Coenzyme A (CoA, CoASH, or HSCoA) is adapted from beta-mercaptoethylamine, panthothenate 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 or thioesters. A molecule of coenzyme A carrying an acetyl group is also referred to as acetyl-CoA. It is sometimes referred to as 'CoASH' or 'HSCoA' because when it's not attached to a molecule such as an acetyl group, it is attached to a thiol group, -SH.
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 an acetyl group to choline to produce acetylcholine, in a reaction 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 (thiamin pyrophosphate) 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+.
Coenzyme A is very central to the balance between carbohydrate metabolism and fat metabolism. Carbohydrate metabolism needs some CoA for the citric acid cycle to continue, and fat metabolism needs a larger amount of CoA for breaking down fatty acid chains during beta-oxidation. Fats can only supply energy by being broken down into small units, and our cells have only one way of doing this: breaking fats down into acetyl-CoA molecules, each of which needs a CoA molecule for its production.
Carbohydrate metabolism recycles CoA in the citric acid cycle; in fat metabolism, which does not involve the citric acid cycle, all of the CoA is trapped in the form of acetyl-CoA. The only way to release enough CoA is to combine acetyl-CoA with each other to create ketone bodies and detach a CoA. (This is why excess ketone bodies are produced in diabetes and during starvation or low-carbohydrate diets.)
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