Question, what are the fates of pyruvate?

In the presence of oxygen (aerobic condition) pyruvate is converted to acetyl-CoA by

the enzyme pyruvate dehydrogenase which enters the TCA or Kerb cycle where large

(most) of ATP molecules is generated.

In the absence of oxygen (anaerobic conditions) pyruvate undergoes fermentation

either lactic acid fermentation or alcohol fermentation. In these fermentation reaction

NO ATP molecules is generated, however reduced NAD+ is generated from

fermentation. The NAD+ regenerated is used in the glycolysis process to make ATP.

Therefore these cells only get energy (2 ATP) from glycolysis and not from the TCA

cycle. Example of such cell is red blood cells.

LACTATE FERMENTATION: occurs in muscle, erythrocytes, and some other cells.

The pyruvate made in red blood cells is converted to lactate by the enzyme lactate

dehydrogenase. This might be confusing since there are a lot of oxygen in red blood so

why pyruvate isn’t converted to acetyl CoA and enter the TCA cycle. Pyruvate made

from glycolysis does not enter the TCA cycle because the red blood cells do not have

mitochondria which are the site for the TCA cycle (the TCA cycle occurs in the matrix of

the mitochondria). Since NAD+ concentration is low the main purpose of fermentation in

red blood cell is to regenerate NAD+ to enter the glycolysis pathway to yield ATP, since

glycolysis the main manufacture of energy (ATP) for red blood cells.

Moreover lactate is made in muscles. This occurs when the muscles are under vigorous

muscle contraction due to exercise activities, when this occurs lactic acid builds up in

the muscles causing cramps and pain. This is one way the brain is telling the body to

STOP!!! When oxygen become present in the muscle the pyruvate is convert to acetyl

CoA which may enter the TCA cycle to generate ATP.

ETHANOL FERMENTATION: is a two step reaction which uses 2 enzymes; pyruvate

decarboxylase and alcohol dehydrogenase. TPP is an co-factor for both of these

enzymes.

REMINDER:  2 molecules of pyruvate made from one molecules of glucose therefore

two molecules of NAD+ is regenerated.