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

(1 -->3),(1 -->4)-beta -D-Glucans represent an important component of cell walls in the Poaceae family of higher plants. A number of glycoside endo- a nd exohydrolases is required for the depolymerization of (1 -->3),(1 -->4)- beta -D-glucans in germinated grain or for the partial hydrolysis of the po lysaccharide in elongating vegetative tissues. The enzymes include (1 -->3) ,(1 -->4)-beta -D-glucan endohydrolases (EC 3.2.1.73), which are classified as family 17 glycoside hydrolases, (1 -->4)-beta -D-glucan glucohydrolases (family 1) and beta -D-glucan exohydrolases (family 3). Kinetic analyses o f hydrolytic reactions enable the definition of action patterns, the thermo dynamics of substrate binding, and the construction of subsite maps. Mechan ism-based inhibitors and substrate analogues have been used to study the sp atial orientation of the substrate in the active sites of the enzymes, at t he atomic level. The inhibitors and substrate analogues also allow us to de fine the catalytic mechanisms of the enzymes and to identify catalytic amin o acid residues. Three-dimensional structures of (1 -->3),(1 -->4)-beta -D- glucan endohydrolases, (1 -->4)-beta -D-glucan glucohydrolases and beta -D- glucan exohydrolases are available or can be reliably modelled from the cry stal structures of related enzymes. Substrate analogues have been diffused into crystals for solving of the three-dimensional structures of enzyme-sub strate complexes. This information provides valuable insights into potentia l biological roles of the enzymes in the degradation of the barley (1 -->3) ,(1 -->4)-beta -D-glucans during endosperm mobilization and in cell elongat ion.