Coenzyme A
|
|
Acetyl_Coenzyme_A_1.jpg
Coenzyme A (CoA, CoASH, or HSCoA) is adapted from β-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 as 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 sulfhydryl 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 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+.
Coenzyme A is very central to the balance between carbohydrate metabolism and fat metabolism. Carb metabolism needs some CoA for the citric acid cycle to continue, and fat metabolism needs a lot of CoA for breaking down fatty acid chains during β-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 tons of acetyl-CoA molecules, each of which needs its own new CoA molecule.)
However, CoA can only come from certain processes: Carb metabolism gets CoA from its own citric acid cycle, where it's constantly recycled; but during fat metabolism, since the citric acid cycle is stopped (as it is during starvation), all of the CoA is trapped in those tons 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-carb diets.)
See also
References
- Karl Miller (1998). Beta Oxidation of Fatty Acids (http://www.gwu.edu/~mpb/betaox.htm). Retrieved May 18, 2005.
- Charles Ophard (2003). Acetyl-CoA Crossroads (http://www.elmhurst.edu/~chm/vchembook/623acetylCoAfate.html). Retrieved May 18, 2005.
Template:Biochem-stubde:Acetyl-CoA
eo:Acetila Koenzimo A
vi:Coenzyme A