Structure of maltotetraose-forming amylase from Pseudomonas saccharophila STB07 provides insights into its product specificity

The maltooligosaccharide-forming amylases (MFAses) degrade starch into maltooligosaccharides which potentially benefit human diet and grow popular in food processing, but little has been studied about their product specificity and structures. We focused on this topic and provide evidence through an X-ray crystal structure of the maltotetraose (G4)-forming amylase from Pseudomonas saccharophila STB07 (MFAps), as well as co-crystal structures of MFAps with G4 and with pseudo-maltoheptaose (pseudo-G7) determined at up to 1.1 Å resolution. G4 and pseudo-G7 occupy active cleft subsites −4 to −1 and −4 to +3 respectively. Binding induces conformational changes in the active sites except Asp193, working as the base catalyst. Comparison of the MFAps structure with those of other α-amylases revealed obvious differences in the loop structures providing dominant interactions between protein and substrate in the non-reducing side of the active sites cleft. These structures at the non-reducing end may govern the G4 specificity of MFAps and also be relevant to its exo-type action pattern.