Lipoamida

Lipoamida, or lipoyl amide, is the covalent amide derivative of lipoic acid formed when the carboxyl group of lipoic acid bonds to the ε-amino group of a lysine residue on a protein. In cells, lipoamide is typically found as a prosthetic group on several enzyme complexes, notably the pyruvate dehydrogenase (PDH) complex, the α-ketoglutarate dehydrogenase complex, and the branched-chain α-ketoacid dehydrogenase complex, within the E2 subunits. It is also part of the H-protein of the glycine cleavage system.

Mechanism: The lipoyl moiety cycles between an oxidized disulfide form and a reduced dihydrolipoyl form during catalysis, acting as a flexible swinging arm to transfer acyl groups or methylene units between enzyme active sites. In PDH and related complexes, the acyl group is transferred from the E1 enzyme to CoA via the lipoyl arm, and the reduced form is reoxidized by dihydrolipoamide dehydrogenase (E3) with FAD and NAD+ as electron acceptors.

Biosynthesis and occurrence: In bacteria, enzymes such as lipoate-protein ligase (LplA) attach lipoate to specific lysines, forming lipoamide on target proteins; in mitochondria of eukaryotes, lipoic acid is synthesized de novo and attached by similar machinery, with lipoyl synthase introducing sulfur atoms to complete the lipoyl domain.

Clinical and nutritional notes: Lipoamide is an essential cofactor for energy metabolism; impairment of lipoylation can contribute to metabolic disorders, including pyruvate dehydrogenase deficiency. Lipoic acid supplementation is used in some metabolic conditions, but its efficacy varies and depends on the underlying defect.