Lipoic Acid

Lipoic acid's role as a growth factor for microorganisms and as a cofactor for biochemical reactions in all organisms is well established. The cofactor was discovered originally in the conversion of pyruvate to acetate and as a factor essential for the oxidation of pyruvate. Lipoic acid is known to occur in a-keto acid dehydrogenases from a variety of organisms. It is normally bound to the e-amino group of a lysine residue (analogous to biotin) allowing the cofactor to extend out and away from the enzyme surface as a 'swinging arm.'

Reactivity The reactions taking place in the pyru-vate dehydrogenase complex best reveal the cofac-tor function of lipoic acid. As a cofactor, lipoic acid (6,8 dithiooctanoic acid) exists in both an oxidized (disulfide) and reduced form (Figure 10). The disulfide form oxidizes active acetaldehye bound to TPP simultaneously with the transfer of the acetate product to one of the -SH groups of the now reduced lipoic acid. As a thioester, the acetate group is subsequently transferred to coenzyme A forming acetyl-CoA and regenerating a free -SH group on lipoic acid. The reduced lipoic acid, which still contains the electrons, is then oxidized by a flavoprotein (FAD) restoring the disulfide group of lipoic acid for another round of catalysis. Eventually, FADH2 passes the electrons to NAD+, which links to the electron transport chain of the mitochondria. Lipoic acid is thus an oxidizing agent and a carrier of acetate in the reaction. One can picture the long arm of the cofactor swinging between sites on the subunits of the pyruvate dehy-drogenase complex in order to perform the multiple reactions in synchrony with the catalytic events taking place.

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