Increased activity under acidic conditions and improved thermostability of Aspergillus niger phytase A

Phytases are high-demand enzymes in the production of feed additives for broilers. Both high enzyme activity under acidic pH and high thermostability are required. The medium in the crop and proventriculus/gizzard of broilers is acidic, while the feed pelleting process includes heat treatment. Improvement in both activity and thermostability is a complicated task because an increase in the rigidity of the protein molecule that accompanies the increase in the thermostability generally decreases the activity, and vice versa. In the study, both activity and thermostability of Aspergillus niger phytase A were improved through rational design and site-directed mutagenesis. The activity at pH 2.5 and thermostability at 90 °C were increased by 1.6 and 1.4 times, respectively. Molecular dynamics modeling of the enzyme-substrate complex showed better binding of the substrate that correlated with a 2.3-fold improvement in K_m, while a reduction in fluctuation at 90 °C correlated with the improved thermostability.