Disulfide Bond Engineering for Enhancing the Thermostability of the Maltotetraose-Forming Amylase from Pseudomonas saccharophila STB07

Maltooligosaccharides are a novel type of functional oligosaccharides with potential applications in food processing and can be produced by glycosyl hydrolases hydrolyzing starch. However, the main obstacle in industrial applications is the balance between the high temperature of the process and the stability of enzymes. In this study, based on the structural information and in silico tools (DSDBASE-MODIP, Disulfide by Design2 and FoldX), two disulfide bond mutants (A211C-S214C and S409C-Q412C) of maltotetraose-forming amylase from Pseudomonas saccharophila STB07 (MFA(ps)) were generated to improve its thermostability. The mutation A211C-S214C was closer to the catalytic center and showed significantly improved thermostability with a 2.6-fold improved half-life at 60 °C and the thermal transition mid-point increased by 1.6 °C, compared to the wild-type. However, the thermostability of mutant S409C-Q412C, whose mutation sites are closely to CBM20, did not change observably. Molecular dynamics simulations revealed that both disulfide bonds A211C-S214C and S409C-Q412C rigidified the overall structure of MFA(ps), however, the impact on thermostability depends on the position and distance from the catalytic center.