Characterization and identification of essential residues of the glycoside hydrolase family 64 laminaripentaose-producing-β-1, 3-glucanase

Abstract

Laminaripentaose-producing β-1,3-glucanase (LPHase) from Streptomyces matensis DIC-108 uniquely catalyzes the hydrolysis of β-1,3-glucan to release laminaripentaose as the predominant product. For studying this novel enzyme, the gene of LPHase was reconstructed with polymerase chain reaction and over-expressed in Escherichia coli. The recombinant wild-type enzyme and various mutants were further purified to >90 homogeneity on an ion-exchange chromatograph. The catalysis of the recombinant LPHase is confirmed to follow a one-step single-displacement mechanism with 1H-NMR spectrometry. To determine the amino-acid residues essential for the catalysis, more than ten residues, including five highly conserved residuesAsp 143, Glu 154, Asp 170, Asp 376 and Asp 377, were mutated. Among the mutants, E154Q, E154G, D174N and D174G significantly lost catalytic activity. Further investigation with chemical rescue using sodium azide on E154G and D174G confirmed that Glu 154 functions as the general acid whereas Asp 170 serves as the general base in a catalytic turnover. This work is the first report that provides direct information for the identification of the essential residues of GH-64 through kinetic examination. © 2011 The Author.