Site-Directed Mutagenesis Improves Catalytic Efficiency and Thermostability of Escherichia coli pH 2.5 Acid Phosphatase/Phytase Expressed in Pichia pastoris

Escherichia coli pH 2.5 acid phosphatase gene (appA) and three mutants were expressed in Pichia pastoris to assess the effect of strategic mutations or deletion on the enzyme (EcAP) biochemical properties. Mutants A131N/V134N/D207N/S211N, C200N/D207N/S211N, and A131N/V134N/C200N/D207N/S211N had four, two, and four additional potential N-glycosylation sites, respectively. Extracellular phytase and acid phosphatase activities were produced by these mutants and the intact enzyme r-AppA. The N-glycosylation level was higher in mutants A131N/V134N/D207N/S211N (48%) and A131N/V134N/C200N/D207N/S211N (89%) than that in r-AppA (14%). Despite no enhancement of glycosylation, mutant C200N/D207N/S211N was different from r-AppA in the following properties. First, it was more active at pH 3.5–5.5. Second, it retained more (P < 0.01) phytase activity than that of r-AppA. Third, its specific activity of phytase was 54% higher. Lastly, its apparent catalytic efficiency kcat/Km for either p-nitrophenyl phosphate (5.8 × 105 vs 2.0 × 105 min−1 M−1) or sodium phytate (6.9 × 106 vs 1.1 × 106 min−1 M−1) was improved by factors of 1.9- and 5.3-fold, respectively. Based on the recently published E. coli phytase crystal structure, substitution of C200N in mutant C200N/D207N/S211N seems to eliminate the disulfide bond between the G helix and the GH loop in the α-domain of the protein. This change may modulate the domain flexibility and thereby the catalytic efficiency and thermostability of the enzyme.