Structure-function relationships of β- D-glucan endo- and exohydrolases from higher plants

Abstract

(1→3),(1→4)-β-D-Glucans represent an important component of cell walls in the Poaceae family of higher plants. A number of glycoside endo- and exohydrolases is required for the depolymerization of (1→3),(1→4)-β-Dglucans in germinated grain or for the partial hydrolysis of the polysaccharide in elongating vegetative tissues. The enzymes include (1→3),(1→4)-β-D-glucan endohydrolases (EC 3.2.1.73), which are classified as family 17 glycoside hydrolases, (1→4)-β-D-glucan glucohydrolases (family 1) and β-D-glucan exohydrolases (family 3). Kinetic analyses of hydrolytic reactions enable the definition of action patterns, the thermodynamics of substrate binding, and the construction of subsite maps. Mechanism-based inhibitors and substrate analogues have been used to study the spatial orientation of the substrate in the active sites of the enzymes, at the atomic level. The inhibitors and substrate analogues also allow us to define the catalytic mechanisms of the enzymes and to identify catalytic amino acid residues. Three-dimensional structures of (1→3),(1→4)-β-D-glucan endohydrolases, (1→4)- β-D-glucan glucohydrolases and β-D-glucan exohydrolases are available or can be reliably modelled from the crystal structures of related enzymes. Substrate analogues have been diffused into crystals for solving of the threedimensional structures of enzyme-substrate complexes. This information provides valuable insights into potential biological roles of the enzymes in the degradation of the barley (1→3),(1→4)-β-D-glucans during endosperm mobilization and in cell elongation.