Rheological properties of guar galactomannan solutions during hydrolysis with galactomannanase and α-galactosidase enzyme mixtures

Abstract

Guar galactomannan, a naturally occurring polysaccharide, is susceptible to hydrolysis by three enzymes: β-mannosidase, β-mannanase, and α-galactosidase. The β-mannosidase cleaves a single mannose unit from the nonreducing end of the guar molecule, the β-mannanase cleaves interior glycosidic bonds between adjacent mannose units, and the α-galactosidase cleaves the galactose side branches off the guar. In this study, hydrolysis of guar solutions using hyperthermopilic versions of these enzymes together in different proportions and combinations are examined. The enzymatic reactions are carried out in situ in a rheometer, and the progress of the reaction is monitored through measuring the variation in zero shear viscosity. We find the presence of α-galactosidase to affect the action of both β-mannanase and β-mannosidase with respect to solution rheology. However, this effect is more pronounced when the α-galactosidase and β-mannanase or β-mannosidase enzymes were added sequentially rather than simultaneously. This likely is the result of debranching of the guar, which facilitates attack on β-1,4-linkages by both the β-mannanase and the β-mannosidase enzymes and increases hydrolytic rates by the individual enzymes. A rheology-based kinetic model is developed to estimate the reaction rate constants and interpret synergistic effects of multiple enzyme contributions. The model fits the experimental data well and reveals that both the native and the debranched guar have the same activation energy for β-mannanase action, although debranching considerably increases the frequency of enzyme-guar interactions. © 2007 American Chemical Society.