Structure-Based Engineering of a Maltooligosaccharide-Forming Amylase To Enhance Product Specificity

Maltooligosaccharide-forming amylases (MFAses) are promising tools for a variety of food industry applications because they convert starch into functional maltooligosaccharides. The MFAse from Bacillus stearothermophilus STB04 (BstMFAse) is a thermostable enzyme that preferentially produces maltopentaose and maltohexaose. An X-ray crystal structure of the BstMFAse-acarbose complex suggested that mutation of glycine 109 would increase its maltohexaose specificity. Using site-directed mutagenesis, glycine 109 was replaced with several different amino acids. Mutant-containing asparagine (G109N), aspartic acid (G109D), and phenylalanine (G109F) produced 36.1, 42.4, and 39.0% maltohexaose from starch, respectively, which was greater than that produced by the wild-type (32.9%). These mutants also exhibited substantially altered oligosaccharide hydrolysis patterns in favor of maltohexaose production. Homology models suggested that the mutants form extra interactions with the substrate at subsite -6, which were responsible for the enhanced maltohexaose specificity of BstMFAse. The results of this study support the proposition that binding of the substrate's nonreducing end in the nonreducing end-subsite of the MFAse active center plays a crucial role in its product specificity.