Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Cofactors
Carboxyl Group Carriers

Carboxylation-decarboxylation and transcarboxylation reactions are mediated by Enzymes, many of which require biotin as a cofactor. Biotin (vitamin H) serves as a prosthetic group for enzymes, forming a covalent bond with their protein moiety via the ε-amino group of Lysine (Fig. 7.7). The heterocyclic portion of the biotin molecule consists of imidazole (I) and thiophene (T) rings, while the side chain is represented by a valeric acid residue. The active form of the biotin cofactor is N5-carboxybiotin (“active carboxyl”).

Biotin-containing enzymes are involved in carbon dioxide fixation, as well as The Biosynthesis of Lipids, Amino Acids, CARBOHYDRATES, Nucleic Acids, and Other Compounds. The Role of biotin is to couple ATP Hydrolysis with carboxylation: in The First stage, bicarbonate (HCO3-) or CO2 is attached to biotin to form carboxybiotin, a process that requires ATP hydrolysis. In the second stage, carboxybiotin transfers the carboxyl group to the substrate, acting as a flexible “swinging arm” within the enzyme complex.

Only one biotin-containing enzyme is known that does not require ATP for catalysis—propionyl-CoA carboxylase from propionic acid Bacteria. However, even in this case, biotin acts as the carboxyl group carrier.

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Fig. 7.7. Structure of biotin and “active carboxyl”



Last update: 06/08/2026

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