Once pyruvate is formed from glucose it can be used in multiple pathways. Gluconeogenesis is the anabolic synthesis of glucose from pyruvate, but it is not the exact reverse of glycolysis. Pyruvate can also participate in two different catabolic processes: fermentation and slow cellular respiration. Fermentation happens when oxygen is low or energy is needed quickly, and there are two kinds: homolactic fermentation, which occurs in muscle cells, red blood cells, and some bacteria, and alcoholic fermentation, which occurs in yeast and some bacteria. The purpose of this fast anaerobic respiration is to regenerate NAD+ so that glycolysis can continue, and it does not generate additional ATP. Slow cellular respiration occurs when there is enough oxygen and the need for energy is not urgent. In slow respiration, pyruvate is used to synthesize acetyl-CoA to be further oxidized. Acetyl-CoA is an acetyl group attached to coenzyme A and is the central molecule in metabolism of fuel molecules. Formation of acetyl-CoA from pyruvate by a pyruvate dehydrogenase complex is the “point of no return” in carbohydrate metabolism. It cannot be converted back to pyruvate.
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