title: X-ray structure of an anti-fungal chitosanase from streptomyces N174.
abstract: We report the 2.4 A X-ray crystal structure of a protein with chitosan endo-hydrolase activity isolated from Streptomyces N174. The structure was solved using phases acquired by SIRAS from a two-site methyl mercury derivative combined with solvent flattening and non-crystallographic two-fold symmetry averaging, and refined to an R-factor of 18.5%. The mostly alpha-helical fold reveals a structural core shared with several classes of lysozyme and barley endochitinase, in spite of a lack of shared sequence. Based on this structural similarity we postulate a putative active site, mechanism of action and mode of substrate recognition. It appears that Glu 22 acts as an acid and Asp 40 serves as a general base to activate a water molecule for an SN2 attack on the glycosidic bond. A series of amino-acid side chains and backbone carbonyl groups may bind the polycationic chitosan substrate in a deep electronegative binding cleft.
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas, Austin 78712, USA.
authors.forename: E M
authors.initials: EM
authors.lastname: Marcotte
authors.forename: A F
authors.initials: AF
authors.lastname: Monzingo
authors.forename: S R
authors.initials: SR
authors.lastname: Ernst
authors.forename: R
authors.initials: R
authors.lastname: Brzezinski
authors.forename: J D
authors.initials: JD
authors.lastname: Robertus
mesh_terms: D001665:Binding Sites; D002240:Carbohydrate Sequence; D002686:Chitin / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D048271:Chitosan; D018360:Crystallography, X-Ray; D006026:Glycoside Hydrolases / Q000737:chemistry* / Q000378:metabolism; D006868:Hydrolysis; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D013237:Stereoisomerism; D013302:Streptomyces / Q000201:enzymology*
chemical_list: D002686:Chitin; D048271:Chitosan; D006026:Glycoside Hydrolases; C019868:chitosanase