β-Propeller Phytase Hydrolyzes Insoluble Ca2+-Phytate Salts and Completely Abrogates the Ability of Phytate To Chelate Metal Ions

Phytate is an antinutritional factor that influences the bioavailability of essential minerals by forming complexes with them and converting them into insoluble salts. To further our understanding of the chemistry of phytate's binding interactions with biologically important metal cations, we determined the stoichiometry, affinity, and thermodynamics of these interactions by isothermal titration calorimetry. The results suggest that phytate has multiple Ca2+-binding sites and forms insoluble tricalcium- or tetracalciumphytate salts over a wide pH range (pH 3.0-9.0). We overexpressed the beta-propeller phytase from Hahella chejuensis (HcBPP) that hydrolyzes insoluble Ca2+-phytate salts. Structure-based sequence alignments indicated that the active site of HcBPP may contain multiple calcium-binding sites that provide a favorable electrostatic environment for the binding of Ca2+-phytate salts. Biochemical and kinetic studies further confirmed that HcBPP preferentially recognizes its substrate and selectively hydrolyzes insoluble Ca2+-phytate salts at three phosphate group sites, yielding the final product, myo-inositol 2,4,6-trisphosphate. More importantly, ITC analysis of this final product with several cations revealed that HcBPP efficiently eliminates the ability of phytate to chelate several divalent cations strongly and thereby provides free minerals and phosphate ions as nutrients for the growth of bacteria. Collectively, our results provide significant new insights into the potential application of HcBPP in enhancing the bioavailability and absorption of divalent cations.