113Cd NMR as a Probe of the Active Sites of Metalloenzymes

113Cd NMR has been used to study the active site metal ion(s) of the 113Cd(II) derivatives of four Zn(II) metalloenzymes, carboxypeptidase A, carbonic anhydrases, alkaline phosphatase, and superoxide dismutase. The resonances of the enzyme-bound 113Cd(II) ions are extremely sensitive to ligand exchange, including solvent and inhibitor, and to changes in the metal ion coordination sphere. The nature of this behavior can be shown to parallel the known structural properties and proposed roles of the metal ion in the catalytic mechanisms. Models accounting for the exchange mechanisms which may be modulating the chemical shift, linewidth, and coupling are discussed.