carboxybiotiini

Carboxybiotiini, also known as carboxybiotin, is the carboxylated form of biotin (vitamin B7) that serves as a CO2 carrier in biotin-dependent carboxylases. Biotin acts as a prosthetic group covalently linked to a lysine residue in enzymes such as the biotin carboxyl carrier protein. In biotin-dependent carboxylation reactions, bicarbonate is activated by ATP to form a carboxyphosphate intermediate, which transfers CO2 to biotin to generate carboxybiotin. The carboxyl group is then transferred from carboxybiotin to the substrate by the enzyme’s carboxyltransferase domain.

Carboxybiotin operates as a transient intermediate in several essential metabolic pathways. In pyruvate carboxylase, the CO2 is added to pyruvate to form oxaloacetate; in acetyl-CoA carboxylase, CO2 is added to acetyl-CoA to form malonyl-CoA; and in propionyl-CoA carboxylase, CO2 is transferred to propionyl-CoA to form D-methylmalonyl-CoA. These reactions play key roles in gluconeogenesis, fatty acid synthesis, and odd-chain fatty acid and amino acid metabolism.

Biotin-dependent carboxylases and their carboxybiotin intermediates are found across bacteria and eukaryotes. The carboxybiotin state is typically short-lived, being rapidly transferred to substrates and regenerated as biotin for subsequent catalytic cycles. Disturbances in biotin availability or in the biotinylation of carboxyl carrier proteins can impair these carboxylation reactions and affect overall metabolic flux.