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After emerging from glycolysis, the two pyruvate are transported into the mitochondria. There, the pyruvates undergo a transition stage before entering the actual citric acid cycle. In this phase the pyruvate is transformed into acetyl-coenzyme A (acetyl-CoA), the starting product in the citric acid cycle. \(2 Pyruvate + 2 coenzyme A + 2NAD^+ -> 2 acetyl-CoA +2CO_2 + 2 NADH\)
Acetyl-CoA is a common product of carbohydrate, lipid, and protein breakdown. It consists of an acetyl group attached to a coenzyme A molecule. Coenzyme A is a large molecule that contains a molecule of ADP with two side chain groups stemming from its phosphate arms. Acetyl groups attach to the end of these side chains. In this way, coenzyme A acts as a carrier of acetyl groups. When it is broken down by water, large amounts of energy are released, which, as we shall see, drive the citric acid cycle. The most common way that acetyl-CoA is derived in the metabolic pathway is with the help of the pyruvate dehydrogenase multienzyme complex. The pyruvate dehydrogenase multienzyme is a complex of three distinct enzymes that together convert pyruvate into acetyl-CoA with the help of a molecule of coenzyme A and NAD.
Once we have acetyl-CoA, we are ready to enter the citric acid cycle.
