Characterization of the Properties of the Multiple Metal Binding Sites in Alkaline Phosphatase by Carbon-13 Nuclear Magnetic Resonance

Carbon-13 nuclear magnetic resonance (¹³C NMR) of Escherichia coli alkaline phosphatase labeled biosynthetically with β, β-[γ-¹³C] dideuteriohistidine has been used to determine the number and identity of the histidine residues that participate in metal ion coordination at the three classes of binding sites in this dimeric Zn²⁺metalloenzyme. Detailed¹³C NMR titrations of the apoenzyme with¹¹³Cd²⁺and Mg²⁺, in conjunction with parallel¹¹³Cd NMR measurements [Otvos, J. D., & Armitage, I. M. (1980) Biochemistry (third of three papers in this issue)], permitted the assignment of four histidine residues as ligands to the “catalytic”, or A site, metal ions, two coordinated via their N^Timidazole nitrogens and two via N^T. in addition, a fifth histidyl ligand, coordinated through N^T, was shown to be located at the “structural”, or B, sites on the dimer. The “regulatory”, or C, sites do not contain histidyl metal ligands Unambiguous identification of the three histidines coordinated to metal ion via N^Twas provided by the observation of resolved¹¹³Cd-¹³C spin-spin coupling (³J = 12-19 Hz) in their γ-carbon resonances. Once assigned, the¹³C resonances of the five histidyl metal ligands were used to monitor the relative affinities of the A, B, and C sites for Cd²⁺and Zn²⁺. At pH 6.3, Cd²⁺was found to bind to the A sites at least 10 times tighter than to the B or C sites, which have roughly equal affinities. in marked contrast, Zn²⁺was found to have similar affinities for the A and B sites at both pH 6.3 and 8.0. The affinity of the C sites for Zn²⁺and Mg²⁺was shown to be at least an order of magnitude lower. The binding constants of all three sites for Cd²⁺and Zn²⁺are greater than 10⁵M⁻¹. Evidence is also presented that suggests that the A, B, and C sites may be located in close proximity to one another in the monomers.