Biochemical Journal

Research article

Identification of essential active-site residues in the cyanogenic β-glucosidase (linamarase) from cassava (Manihot esculenta Crantz) by site-directed mutagenesis

Zsolt KERESZTESSY, Kate BROWN, M. Alison DUNN, Monica A. HUGHES

Abstract

The coding sequence of the mature cyanogenic β-glucosidase (β-glucoside glucohydrolase, EC 3.2.1.21; linamarase) was cloned into the vector pYX243 modified to contain the SUC2 yeast secretion signal sequence and expressed in Saccharomyces cerevisiae. The recombinant enzyme is active, glycosylated and showed similar stability to the plant protein. Michaelis constants for hydrolysis of the natural substrate, linamarin (Km = 1.06mM) and the synthetic p-nitrophenyl β-D-glucopyranoside (PNP-Glc; Km = 0.36mM), as well as apparent pKa values of the free enzyme and the enzymeŐsubstrate complexes (pKE1 = 4.4Ő4.8, pKE2 = 6.7Ő7.2, pKES1 = 3.9Ő4.4, pKES2 = 8.3) were very similar to those of the plant enzyme. Site-directed mutagenesis was carried out to study the function of active-site residues based on a homology model generated for the enzyme using the MODELLER program. Changing Glu-413 to Gly destroyed enzyme activity, consistent with it being the catalytic nucleophile. The Gln-339Glu mutation also abolished activity, confirming a function in positioning the catalytic diad. The Ala-201Val mutation shifted the pKa of the acid/base catalyst Glu-198 from 7.22 to 7.44, reflecting a change in its hydrophobic environment. A Phe-269Asn change increased Km for linamarin hydrolysis 16-fold (16.1mM) and that for PNP-Glc only 2.5-fold (0.84mM), demonstrating that Phe-269 contributes to the cyanogenic specificity of the cassava β-glucosidase.

  • catalytic nucleophile
  • cyanogenesis
  • linamarin binding
  • molecular modelling
  • pH dependence