X-RAY STRUCTURE OF AN ANTIFUNGAL CHITOSANASE FROM STREPTOMYCES N174

We report the 2.4 Angstrom X-ray crystal structure of a protein with c hitosan endo-hydrolase activity isolated from Streptomyces N174, The s tructure was solved using phases acquired by SIRAS from a two-site met hyl mercury derivative combined with solvent flattening and non-crysta llographic two-fold sym metry averaging, and refined to an R-factor of 18.5 %. The mostly alpha-helical fold reveals a structural core share d with several classes of lysozyme and barley endochitinase, in spite of a lack of shared sequence. Based on this structural similarity we p ostulate a putative active site, mechanism of action and mode of subst rate recognition. It appears that Glu 22 acts as an acid and Asp 40 se rves as a general base to activate a water molecule for an S(N)2 attac k on the glycosidic bond. A series of amino-acid side chains and backb one carbonyl groups may bind the polycationic chitosan substrate in a deep electronegative binding cleft.