it's a little bit misleading. 01:12:11.700 --> 01:12:15.600 And you will see that in general, dehydrogenases 01:12:15.600 --> 01:12:19.470 are classes of enzymes that catalyze 01:12:19.470 --> 01:12:23.110 oxidation-reduction reactions, and as a result, 01:12:23.110 --> 01:12:27.090 often involve NAD/NADH as cofactors 01:12:27.090 --> 01:12:33.560 because they facilitate those electron transfer reactions. 01:12:33.560 --> 01:12:34.790 OK. 01:12:34.790 --> 01:12:36.860 So now we're able to describe how 01:12:36.860 --> 01:12:41.670 GAPDH works in great detail. 01:12:41.670 --> 01:12:47.520 And so the chemistry of GAPDH works as follows. 01:12:47.520 --> 01:12:52.420 And so in the active site of GAPDH, 01:12:52.420 --> 01:12:54.420 there's a cystine residue. 01:12:54.420 --> 01:12:58.570 That cystine residue has a sulfur shown here. 01:12:58.570 --> 01:13:03.900 Also bound near the active site is in NAD molecule 01:13:03.900 --> 01:13:07.020 to catalyze the reaction shown. 01:13:07.020 --> 01:13:11.400 That NAD has to be in the oxidized state, so it's NAD+. 01:13:11.400 --> 01:13:18.300 And so glyceraldehyde 3-phosphate binds 01:13:18.300 --> 01:13:19.320 in the active site. 01:13:27.620 --> 01:13:29.660 There's glyceraldehyde 3-phosphate. 01:13:33.710 --> 01:13:39.850 By the way, I will show you how many reactions in metabolism 01:13:39.850 --> 01:13:41.800 work chemically. 01:13:41.800 --> 01:13:45.790 Obviously we don't have time in this class, an overview class 01:13:45.790 --> 01:13:48.970 to go into all the great chemical details of what's 01:13:48.970 --> 01:13:51.465 going on and exactly how everything works, 01:13:51.465 --> 01:13:52.840 but I'll at least try to give you 01:13:52.840 --> 01:13:54.490 a flavor of some of the chemistry that 01:13:54.490 --> 01:13:56.930 happens throughout the course. 01:13:56.930 --> 01:13:57.430 All right. 01:13:57.430 --> 01:13:58.750 So once you have, then-- 01:14:19.900 --> 01:14:20.400 OK. 01:14:20.400 --> 01:14:22.530 So now you have this intermediate bound 01:14:22.530 --> 01:14:25.860 into the enzyme active site. 01:14:25.860 --> 01:14:33.460 You can now carry out the oxidation of that carbon. 01:14:33.460 --> 01:14:39.810 So these are two electrons being removed as a hydride ion. 01:14:39.810 --> 01:14:43.350 They can be transferred to NAD in the active site. 01:14:43.350 --> 01:14:46.500 That generates NADH by the exact chemistry 01:14:46.500 --> 01:14:47.940 that I just showed you. 01:15:04.690 --> 01:15:05.950 OK. 01:15:05.950 --> 01:15:11.440 That NADH now needs to be exchanged out 01:15:11.440 --> 01:15:18.490 or the enzyme will not be able to catalyze 01:15:18.490 --> 01:15:21.855 the next series of reactions, and that NADH will then 01:15:21.855 --> 01:15:23.230 go somewhere else, and of course, 01:15:23.230 --> 01:15:26.050 has to be cycled back to NAD, but that doesn't happen 01:15:26.050 --> 01:15:28.000 in this reaction, per se. 01:15:58.030 --> 01:15:59.550 OK. 01:15:59.550 --> 01:16:08.030 Next, this here, you'll notice, is that by the way 01:16:08.030 --> 01:16:11.690 this chemistry worked, we formed this thioester bond 01:16:11.690 --> 01:16:13.500 in the active site. 01:16:13.500 --> 01:16:16.580 This is another thing that you will see over and over again 01:16:16.580 --> 01:16:18.020 in metabolism. 01:16:18.020 --> 01:16:21.060 This sulfur in the thioester bond is a good leaving group, 01:16:21.060 --> 01:16:24.410 and so it's really setting up a situation that will now help 01:16:24.410 --> 01:16:27.570 with the phosphate addition. 01:16:27.570 --> 01:16:30.920 So here, I'll draw out inorganic phosphate. 01:16:40.830 --> 01:16:49.970 And that, of course, will give us the identical enzyme active 01:16:49.970 --> 01:16:58.310 site that we started with plus 1,3-Bisphosphoglycerate. 01:17:03.990 --> 01:17:14.070 And that 1,3-Bisphosphoglycerate can now be coupled to ATP 01:17:14.070 --> 01:17:18.900 synthesis to make 3-phosphoglycerate. 01:17:18.900 --> 01:17:23.550 And so you can see, in essence, what this reaction has done is 01:17:23.550 --> 01:17:28.950 that it has coupled oxidation of the aldehyde to the acid, 01:17:28.950 --> 01:17:32.610 the aldehyde in 3-phosphate to the acid 01:17:32.610 --> 01:17:35.700 in 1,3-Bisphosphoglycerate, which can then do 01:17:35.700 --> 01:17:38.920 phosphorylation of ADP to make ATP. 01:17:38.920 --> 01:17:47.500 So in essence, this is oxidative phosphorylation. 01:17:52.700 --> 01:17:55.060 Now I realize in high school or in other classes, 01:17:55.060 --> 01:17:57.850 oxidative phosphorylation means something different to you, 01:17:57.850 --> 01:18:00.280 and of course, we will discuss the process that 01:18:00.280 --> 01:18:02.950 is called oxidative phosphorylation in great detail 01:18:02.950 --> 01:18:04.880 later in this course. 01:18:04.880 --> 01:18:07.360 But I want to point out that the chemistry that's actually 01:18:07.360 --> 01:18:12.430 happening here in glycolysis is also oxidative phosphorylation. 01:18:12.430 --> 01:18:17.890 It's basically how you coupled this favorable carbon oxidation 01:18:17.890 --> 01:18:20.620 that's occurring at this GAPDH step 01:18:20.620 --> 01:18:25.060 in a way that allows you to synthesize ATP 01:18:25.060 --> 01:18:30.580 at a high ATP-ADP ratio, which in the end will allow cells 01:18:30.580 --> 01:18:34.350 to do otherwise unfavorable things. 01:18:34.350 --> 01:18:35.700 OK. 01:18:35.700 --> 01:18:42.900 Now we're ready to discuss these reactions, the GAPDH reaction, 01:18:42.900 --> 01:18:45.990 the pyruvate kinase reaction, the phosphoglycerate kinase 01:18:45.990 --> 01:18:49.680 reaction, and how they fit into glycolysis 01:18:49.680 --> 01:18:54.300 as a metabolic pathway in a way that allows cells to use 01:18:54.300 --> 01:18:57.120 glucose metabolism-- that is, the breakdown of glucose, 01:18:57.120 --> 01:19:01.350 the burning of glucose as a way to get energy. 01:19:01.350 --> 01:19:06.210 And that pathway is, of course, glycolysis or glucose lysis 01:19:06.210 --> 01:19:08.520 that we mentioned earlier. 01:19:08.520 --> 01:19:12.420 Glycolysis is a very ancient and ubiquitous pathway. 01:19:12.420 --> 01:19:15.630 It's used by essentially all life on Earth 01:19:15.630 --> 01:19:21.200 and probably has been for at least 2 to 3 billion years. 01:19:21.200 --> 01:19:23.780 Glycolysis, I guess, as a pathway was, 01:19:23.780 --> 01:19:27.860 quote-unquote, "discovered" mid-19th century 01:19:27.860 --> 01:19:30.530 by Louis Pasteur, and Louis Pasteur, 01:19:30.530 --> 01:19:34.670 of course, described life, microorganisms 01:19:34.670 --> 01:19:38.420 as being responsible for the process of fermentation. 01:19:38.420 --> 01:19:42.980 That is, conversion of glucose into alcohol, something 01:19:42.980 --> 01:19:45.290 many college students are interested in, as well 01:19:45.290 --> 01:19:48.950 as other organic acids. 01:19:48.950 --> 01:19:52.310 And of course, fermentation to make alcohol 01:19:52.310 --> 01:19:57.740 or to store other types of organic acids 01:19:57.740 --> 01:19:59.870 has been used in food preservation 01:19:59.870 --> 01:20:03.560 for centuries, which is really why Pasteur's discoveries were 01:20:03.560 --> 01:20:06.890 so impactful at the time. 01:20:06.890 --> 01:20:08.600 Now even though Pasteur discovered this, 01:20:08.600 --> 01:20:11.330 he didn't really understand chemically how it worked, 01:20:11.330 --> 01:20:14.670 and it actually took decades to figure that out. 01:20:14.670 --> 01:20:17.360 And when it was finally figured out and described 01:20:17.360 --> 01:20:19.670 as the pathway called glycolysis, 01:20:19.670 --> 01:20:23.510 this was done by two chemists, two chemists in Germany, Embden 01:20:23.510 --> 01:20:24.710 and Meyerhof. 01:20:24.710 --> 01:20:28.040 And basically what they did is they pieced together 01:20:28.040 --> 01:20:31.430 several enzymatic or chemical activities that 01:20:31.430 --> 01:20:35.570 have been purified from cell lysates mostly from yeast 01:20:35.570 --> 01:20:39.290 and used those to build a pathway that described 01:20:39.290 --> 01:20:43.370 chemically how you could start with glucose 01:20:43.370 --> 01:20:47.030 and chemically break down that glucose in a way that made 01:20:47.030 --> 01:20:50.130 sense in this fermentation pathway, 01:20:50.130 --> 01:20:53.750 and that is why glycolysis is sometimes called 01:20:53.750 --> 01:20:56.780 the Embden-Meyerhof pathway. 01:21:32.910 --> 01:21:35.820 Now, glycolysis does not completely 01:21:35.820 --> 01:21:38.940 convert glucose to CO2. 01:21:38.940 --> 01:21:44.310 Instead, what glycolysis does is it converts glucose-- 01:21:54.400 --> 01:21:58.540 so I will draw here in the pyranose form. 01:21:58.540 --> 01:22:05.960 It converts glucose by a number of steps 01:22:05.960 --> 01:22:12.890 into two molecules of the organic acid pyruvate, which, 01:22:12.890 --> 01:22:19.040 of course, was the product of that pyruvate kinase reaction 01:22:19.040 --> 01:22:20.495 that we showed earlier. 01:22:29.080 --> 01:22:34.450 Note that the fermentation that Louis Pasteur described 01:22:34.450 --> 01:22:39.220 was further metabolism of that pyruvate into something else. 01:22:39.220 --> 01:22:50.840 In the case of yeast, it was ethanol or ethyl alcohol, 01:22:50.840 --> 01:22:51.920 or something else. 01:22:51.920 --> 01:23:04.210 The fermentative product in mammals 01:23:04.210 --> 01:23:08.770 is this organic acid lactate. 01:23:08.770 --> 01:23:11.500 And what we will see next time is 01:23:11.500 --> 01:23:15.280 that this fermentation of pyruvate 01:23:15.280 --> 01:23:18.200 into one of these other molecules 01:23:18.200 --> 01:23:22.010 ends up being an alternative to the further oxidation 01:23:22.010 --> 01:23:26.000 of pyruvate to CO2 which requires oxygen, 01:23:26.000 --> 01:23:30.050 and the reason oxidizing pyruvate to CO2 01:23:30.050 --> 01:23:33.230 requires oxygen is because ultimately we 01:23:33.230 --> 01:23:37.820 have to regenerate that NADH we made in glycolysis to allow 01:23:37.820 --> 01:23:41.780 that cofactor to cycle and glycolysis to keep moving. 01:23:41.780 --> 01:23:45.930 Fermentation gives us an alternative way 01:23:45.930 --> 01:23:49.290 to do this, which we'll discuss in great detail 01:23:49.290 --> 01:23:51.180 in the next lecture. 01:23:51.180 --> 01:23:56.570 But first I want to focus on glycolysis 01:23:56.570 --> 01:23:59.690 and what we can describe next time as really how 01:23:59.690 --> 01:24:03.020 we can build a pathway to turn glucose into pyruvate. 01:24:03.020 --> 01:24:06.170 This glycolysis, this Embden-Meyerhof pathway, 01:24:06.170 --> 01:24:08.450 and some of the details of that pathway 01:24:08.450 --> 01:24:11.820 are shown here on this slide. 01:24:11.820 --> 01:24:14.720 And this is where we'll start off the next lecture, 01:24:14.720 --> 01:24:19.910 discussing how each of these steps in glycolysis function 01:24:19.910 --> 01:24:26.120 in a way that allow a pathway to generate phosphoenolpyruvate, 01:24:26.120 --> 01:24:30.050 1,3-Bisphosphoglycerate, incorporate this GAPDH 01:24:30.050 --> 01:24:34.220 reaction, and do so in a way that's overall energetically 01:24:34.220 --> 01:24:37.640 favorable to allow the cell to couple this oxidation 01:24:37.640 --> 01:24:42.710 of glucose to keeping ATP-ADP ratio in the right range 01:24:42.710 --> 01:24:46.720 to support other unfavorable functions in the cell. 01:24:46.720 --> 01:24:48.610 OK, thanks.