A MULTINUCLEAR LIGAND NMR INVESTIGATION OF CYANIDE, CYANATE, AND THIOCYANATE BINDING TO ZINC AND COBALT CARBONIC-ANHYDRASE

Relaxation rate measurements on C-13-enriched NCO-, NCS-, and CN- as well as on N-15-enriched NCS- have been performed in presence of cobalt(II)-substituted carbonic anhydrase (CA). It is shown on firm basis that NCO- and NCS- do bind cobalt although with different geometries. The data of the NCS- case are consistent with the available X-ray data. No information could be obtained on CN- because of slow exchange between its free and enzyme-bound forms. C-13 relaxation measurements have been performed on (CN-)C-13-N-15 in the presence of (ZnCA)-Zn-67. The large quadrupolar moment of Zn-67(II) induces relaxation on C-13, thus showing that there is direct coordination. NMR spectroscopy provides strong important pieces of information on the present debate on the way of binding of anions to carbonic anhydrase.