Properties of the Glucan Branching Enzyme of the Hyperthermophilic Bacterium Aquifex aeolicus

Glucan branching enzymes are responsible for the synthesis of α(1→6) glycosidic bonds in glycogen and amylopectin. The glucan branching enzyme of the hyperthermophile Aquifex aeolicus is the most thermoactive and thermostable glucan branching enzyme described. The gene encoding this glucan branching enzyme was overexpressed in E. coli and purified using γ-cyclodextrin affinity chromatography. Subsequently, the enzyme was subjected to a biochemical characterization. The optimum temperature for activity was 80°C, and the enzyme was stable up to 90°C. Its thermostability may be explained by the relatively high number of aromatic amino acid residues present, in combination with a relatively low number of glutamine/asparagine residues. The Km for amylose was 4 µM and the Vmax was 4.9 U/mg of protein (at optimal pH and temperature). The side-chain distribution of the branched glucan formed from amylose was determined.