Propionyl-CoA

Propionyl-CoA is a coenzyme A derivative of propionic acid.

Metabolism in animals

Production

There are several different ways in which it is formed:

• It is formed as a product of beta-oxidation of odd-chain fatty acids, which occur in meat of ruminant animals.
• It is also a product of metabolism of isoleucine and valine.
• It is a product of alpha-ketobutyric acid, which in turn is a product of digestion of threonine and methionine.

Metabolic fate

In mammals, propionyl-CoA is converted to (S)-methylmalonyl-CoA by propionyl-CoA carboxylase, a biotin-dependent enzyme also requiring bicarbonate and ATP.

This product is converted to (R)-methylmalonyl-CoA by methylmalonyl-CoA racemase.

(R)-Methylmalonyl-CoA is converted to succinyl-CoA by methylmalonyl-CoA mutase, an enzyme requiring cobalamin to catalyze the carbon-carbon bond migration.

A defect in methylmalonyl-CoA mutase enzyme results in methylmalonic aciduria, a dangerous disorder that causes a lowering of blood pH.

Metabolism in plants and insects

In plants and insects propionyl-CoA is metabolized to acetate in a very different way, similar to beta oxidation.

Not all details of this pathway have been worked out, but it appears to involve formation of acrylyl-CoA, then 3-hydroxypropionyl-CoA.

This is metabolized with loss of carbon 1 of 3-hydroxypropionyl-CoA as carbon dioxide, while carbon 3 becomes carbon 1 of acetate.

References

• Halarnkar P, Blomquist G (1989). "Comparative aspects of propionate metabolism". Comp. Biochem. Physiol., B 92 (2): 227–31.