On the Interpretation of Solvent Proton Magnetic Relaxation Data with Particular Application to the Structure of the Active Site of Mn-Carboxypeptidase A

Mn++ can be substituted for the single Zn++ ion per molecule of carboxypeptidase A with the retention of some enzymatic activity (Coleman and Vallee, 1960). Studies at one value of magnetic field of the effect of Mn++-substituted carboxypeptidase A (CPA(Mn)) on T−11 and T−12, the longitudinal and transverse spin relaxation rates respectively of solvent water protons, have shown that there is at least one water ligand on the Mn++ (Shulman et al., 1966), that F− can also bind without influencing the water interaction (Navon et al., 1970), and that the inhibitor β-phenylpropionate markedly reduces the water interaction (Navon et al., 1968) and affects the apparent F− interaction (Navon et al., 1970). These results were difficult to quantitate because of a lack of knowledge of the correlation time that characterizes the interaction between the relaxing nuclei and the electronic spin moment of the Mn++ ion.