Mechanistic Studies of Lipoic Acid Synthase

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DESCRIPTION (provided by applicant): Lipoic acid is an essential sulfur-containing cofactor of several multienzyme complexes that are involved in primary metabolism. It has been known for a number of years to be derived directly from octanoic acid, but the mechanism of sulfur insertion into the unactivated fatty acid has remained obscure. Recently, a gene has been cloned which appears to be the catalytic core of the enzymatic activity that is involved in the sulfur insertion reaction. Signature motifs within its deduced primary sequence and initial spectroscopic characterization of the protein collectively indicate that this "lipoic acid synthase" belongs to an emerging class of enzymes that contain iron-sulfur clusters, and that use S-adenosyl-L-methionine as a means of generating carbon-centered radical intermediates in enzymatic reactions. The roles of S-adenosyl-L-methionine and iron-sulfur clusters in this class of enzymes represent departures from the roles that they have traditionally been assigned in biochemical textbooks. Iron-sulfur clusters have traditionally been thought of as electron transfer agents or Lewis acid catalysts, while S-adenosyl-L-methionine is still considered to be mainly a cellular methylating agent. The experiments proposed herein seek to reveal the "new" molecular enzymology associated with these cofactors. Several experimental mechanisms are discussed, and experiments are proposed that will distinguish among them. Aside from the involvement of lipoic acid as a cofactor in enzyme complexes of energy metabolism, it is known to modulate glucose metabolism in patients with type II diabetes and to serve as a general cellular antioxidant, among many other things. There is significant evidence that lipoic acid can be endogenously synthesized in mammalian cells. Experiments outlined in this proposal will lay a foundation upon which studies to address whether the inability to synthesize lipoic acid endogenously can lead to a compromising of cellular function.