The Importance of Surface-Binding Site towards Starch-Adsorptivity Level in α -Amylase: A Review on Structural Point of View.

Starch is a polymeric carbohydrate composed of glucose. As a source of energy, starch can be degraded by various amylolytic enzymes, including α -amylase. In a large-scale industry, starch processing cost is still expensive due to the requirement of high temperature during the gelatinization step. Therefore, α -amylase with raw starch digesting ability could decrease the energy cost by avoiding the high gelatinization temperature. It is known that the carbohydrate-binding module (CBM) and the surface-binding site (SBS) of α -amylase could facilitate the substrate binding to the enzyme's active site to enhance the starch digestion. These sites are a noncatalytic module, which could interact with a lengthy substrate such as insoluble starch. The major interaction between these sites and the substrate is the CH/pi-stacking interaction with the glucose ring. Several mutation studies on the Halothermothrix orenii , SusG Bacteroides thetaiotamicron , Barley , Aspergillus niger , and Saccharomycopsis fibuligera α -amylases have revealed that the stacking interaction through the aromatic residues at the SBS is essential to the starch adsorption. In this review, the SBS in various α -amylases is also presented. Therefore, based on the structural point of view, SBS is suggested as an essential site in α -amylase to increase its catalytic activity, especially towards the insoluble starch.