3-Hydroxy-3-methylglutaryl-CoA lyase: catalysis of acetyl coenzyme A enolization

Publisher Website
Google Scholar
Abstract
3-Hydroxy-3-methylglutaryl-CoA [HMG-CoA] lyase [duck liver], which performs the cleavage of HMG-CoA to acetoacetate and acetyl-CoA by a Claisen-type reaction, also catalyzes enolization of acetyl-CoA. The rate of detritiation of methyl-labeled acetyl-CoA is proportional to enzyme concentration and is diminished by an antiserum that also inhibits the cleavage reactions. The 3H-exchange reaction requires both divalent cation and acetoacetate. An analog of HMG-CoA, 3-hydroxyglutaryl-CoA, was prepared by reaction of acetonedicarboxylic anhydride with CoASH and reduction of the ketoacyl-CoA product with cyanohydridoborate. While 3-hydroxyglutaryl-CoA does not appear to be a substrate for HMG-CoA lyase, it competitively inhibits both the cleavage reaction (Ki = 50 .mu.M) and the 3H exchange from acetyl-CoA (Ki = 95 .mu.M). Agreement between the Ki values measured for cleavage and for 3H exchange supports the hypothesis that the slow 3H exchange is a lyase-dependent reaction. Initial attempts to demonstrate complete reversibility of the cleavage reaction were not successful. However, the cleavage of HMG-CoA is at least partially reversible; the enolization of acetyl-CoA may be dependent upon a conformational change of HMG-CoA lyase, induced by binding of acetoacetate, in a manner analogous to the keto acid dependent 3H exchange catalyzed by a malate synthase and citrate synthase.