Mutations in two amino acids in phyI1s from <Emphasis Type="Italic">Aspergillus niger</Emphasis> 113 improve its phytase activity

We applied in vitro mutagenesis and colony screening, using the wild type phyI1s gene from Aspergillus niger 113 as the template, and obtained two mutant phyI1s (gene products) after one round of screening. The two mutants had mutations at two nucleic acid sites, resulting in changes in two amino acids: K41E, E121F. None of the amino acid substitutions in the two mutants was in a position reported to be important for catalysis or substrate binding. Kinetic analysis of the phytase activity of the two mutants indicated that the substitutions gave rise to 2.5- and 3.1-fold increased specific activity, and a 1.78- and 3.24-fold reduced affinity for sodium phytate. In addition, the overall catalytic efficiency (k _cat/K _m) of the two mutants was changed by 0.52-fold and 0.68-fold compared to that of the wild type. Such mutants will be instrumental for the structure–function study of the enzyme and for industrial application.