Fate of Pyruvate – Glycolysis and Pyruvate Metabolism

by Kevin Ahern, PhD

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    00:00 Now the last thing I wanna consider here is the fate of pyruvate.

    00:04 Pyruvate is the end product of this pathway but we remember that pathways don't occur an isolation.

    00:11 Pyruvate goes on to something else. And so pyruvate can go on to a variety of things and the things that it goes into depends upon the circumstances in the cell in which its found.

    00:21 Okay. So let's look a little bit closer to see what happens with this pathway? As I say it's not an end point and it's important for a variety of things. It's important to make alanine.

    00:30 It's important for energy and it's an important source of keeping glycolysis going as we are going to see in just a second.

    00:38 Okay? Now let's consider the three different things that pyruvate can go to that are shown on the screen.

    00:45 Pyruvate in bacterial cells and yeast cells can be converted into a molecule called acetaldehyde.

    00:52 An acetaldehyde can be converted into ethanol.

    00:57 Beer drinkers of course like that reactions because that reaction is known as fermentation and its the way we brew. We make beer. We make wine so forth with this reaction.

    01:07 And we make this reaction by closing off the vessel in which the reaction is occurring and the loss of oxygen in that vessel favors the production of ethanol.

    01:21 Now you probably didn't think about this that your cells that is human, also ferment. Now fermentation in us is different than it is in yeast and bacteria otherwise we wouldn't have to go out and get beer. Right? To convert pyruvate into lactate, there is a reason we are doing this as we will see.

    01:39 But our cells when they run out of oxygen, they make lactate.

    01:44 Also known as lactic acid and that becomes important for the production of NAD as we shall see.

    01:52 You notice that in the acetaldehyde ethanol reaction that NAD was also produced.

    01:57 Now what about aerobic conditions? Under aerobic conditions acetyl-CoA is sent to the citric acid cycle for oxidation. Oxidation requires oxygen and so this make sense.

    02:09 No oxygen we have alternative pathways. We have oxygen we uses it best as we can.

    02:15 Now the difference between oxidation using the citric acid cycle and oxidation using the fermentation pathways is enormous.

    02:27 If we start with glucose and we go through a fermentation pathway the net products of that reaction are 2 ATPs.

    02:34 If we start with glucose and we go through the citric cycle and oxidize it completely we get 38 ATPs.

    02:42 That's 19 times more energy under oxygen conditions than under oxygen absence.

    02:49 You can see why cells really like to have oxygen and you can see why our body are setup to deliver oxygen as much as it can.

    02:56 It doesn't always succeed in that. But it does as much as it can.

    03:01 Okay, well why is that fermentation is even necessary? Well you probably didn't think about it as I was going though describing the lecture but I noted that glyceraldehyde-3-phosphate is converted into 1,3-bisphosphoglycerate in the only oxidation reaction of glycolysis.

    03:17 That reaction is important because any oxidation requires an electron carrier and the electron carrier used here is NAD+ which becomes NADH.

    03:27 The cell doesn't have an infinite amount of NAD.

    03:31 If it converts its NAD into NADH it has to be able to convert NADH back to NAD Fine and dandy. Okay? How does a cell do that? Usually the cell does that in a process that requires oxygen.

    03:46 It's called the electron transport system. And, what does the electron transport system require? Oxygen.

    03:52 So if there is no oxygen, the electron transport system can't function.

    03:56 And what will happen to the concentration of NADH? It will go high and the concentration of NAD will go low.

    04:04 Glycolysis can't keep running under those conditions.

    04:09 But imagine you are out jogging and your blood supply is not delivering oxygen as rapidly as you need it, then what's going to happen in that case? When that happens then glycolysis is gonna stop and glycolysis is the only thing that's providing those muscles cells with energy.

    04:26 You got a problem. Well fortunately muscle cells and yeast cells, and all these things have ways of regenerating that NAD and that's fermentation.

    04:35 We can see here that the end product of glycolysis is pyruvate.

    04:39 So pyruvate in a bacterial system is converted into acetaldehyde as I noted and then acetaldehyde is converted into ethanol. That's why the ester is making ethanol.

    04:50 In the ethanol reaction, NAD+ is produced and guess where it's used.

    04:56 Back in the glycolysis reaction. So by cycling back and forth between those two reactions, NAD is generated, glycolysis is kept going and the cell stays alive.

    05:06 Now animal cells don't convert pyruvate into acetaldehyde instead they convert pyruvate into lactate or lactic acid. You can see this reaction here catalyzed by the lactate dehydrogenase. And the same thing is happening here that was happening in the yeast.

    05:20 It takes electrons from NADH and transfers them onto pyruvate to make lactate.

    05:27 That regenerates NAD and that NAD that's been made goes right back to the glycolysis and keeps it going.

    05:32 So we ferment for the same reasons that yeast and bacteria do but we don't have the same end product.

    05:39 In this set of talks, well I hope I have given you is a good overview of the pathway of glycolysis the energy needs of the cell and how energy factors into the overall ways in which the reaction occurs and the overall controls that enable the cells to efficiently make this process happen.

    About the Lecture

    The lecture Fate of Pyruvate – Glycolysis and Pyruvate Metabolism by Kevin Ahern, PhD is from the course Carbohydrate Metabolism.

    Included Quiz Questions

    1. It is made into lactate during fermentation
    2. It is converted into acetyl-CoA when oxygen is limiting
    3. It uses NADH to produce acetaldehyde
    4. It is an intermediate in the citric acid cycle
    1. It requires alcohol dehydrogenase
    2. It occurs when cells are short of oxygen
    3. It occurs when cells are short of NAD+
    4. It has different products in bacteria and humans
    1. Acetyl CoA
    2. 2,3-bisphosphoglycerate
    3. Lactate
    4. Acetaldehyde
    5. Ethanol

    Author of lecture Fate of Pyruvate – Glycolysis and Pyruvate Metabolism

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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