NITRIC-OXIDE AND CARBON-MONOXIDE EQUILIBRIA OF HORSE MYOGLOBIN AND (N-METHYLIMIDAZOLE PROTO-HEME - EVIDENCE FOR STERIC INTERACTION WITH THE DISTAL RESIDUES

The Soret absorption maxima and extinction coefficients of the CO and NO complexes of horse myoglobin and (NMelm)protoheme (NMelm=1-methylimidazole) have been determined. The partition coefficient N, equal to the ratio P1/2(CO)/P1/2(NO), has been determined spectrophotometrically for horse myoglobin and (NMelm)protoheme. p,1/2-(NO) values calculated from the partition coefficients are 5.7ËŸ10-7 mmHg for (NMelm)protoheme and 1.1ËŸ10 -6 mmHg for horse myoglobin. The ratio of P1/2(NO) values for protein and model is 1.9 which is similar to a value of 1.6 reported for the ratio of P1/2(O2) values. These values may be compared to a ratio of 15 for CO binding to protein and model complexes. This different ratio for CO provides further evidence for steric interaction of the bound CO with the protein based on a consideration of the preferred nonlinear geometry of Fe-NO and Fe-O2 and the linear geometry of Fe-CO. © 1979, American Chemical Society. All rights reserved.