Determination by cadmium-113 nuclear magnetic resonance of the structural basis for metal ion dependent anticooperativity in alkaline phosphatase.

Cadmium-113 nuclear magnetic resonance (113Cd NMR) has been used to probe the binding characteristics of 113Cd2+ to the three classes of metal binding sites in Escherichia coli alkaline phosphatase to help elucidate the molecular origin of the metal ion dependent "half-sites" reactivity exhibited by this dimeric Zn2+ metalloenzyme [Otvos, J.D., Armitage, I.M., Chlebowski, J.F., & Coleman, J.E. (1979) J. Biol. Chem. 254, 4707-4713]. In the absence of phosphate, the first two 113Cd2+ ions added to the apodimer give rise to a single 113Cd resonance (169 ppm), indicating selective binding to the pair of symmetrically disposed A sites. Resonances arising from additional 113Cd2+ bound to the B and C sites cannot be observed; B- and/or C-site occupation also renders the A-site 113Cd resonance undetectable. Both these observations have been attributed to severe chemical exchange broadening in the A-, B-, and C-site 113Cd signals induced by an unknown modulation process(es). Interestingly, covalent phosphorylation of the active-site serine residues abolishes this exchange modulation, allowing three separate resonances to be detected and assigned to 113Cd2+ located at each of the three classes of metal binding sites in the enzyme. By varying the metal composition of the phosphorylated enzyme, we have characterized the correlations that exist between the chemical shifts ana intensities of these 113Cd resonances and the metal occupancies of the A, B, and C sites in the individual subunits. This information has allowed us to conclude that the half-sites phosphorylation of the Cd2 2+ enzyme is accompanied by a slow migration of half the Cd2+ originally located at the A sites to the B sites on the phosphorylated subunits. The driving force for this metal redistribution, which at equilibrium leaves half the subnits devoid of metal ion and thereby incapable of binding phosphate, is apparently the dramatic stabilization of the complex of Cd2+ with the B sites, which was demonstrated to occur in those subunits that become phosphorylated. From the kinetics of both phosphorylation and metal redistribution in Cd2 2+ enzyme, we suggest that population of the A and B sites in a subunit, rather than the A site alone, constitutes the minimum requirement for induction of catalytic function in alkaline phosphatase. The spin relaxation properties of the enzyme-bound 113Cd2+ ions are also briefly discussed.