Kinetic Characterization of O-Phospho-L-tyrosine Phosphohydrolase Activity of Two Fungal Phytases

摘要

Fungal phytases belonging to "histidine acid phosphatase" or HAP class of phosphohydrolases that catalyze the hydrolysis of phytic acid could also hydrolyze O-phospho-L-tyrosine, which' is also called phosphotyrosine. Two phytases from Aspergillus niger and Aspergillus awamori with pH optima 2.5 were tested for phosphotyrosine hydrolase activity; both enzymes cleaved the phosphomonoester bond of phosphotyrosine efficiently at acidic pH. The K_m for phosphotyrosine ranged from 465 to 590 μM as opposed to 135 to 160 μM for phytate. The V_(max), however, is 2-4 times higher for phosphotyrosine than it is for phytate. The catalytic efficiency of phytase for phosphotyrosine is on the same order as it is for phytate (3.5 x 10~6 to 1.6 x 10~7 M~(-1) s~(-1)); the pH versus activity profile for phosphotyrosine is, however, different from what it is for phytate. The temperature optima shifted 5 °C higher to 70 °C when phosphotyrosine was used as the substrate. Taken together, the kinetic data show that fungal HAPs that are known as PhyB are capable of cleaving the phosphomonoester bond in phosphotyrosine. This is the first time that phosphotyrosine phosphatase (PTPase) activity has been reported for the subgroup of HAP known as phytase.