A protein’s biological activity can be modified by non-protein co-factors. Co-factors bind to or are covalently linked to an enzyme to assist in biochemical transformations. An enzyme and a co-factor together form an enzymatically active conjugated protein called the holozyme. Co-factors can be organic (coenzymes) or inorganic (metals). They can also be classified according to their ability to bind to enzymes. Loosely-bound cofactors are termed coenzymes and tightly-bound cofactors are termed prosthetic groups. The primary difference between a prosthetic group and coenzyme is; the prosthetic group remains attached to the apoenzyme while undergoing oxidation and reduction while coenzymes may undergo reduction while attached to one apoenzyme, and then migrate to another apoenzyme where it can be oxidized. NAD, NADP and CoA are examples of coenzymes whereas hemes, flavins and biotin are prosthetic groups.
Biotin is a cofactor responsible for carbon dioxide transfer in several carboxylase enzymes. It is covalently attached to the active sites of the metabolic carboxylases. Using biotin cofactor as a mobile carboxyl carrier these metabolic enzymes generally capture CO2 from bicarbonate ion and catalyze transfer of this carboxylate to organic acids to form various cellular metabolites .
Biotin protein ligase (BPL), also known as holocarboxylase synthetase (EC 126.96.36.199), is the enzyme that enables covalent attachment of biotin to the carboxylases.
Post-translationally, biotin forms an amide linkage with specific lysine residue of newly synthesized carboxylases with the help of BPL. The image below shows steps of the biotin protein ligase reaction.
Role of biotin in carboxyl group transfer.
Biotin is the only prosthetic group that facilitates the transfer of a carboxyl group. The role of biotin is to act as a mobile carboxyl group carrier, transporting the carboxyl group from the site of the carboxyl donor to the carboxyl group acceptor enzyme. Common carboxyl group donors are HCO3-, oxaloacetate, or methylmalonyl CoA, and carboxyl group accepter enzymes are pyruvate, acetyl CoA, propionyl CoA.
The four biotin-dependent carboxylases in mammals are acetyl-CoA carboxylase (E.C. 188.8.131.52), pyruvate carboxylase (E.C. 184.108.40.206), propionyl-CoA carboxylase, (E.C. 220.127.116.11), and b-methylcrotonyl-CoA carboxylase (E.C. 18.104.22.168). Acetyl-CoA carboxylase (ACC) is found mainly in the cytosol while pyruvate carboxylase (PC), propionyl-CoA carboxylase (PCC) and methylcrotonyl-CoA carboxylase (MCC) are present in the mitochondria  (Figure 1).
The biotin-dependent carboxylases play a crucial role in cell metabolism. ACC controls fatty acid synthesis in the cell cytosol by providing the substrate malonyl-CoA (Figure 1). ACC may also play an important role in biotin storage . PC is a key enzyme in gluconeogenesis  and provides a tricarboxylic acid cycle intermediate . PCC catalyzes an essential step in the metabolism of amino acids such as isoleucine and methionine, odd-chain fatty acids, and breakdown products of dietary carbohydrates . MCC carboxylase catalyzes an essential step in leucine metabolism.