Lipoyl synthase requires two equivalents of S-adenosyl-L-methionine to synthesize one equivalent of lipoic acid

Lipoyl synthase (LipA) catalyzes the formation of the lipoyl cofactor, which is employed by several multienzyme complexes for the oxidative decarboxylation of various α-keto acids, as well as the cleavage of glycine into CO₂ and NH₃, with concomitant transfer of its α-carbon to tetrahydrofolate, generating N⁵,N ¹⁰-methylenetetrahydrofolate. In each case, the lipoyl cofactor is tethered covalently in an amide linkage to a conserved lysine residue located on a designated lipoyl-bearing subunit of the complex. Genetic and biochemical studies suggest that lipoyl synthase is a member of a newly established class of metalloenzymes that use S-adenosyl-L-methionine (AdoMet) as a source of a 5′-deoxyadenosyl radical (5′-dA^.), which is an obligate intermediate in each reaction. These enzymes contain iron-sulfur clusters, which provide an electron during the cleavage of AdoMet, forming L-methionine in addition to the primary radical. Recently, one substrate for lipoyl synthase has been shown to be the octanoylated derivative of the lipoyl-bearing subunit (E₂) of the pyruvate dehydrogenase complex [Zhao, S., Miller, J. R., Jian, Y., Marletta, M. A., and Cronan, J. E., Jr. (2003) Chem. Biol. 10, 1293-1302]. Herein, we show that the octanoylated derivative of the lipoyl-bearing subunit of the glycine cleavage system (H-protein) is also a substrate for LipA, providing further evidence that the cofactor is synthesized on its target protein. Moreover, we show that the 5′-dA^. acts directly on the octanoyl substrate, as evidenced by deuterium transfer from [octanoyl-d₁₅]H-protein to 5′-deoxyadenosine. Last, our data indicate that 2 equiv of AdoMet are cleaved irreversibly in forming 1 equiv of [lipoyl]H-protein and are consistent with a model in which two LipA proteins are required to synthesize one lipoyl group.