CRYSTAL-STRUCTURE OF BETA-KETOACYL-ACYL CARRIER PROTEIN SYNTHASE-II FROM ESCHERICHIA-COLI REVEALS THE MOLECULAR ARCHITECTURE OF CONDENSING ENZYMES

In the biosynthesis of fatty acids, the beta-ketoacyl-acyl carrier pro tein (ACP) synthases catalyze chain elongation by the addition of two- carbon units derived from malonyl-ACP to an acyl group bound to either ACP or CoA. The crystal structure of beta-ketoacyl synthase II from E scherichia coli has been determined with the multiple isomorphous repl acement method and refined at 2.4 Angstrom resolution, The subunit con sists of two mixed five-stranded beta-sheets surrounded by alpha-helic es, The two sheets are packed against each other in such a way that th e fold can be described as consisting of five layers, alpha-beta-alpha -beta-alpha. The enzyme is a homodimer, and the subunits are related b y a. crystallographic 2-fold axis, The two active sites are located ne ar the dimer interface but are similar to 25 Angstrom apart. The propo sed nucleophile in the reaction, Cys163, is located at the bottom of a mainly hydrophobic pocket which is also lined with several conserved polar residues, In spite of very low overall sequence homology, the st ructure of beta-ketoacyl synthase is similar to that of thiolase, an e nzyme involved in the beta-oxidation pathway, indicating that both enz ymes might have a common ancestor.