coenzyme A

Coenzyme A is a thiol-containing cofactor consisting of a β-mercaptoethylamine group, pantothenic acid (vitamin B5), and an ADP moiety with a 3'-phosphate. Its reactive sulfhydryl (-SH) group forms high-energy thioester bonds with acyl groups, most importantly acetyl groups, to create acetyl-CoA.

Biosynthesis: Pantothenic acid (vitamin B5) is phosphorylated and combined with cysteine, then decarboxylated and adenylated in a five-step pathway to form CoA. Deficiency of pantothenic acid is rare but can cause paresthesias ('burning feet syndrome').

Metabolic roles: Acetyl-CoA is the key entry point into the TCA (Krebs) cycle, linking glycolysis, fatty acid oxidation, and amino acid catabolism to oxidative energy production. CoA derivatives are also essential in: fatty acid synthesis and β-oxidation (via acyl-CoA intermediates), ketone body synthesis (HMG-CoA pathway), cholesterol synthesis (via HMG-CoA reductase, the rate-limiting step targeted by statins), heme synthesis (succinyl-CoA as a precursor), and the citric acid cycle (succinyl-CoA, acetyl-CoA).

Clinical relevance: Enzymes and pathways involving CoA are frequent board topics, including: Pyruvate dehydrogenase complex (converts pyruvate to acetyl-CoA, requires B1, B2, B3, B5, lipoic acid) — deficiency causes lactic acidosis. HMG-CoA reductase inhibitors (statins) — reduce cholesterol synthesis. Fatty acid oxidation disorders (e.g., MCAD deficiency) — due to problems metabolizing acyl-CoA species, presenting with hypoglycemia and hypoketosis. Propionic acidemia and methylmalonic acidemia — involve propionyl-CoA metabolism defects.

Understanding CoA's central role helps integrate carbohydrate, lipid, and amino acid metabolism, which is heavily tested on Step 1.