Characterization and overproduction of the Escherichia coli appA encoded bifunctional enzyme that exhibits both phytase and acid phosphatase activities

The appA gene that was previously shown to code for an acid phosphatase instead codes for a bifunctional enzyme exhibiting both acid phosphatase and phytase activities. The purified enzyme with a molecular mass of 44 708 Da was further separated by chromatofocusing into two isoforms of identical size with isoelectric points of 6.5 and 6.3. The isoforms had identical pH optima of 4.5 and were stable at pH values from 2 to 10. The temperature optimum for both phytase isoforms was 60 C. When heated at different pH values the enzyme showed the greatest thermal resistance at pH 3. The pH 6.5 isoform exhibited K-m and V-max values of 0.79 mM and 3165 U.mg(-1) of protein for phytase activity and 5.5 mM and 712 U.mg(-1) of protein for acid phosphatase, respectively. The pH 6.3 isoform exhibited slightly lower K-m and V-max values. The enzyme exhibited similar properties to the phytase purified by Greiner et al. (1993), except the specific activity of the enzyme was at least 3.5-fold less than that previously reported, and the N-terminal amino acid sequence was different. The Bradford assay, which was used by Greiner et al. (1993) for determination of enzyme concentration was, in our hands, underestimating protein concentration by a factor of 14. Phytase production using the T7 polymerase expression system was enhanced by selection of a mutant able to grow in a chemically defined medium with lactose as the carbon source and inducer. Using this strain in fed-batch fermentation, phytase production was increased to over 600 U.mL(-1). The properties of the phytase including the low pH optimum, protease resistance, and high activity, demonstrates that the enzyme is a good candidate for industrial production as a feed enzyme.