Fates of pyruvate:

-   Entry into the citric acid cycle.    The Glycolysis   releases  associatively   little  of  the energy  present  in a glucose  molecule;  much more is released  through the subsequent   operation   of  the  oxidative   phosphorylation  and citric  acid  cycle.

Following   this route under aerobic conditions and pyruvate is converted to acetyl CoA through the enzyme pyruvate dehydrogenase and the acetyl CoA then enters the citric acid cycle. A pyruvate dehydrogenase reaction is an oxidative decarboxylation:

-   Conversion to fatty acid or ketone bodies.    When the cellular energy stage is high (ATP  in  excess),  the  rate  of  the  citric  acid  cycle decreases  and acetyl CoA starts to accumulate.  Under these conditions the acetyl CoA can be used of ketone bodies for fatty acid synthesis or the synthesis.

-   Conversion to lactate.   The NAD⁺ used during glycolysis (in the formation of 1, 3-bisphosphoglycerate   through   glyceraldehyde    3-phosphate    dehydrogenase) must be regenerated if glycolysis is to continue. Under aerobic conditions  NAD⁺ is  reproduce  through  the  reoxidation  of  NADH  through  the  electron transport chain. When oxygen is limiting, as in muscle during vigorous  contraction,  the  reoxidation  of  NADH  to  NAD⁺  through  the  electron transport   chain  becomes  insufficient   to  maintain  glycolysis.   Under these conditions, NAD⁺ is reproducing alternativaly through conversion of the pyruvate to lactate through lactate dehydrogenase:

When sufficient oxygen becomes available once more, NAD⁺ levels rise by operation of the electron transport chain. Lactate dehydrogenase reaction  then reverses  to regenerate  pyruvate  which is converted  through pyruvate dehydrogenase   to  acetyl  CoA  that  can  enter  the  citric  acid  cycle. Therefore the operation of lactate dehydrogenase  in mammals is a mechanism  for  the  reoxidation  of  NADH  to  NAD⁺ while  permitting  glycolysis  to continue  and ATP to be create  under  anaerobic  conditions.  The process is even more difficult in the case of vigorously contracting skeletal muscle. Now  the  lactate  produced  is  transported  in  the  bloodstream   to  the  liver where it is converted back to glucose and can return once again through the bloodstream  to the skeletal muscle to be metabolized  to yield energy. At last, in some microorganisms and lactate is the normal product from pyruvate.

-   Conversion to ethanol.   In yeast and some other microorganisms under anaerobic conditions, the NAD⁺ needs for the continuation of glycolysis are regenerated through a procedure known as alcoholic fermentation.  The pyruvate is translated to acetaldehyde (through pyruvate decarboxylase) and then to ethanol (through alcohol dehydrogenase) and latter reaction reoxidizing the NADH to NAD⁺    :