Coenzyme A (CoA, CoASH, or HSCoA) is adapted from pantothenic acid and adenosine triphosphate and used in metabolism in areas such as fatty acid oxidization and the citric acid cycle. Its main function is to carry acyl groups such as acetyl as thioesters. A molecule of coenzyme A carrying an acetyl group is also referred to as acetyl-CoA.
Acetyl-CoA is an important molecule itself. It is the precursor to HMG CoA, which is a vital component in cholesterol and ketone synthesis. Furthermore, it contributes the acetyl group to acetylcholine; the addition of the acetyl group to choline a reaction that is catalysed by choline acetyltransferase. Its main task is conveying the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production.
The conversion of pyruvate into Acetyl-CoA is referred to as the Pyruvate Dehydrogenase Reaction. It is catalyzed by an enzyme-complex called pyruvate dehydrogenase. The enzyme consists of 60 subunits: 24 pyruvate dehydrogenase, 24 dihydrolipoyl transacetylase , and 12 dihydrolipoyl dehydrogenase (commonly denoted E1, E2, and E3). 24 pyruvate dehydrogenase has the coenzyme TPP incorporated into it, 24 dihydrolipoyl transacetylase has lipoate and coenzyme A, and 12 dihydrolipoyl dehydrogenase has the coenzymes FAD and NAD+. Through a complex reaction, pyruvate is decarboxylated and turned into acetaldehyde, then attached to coenzyme A while NAD+ is subsequently reduced to NADH and H+.