Improving catalytic efficiency and maximum activity at low pH of Aspergillus neoniger phytase using rational design

A preferable phytase for use in animal feeds for industrial applications should have high, optimal activity at low pH in the monogastric gut environment and high thermostability. To obtain enzymes with enhanced catalytic efficiency (pH 5.5) and excellent activity in acidic pH range, we performed structure-based rational design of a thermostable phytase (PhyAn). For this, six mutants based on different rational design strategies were constructed and heterologously expressed in Pichia pastoris. Particularly, the extracellular enzymatic activity was assessed to ensure that the produced enzymes met requirements of further analyses. Several positive mutants with enhanced catalytic efficiency or pH-profile shifts were carefully examined. Biochemical and kinetic investigations of purified mutants revealed that E79K, E80K, E79K + E80K and D68K had higher catalytic efficiency than the parent enzyme by approximately 49%, 67%, 86% and 15%, respectively. Moreover, the optimum pH of mutant Y65H was shifted from 5.0 to 3.0, and the peak of D68K shifted to pH 5.5. Analysis of the structural–functional relationships revealed that changes in amino acid charges, structural flexibility and space hindrance could significantly influence certain enzyme characteristics. Our results illustrate the feasibility and present a structural foundation for enhancing the phytase-catalytic efficiency and acid resistance by assembling mutations derived using rational design.