Catalytic characterization of phytase (myo-inositolhexakisphosphate phosphohydrolase) from Aspergillus niger van Teighem: Glycosylation pattern, kinetics and molecular properties

A phytase with a high specific activity from Aspergillus niger van Teighem was purified to near homogeneity and characterized in terms of different catalytic properties. The N-terminal sequence of purified phytase was determined to be FYYGAALPQS. The purified phytase was a glycosylated protein as judged by positive PAS staining with pI of 3.8 as estimated by two-dimensional gel electrophoresis. The kinetic studies revealed that the enzyme was competitively inhibited by myo-inositolhexasulphate (MIHS), a structural analogue of phytic acid, with apparent Ki of 0.05 mM. The Km of the phytase increased from 0.625 to 1.898 mM in the presence MIHS with 50% inhibition of phytase activity at 100 μM MIHS. The enzyme was significantly inhibited by inorganic phosphorus in uncompetitive manner (Ki = 0.16 mM) with 50% inhibition of phytase activity at 0.2 mM Pi. This phytase protein was approx. three-fold more sensitive to MIHS inhibition than inorganic phosphorus.