Fates of Pyruvate
Fates of Pyruvate

Of course, now that we've covered glycolysis, I'm sure you are all wondering what happens to pyruvate. After all, glycolysis, if you'll remember, gave us a net total of two ATP, two NADH, and two pyruvate molecules. We know that the cell can use the ATP, and we'll soon see that the NADH is very important as well. But what about the pyruvate? What happens to it?

Well, that depends on the organism in question. But before we tackle that, let's ask ourselves why we need to worry about pyruvate.

Looking over glycolysis, we can see that there are certain molecules it needs to operate. It is easy to see that there are a number of cofactors required, but those aren't consumed in the reaction so the cell isn't concerned about replacing them. There are also ATP produced from ADP. The cell isn't worried about getting enough ADP either, because when it needs energy it will burn ATP and get back ADP, allowing it to undergo glycolysis once again. But what about NAD+ and NADH? This is something that may not have been discussed in early biochemistry courses despite it's significance.

Obviously, the cell needs to obtain NAD+ from somewhere. The obvious solution of course is to simply regenerate it from NADH, much like the cell regenerated ADP from ATP. But how? This question is answered best by examining the fate of pyruvate.

In aerobes (oxygen breathers, like you and me etc.) it is something we've all learned at one point or another. Pyruvate is combined with oxygen to produce Acetyl-CoA which then goes through the citric acid cycle to form CO2 and H2O. Eventually, through oxidative phosphorylation, NAD+ is regenerated from NADH and even more ATPs are made from ADP. This is something that we'll see in much more detail later on, so we won'd delve too deeply into it at this point in time.

Anaerobes have two different usually exclusive pathways. In Lactic Acid Fermentation NAD+ is regenerated from NADH by an enzyme called Lactate dehydrogenase. Lactate dehydrogenase (or, from now on, DH = dehydrogenase) adds a water molecule across the C=O of pyruvate. This produces the molecule lactate, which builds up in the muscles of aerobes under anaerobic conditions (lactic acid "burn"). Erythrocytes and some microorganisms also undergo this process.

The other anaerobic fate of pyruvate is Alcohol Fermentation. Here NAD+ is regenerated by the fermentation of pyruvate to ethanol and carbon dioxide. However, that is not the end story of this pathway. This pathway requires the enzyme pyruvate decarboxylase and alcohol dehydrogenase, both of which we'll discuss here.

Pyruvate decarboxylase is an enzyme which decarboxylates pyruvate (duh). What this means is that the carboxyl group of pyruvate loses a carbon dioxide molecule. This results in Acetaldehyde, which serves as the substrate for the next enzyme in the pathway. Pyruvate decarboxylase requires Mg2+ as a cofactor and also needs TPP as a coenzyme. TPP, or Thiamine PyroPhosphate, is derived from thiamin (vitamin B1) and contains a thiazolium ring which acts as an electron "sink" at carbon two of pyruvate. For more information, please view the pyruvate image gallery.

Alcohol Dehydrogenase is the other enzyme required in this process. It takes acetaldehyde and NADH and forms NAD+, ethanol, and Carbon dioxide. And this is where human consumed alcohol comes from.

It should be briefly noted here that lactate and ethanol are not the only products of microbial fermentation. Many other commercially useful products are created and exploited. For more information on this, please check another source. More information may be available in the book store.