Prof. Jisnuson Svasti, Ph.D.

Research Interest

Plant Glycosidase Enzymes

1) Introduction

Glycosidase enzymes hydrolyze carbohydrate compounds, such as oligosaccharides and glycosides. These molecules can function in cellular recognition, or act as components of drugs, toxins, antibiotics, flavors and scents. We became interested in these enzymes, because they not only hydrolyse glycoside substrates, but can be used to synthesize glycosides by transglycosylation or reverse hydrolysis (Figure 1). Our studies began with the screening and purification of several glycosidase enzymes form Thai plants (1-3), but we are now concentrating on the beta-glucosidases (4-8) as a model for studying structure and function relationships, since these enzymes catalyze similar reactions, but show different catalytic properties in terms of hydrolysis, reverse hydrolysis and transglucosylation (9).

The b-glucosidases (EC 3.2.1.21) are a heterogeneous group of enzymes, hydrolyzing various substrates. b-Glucosidases involved in defense generally produce toxic compounds, such as hydrogen cyanide, saponins, coumarins and naphthoquinones by deglycosylation of their substrates. Some b-glucosidases hydrolyze oligosaccharides and may be involved in cell wall remodeling. Other b-glucosidases regulate plant growth by releasing cytokinin growth factors from their glucosides and act in stress response. The active site of enzymes in this family are well studied, and involve catalysis by two conserved glutamic acid residues, one acting as the catalytic acid/base and the other, the catalytic nucleophile. Although many amino acid residues found in the active site pocket are well conserved in plant beta-glucosidases, consistent with their roles in catalysis and glucose binding, the remaining residues show many substitutions, consistent with their adaptation to many divergent functions. Thus, it is difficult to predict enzyme function from the sequences of these enzymes, so to define the biological function of plant beta-glucosidases, sequence and structural studies of these enzymes must be combined with studies of protein expression, cellular location, and substrate specificity.

Since the b-glucosidases differs in substrate specificity and differ in their catalytic activity and specificity for the reverse hydrolysis and transglucosylation reactions, they serve as excellent models for studying structure-function relationships in proteins, which is a major objective of the Center. Our group works closely with the groups of Dr. James Ketudat-Cairns at Suranaree University of Technology, Dr. Prachumporn T. Kongsaeree at Kasetsart University, and Dr. Dumrongkiet Arthan at the Faculty of Tropical Medicine, Mahidol University.

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