Effect of magnesium on the properties of zinc alkaline phosphatase

Alkaline phosphatase of Escherichia coli, isolated by procedures which do not alter its intrinsic metal content, contains 4.0 +/- 0.3 g-atoms of tightly bound zinc per mole (Kd less than 1 muM) and 1.3 +/- 0.2 g-atoms of magnesium per mole (Bosron, W.F., Kennedy, F.S., and Vallee, B.L. (1975), Biochemistry 14, 2275-2282). Importantly, the binding of magnesium is dependent both upon pH and zinc content. Hence, the failure to assign the maximal magnesium stoichiometry to enzyme isolated by conventional procedures may be considered a consequence of the conditions chosen for optimal bacterial growth and purification of the enzyme which are not the conditions for optimal binding of magnesium to alkaline phosphatase. Under the conditions employed for the present experimental studies, a maximum of six metal sites are available to bind zinc and magnesium, i.e., four for zinc and two for magnesium. Magnesium alone does not activate the apoenzyme, but it regulates the nature of the zinc-dependent restoration of catalytic activity to apophosphatase, increasing the activity of enzyme containing 2-g-atoms of zinc five-fold and that of enzyme containing 4-g-atoms of zinc 1.4-fold. Moreover, hydrogen-tritium exchange reveals the stabilizing effects of magnesium on the structural properties of phosphatase. However, neither the KM for substrate nor the phosphate binding stoichiometry and Ki are significantly altered by magnesium. Hence, magnesium, which is specificially bound to the enzyme, both stabilizes the dynamic protein structure and regulates the expression of catalytic activity by zinc in alkaline phosphatase.