Doctor of Philosophy (PhD)
Acetyl-CoA carboxylase catalyzes the first step in the synthesis of fatty acids. The Escherichia coli form of the enzyme consists of a biotin carboxylase protein, a biotin carboxyl carrier protein, and a carboxyltransferase protein. This enzyme uses the cofactor biotin as a carboxyl carrier. In order for the carboxylation of biotin to occur, biotin must be deprotonated at its N-1 position. It has been proposed that the active site residues cysteine 230 and lysine 238 act as an acid-base pair to deprotonate biotin. To test this hypothesis, site-directed mutagenesis was used to mutate cysteine 230 to alanine (C230A) and lysine 238 to glutamine (K238Q). Mutations at either residue resulted in a 50-fold increase in the Km for ATP. The C230A mutation had no effect on the formation of carboxybiotin, indicating that cysteine 230 does not play a role in the deprotonation of biotin. However, the K238Q mutation resulted in no formation of carboxybiotin, which showed that lysine 238 has a role in the carboxylation reaction. However, the pK value for lysine 238 was 9.4 or higher, suggesting lysine 238 is not a catalytic base. Thus, the results suggest that cysteine 230 and lysine 238 do not act as an acid-base pair in the deprotonation of biotin. A bisubstrate analog inhibitor of carboxyltransferase was synthesized by covalently linking biotin to Coenzyme A via an acyl bridge between the sulfur of Coenzyme A and the N-1 of biotin. The inhibitor was found to have an inhibition constant of 23 ± 2 ìM, which means it binds the enzyme 350-times tighter than biotin. The bisubstrate analog demonstrated competitive inhibition versus malonyl-CoA and noncompetitive inhibition versus biocytin. This is consistent with an ordered kinetic mechanism with malonyl-CoA binding first. A precursor to the inhibitor, chloroacylated biotin, was capable of inhibiting the differentiation of 3T3-L1 cells in a dose-dependent manner. Treatment with chloroacylated biotin resulted in a decrease in acetyl-CoA carboxylase activity and inhibited lipid accumulation. Our results support recent studies that indicate acetyl-CoA carboxylase may be a suitable target as an anti-obesity therapeutic.
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Levert, Keith Logan, "The role of cysteine 230 and lysine 238 of biotin carboxylase in the deprotonation of biotin and synthesis of bisubstrate analogy inhibitor of carboxyltransferase" (2002). LSU Doctoral Dissertations. 881.
Grover L. Waldrop