This family includes biotin protein ligase, lipoate-protein ligase A and B. Biotin is covalently attached at the active site of certain enzymes that transfer carbon dioxide from bicarbonate to organic acids to form cellular metabolites. Biotin protein ligase (BPL) is the enzyme responsible for attaching biotin to a specific lysine at the active site of biotin enzymes. Each organism probably has only one BPL. Biotin attachment is a two step reaction that results in the formation of an amide linkage between the carboxyl group of biotin and the epsilon-amino group of the modified lysine. Lipoate-protein ligase A (LPLA) catalyzes the formation of an amide linkage between lipoic acid and a specific lysine residue in lipoate dependent enzymes. The unusual biosynthesis pathway of lipoic acid is mechanistically intertwined with attachment of the cofactor.