Characterisation of the activator effect of glycogen on amylosucrase from Neisseria polysaccharea.

Amylosucrase produces an insoluble alpha-1,4-linked glucan from sucrose, releasing fructose. In addition to polymerisation, in the presence of sucrose as sole substrate, amylosucrase catalyses sucrose hydrolysis and oligosaccharide synthesis in significant proportions. The effects of both glycogen acceptor and sucrose concentrations on the reactions catalysed by the highly purified amylosucrase from Neisseria polysaccharea were investigated. Sucrose hydrolysis decreased strongly with the increase of the concentration of glycogen, as did oligosaccharide synthesis, by glucose transfer onto glucose and fructose. The glucosyl units consumed were then preferentially used for elongation of glycogen chains. The study of the kinetic behaviour of amylosucrase revealed a strong, sucrose concentration dependent activator effect of glycogen. This activation was decreased at high sucrose concentration. The optimal sucrose concentrations increased with glycogen concentration, suggesting competition between sucrose and glycogen, and the presence of a second non-catalytic acceptor binding site which could bind various acceptors (glucose, maltose, glycogen) and also sucrose.