α-Amylases: Interaction with Polysaccharide Substrates, Proteinaceous Inhibitors and Regulatory Proteins

α-Amylases occur widely in plants, animals, and microorganisms. They often act in synergy with other related and degradative enzymes and may also be regulated by proteinaceous inhibitors. Open questions exist on how αamylases interact with polysaccharides. Several enzymes possess secondary carbohydrate binding sites situated on the surface at a certain distance of the active site cleft. The functions of such sites were studied in barley αamylase isozymes by structure-guided mutational analysis and measurement of activity and binding parameters. Two surface sites were assigned distinct roles. One of the sites seems to participate in hydrolysis of polysaccharides by a multiple attack mechanism. Polysaccharide processing enzymes can also contain carbohydrate binding modules, e.g. starch binding domains that assist in the attack on macromolecular substrates and are useful in engineering of enzyme efficiency. The multidomain nature of these enzymes raises questions on the dynamics and structural properties in solution and in substrate complexes.