113Cd NMR of Cd2+-substituted carboxypeptidase. Support for a hexa-coordinate metal ion in the presence of inhibitors.

The liquid-state 113Cd NMR data of carboxypeptidase A in the presence and absence of inhibitors obtained by Gettins (Gettins, P. (1986) J. Biol. Chem. 261, 15513-15518) are analyzed in terms of whether the inhibitors displace water from Cd2+ upon binding to the protein. This question is addressed by applying the single crystal data and the methods introduced by Honkonen and Ellis (Honkonen, R. S., and Ellis, P. D. (1984) J. Am. Chem. Soc. 106, 5488-5497). Calculations based upon these data demonstrate that displacement of water by a carboxyl group should lead to significant shielding of a 113Cd resonance by approximately 100 ppm. Since the observed 113Cd chemical shifts for carboxypeptidase A are modest and deshielding (12-17 ppm), it is argued that the chemical shifts imply that water is not displaced from the Cd2+ center upon binding of inhibitors to carboxypeptidase A. Rather, the Cd2+ ion increases its coordination number from five to six upon binding of the inhibitor.