SUBSITE MAPPING OF HORMOCONIS-RESINAE GLUCOAMYLASES AND THEIR INHIBITION BY GLUCONOLACTONE

The subsite maps of two purified glucoamylases (P and S) from the fungus Hormoconis resinae were determined from kinetic data using sets of linear malto-and isomaltooligosaccharides as substrates. Glucoamylase P, which has an unsually high debranching (1,6-glucosidic) activity, showed a subsite map different from all known subsite maps of glucoamylases. The free energy of binding of maltooligosaccharides was negative at subsite 1, whereas all the others show a positive or zero energy at subsite 1. Inhibition of both glucoamylases P and S acting on either malto- or isomaltohexaose by gluconolactone [D-glucono-(1,5)-lactone] was investigated. Gluconolactone decreased the values of the maximum velocity, V, suggesting it can bind to subsite 1. The size of inhibition constants, identified as dissociation constants of gluconolactone from free enzyme, depended on whether the substrate was maltohexaose or isomaltohexaose. This suggests that gluconolactone has at least two binding sites, and that there are different subsites 1 (or 2) for 1,4- and 1,6-linked substrates. From previously reported results with other glucoamylases, an 'induced fit' model was constructed for glucoamylases hydrolysing oligosaccharides.