succinilCoA

Succinyl-CoA is a high-energy thioester form of coenzyme A that serves as an important intermediate in cellular metabolism. In most organisms, it resides in the mitochondria as part of the citric acid (Krebs) cycle, where it links catabolic reactions to energy production and biosynthetic processes.

In the citric acid cycle, succinyl-CoA is produced by the oxidative decarboxylation of alpha-ketoglutarate via the alpha-ketoglutarate dehydrogenase complex, releasing CO2 and generating NADH. It is then converted to succinate by the enzyme succinyl-CoA synthetase, a reaction that also produces a high-energy phosphate bond (in many tissues as GTP, via substrate-level phosphorylation). This step provides substrate-level energy and contributes to the regeneration of CoA for continued metabolism.

Succinyl-CoA also serves as an entry point for several biosynthetic pathways. In mitochondria, it is a substrate for delta-aminolevulinate synthase, the first step of heme biosynthesis, where succinyl-CoA condenses with glycine to form ALA. It is also produced from propionyl-CoA derived from odd-chain fatty acids, some amino acids, and cholesterol side chains through a sequence involving propionyl-CoA carboxylation and methylmalonyl-CoA mutase, ultimately feeding back into the TCA cycle as succinyl-CoA.

Beyond its metabolic roles, succinyl-CoA participates in post-translational protein modification through succinylation, a regulatory mechanism affecting enzyme activity and metabolism. Defects in related pathways (e.g., propionate metabolism or B12-dependent mutase activity) can alter succinyl-CoA availability and heme synthesis, with clinical consequences.