title: 3-Hydroxy-3-methylglutaryl coenzyme A synthase. Evidence for an acetyl-S-enzyme intermediate and identification of a cysteinyl sulfhydryl as the site of acetylation.
abstract: Homogeneous liver 3-hydroxy-3-methylglutaryl coenzyme A synthase, which catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA, also carries out: (a) a rapid transacetylation from acetyl-CoA to 31-dephospho-CoA and (b) a slow hydrolysis of acetyl-CoA to acetate and CoA. Transacetylation and hydrolysis occur at 50 and 1 percent, respectively, the rate of the synthasecatalyzed condensation reaction. It appears that an acetyl-enzyme intermediate is involved in the transacetylase and hydrolase reactions of 3-hydroxy-3-methylglutaryl-CoA synthase, as well as in the over-all condensation process. Covalent binding to the enzyme of a [14C]acetyl group contributed by [1(-14)C]acetyl-CoA is indicated by migration of the [14C]acetyl group with the dissociated synthase upon electrophoresis in dodecyl sulfate-urea and by precipitation of [14C]acetyl-enzyme with trichloroacetic acid. At 0 degrees and a saturating level of acetyl-CoA, the synthase is rapidly (less than 20 s) acetylated yielding 0.6 acetyl group/enzyme dimer. Performic acid oxidation completely deacetylates the enzyme, suggesting the site of acetylation to be a cysteinyl sulfhydryl group. Proteolytic digestion of [14C]acetyl-S-enzyme under conditions favorable for intramolecular S to N acetyl group transfer quantitatively liberates a labeled derivative with a [14C]acetyl group stable to performic acid oxidation. The labeled oxidation product is identified as N-[14C]acetylcysteic acid, thus demonstrating a cysteinyl sulfhydryl group as the original site of acetylation. The ability of the acetylated enzyme, upon addition of acetoacetyl-CoA, to form 3-hydroxy-3-methylglutaryl-CoA indicates that the acetylated cysteine residue is at the catalytic site.
mesh_terms: D000107:Acetylation; D000818:Animals; D001665:Binding Sites; D001717:Birds; D002852:Chromatography, Ion Exchange; D002855:Chromatography, Thin Layer; D003543:Cysteamine / Q000494:pharmacology; D003545:Cysteine / Q000032:analysis; D004229:Dithiothreitol / Q000494:pharmacology; D004590:Electrophoresis, Paper; D004591:Electrophoresis, Polyacrylamide Gel; D005978:Glutathione / Q000494:pharmacology; D006904:Hydroxymethylglutaryl-CoA Synthase / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D007652:Oxo-Acid-Lyases; D011402:Pronase; D011485:Protein Binding; D013997:Time Factors
chemical_list: D003543:Cysteamine; D006904:Hydroxymethylglutaryl-CoA Synthase; D011402:Pronase; D007652:Oxo-Acid-Lyases; D005978:Glutathione; D003545:Cysteine; D004229:Dithiothreitol