STEREOCHEMISTRY OF CHITIN HYDROLYSIS BY A PLANT CHITINASE LYSOZYME AND X-RAY STRUCTURE OF A COMPLEX WITH ALLOSAMIDIN - EVIDENCE FOR SUBSTRATE ASSISTED CATALYSIS

The plant enzyme hevamine has both chitinase and lysozyme activity. HP LC analysis of the products of the hydrolysis of chitopentaose shows t hat hevamine acts with retention of the configuration, despite the abs ence of a nucleophilic or stabilizing carboxylate. To analyze the stab ilization of a putative oxocarbonium ion intermediate, the X-ray struc ture of hevamine complexed with the inhibitor allosamidin was determin ed at 1.85 Angstrom resolution. This structure supports the role of Gl u127 as a proton donor. The allosamizoline group binds in the center o f the active site, mimicking a reaction intermediate in which a positi ve charge at Cl is stabilized intramolecularly by the carbonyl oxygen of the N-acetyl group at C2.