Thermostable alkaline phytase from Bacillus sp. MD2: effect of divalent metals on activity and stability

Phytate, the major source of phosphorus in seeds, exists as a complex with different metal ions. Alkaline phytases are known to dephosphorylate phytate complexed with calcium ions in contrast to acid phytases that act only on phytic acid. A recombinant alkaline phytase from Bacillus sp. MD2 has been purified and characterized with respect to the effect of divalent metal ions on the enzyme activity and stability. The presence of Ca²⁺ on both the enzyme and the substrate is required for optimal activity and stability. Replacing Ca²⁺ with Ba²⁺, Mn²⁺, Mg²⁺ and Sr²⁺ in the phytase resulted in the expression of > 90% of the maximal activity with calcium-phytate as the substrate, while Fe²⁺ and Zn²⁺ rendered the enzyme inactive. On the other hand, the calcium loaded phytase showed significant activity (60%) with sodium phytate and lower activity (17-20%) with phytate complexed with only Mg²⁺, Sn²⁺ and Sr²⁺, respectively. On replacing Ca²⁺ on both the enzyme and the substrate with other metal ions, about 20% of the maximal phytase activity was obtained only with Mg²⁺ and Sr²⁺, respectively. Only Ca²⁺ resulted in a marked increase in the melting temperature (T_m) of the enzyme by 12-21 °C, while Ba²⁺, Mn²⁺, Sr²⁺ or Cu²⁺ resulted in a modest (2-3.5 °C) increase in T_m. In the presence of 1-5 mM Ca²⁺, the optimum temperature of the phytase activity was increased from 40 °C to 70 °C, while optimum pH of the enzyme shifted by 0.4-1 pH unit towards the acidic region.