LIGAND BINDING TO HEME PROTEINS: AN EVALUATION OF DISTAL EFFECTS
MIMS, MARTHA PRITCHETT
Doctor of Philosophy
Steric interactions between ligand molecules and the valine E11 methyl group of human hemoglobin and sperm whale myoglobin have been examined by high resolution NMR. The methyl proton resonances of this amino acid are shifted markedly upfield by the heme ring current. In all the proteins the position of the E11 valine resonance showed little change in going from bound CO to bound methyl and ethyl isocyanide. In alpha chains and myoglobin, the binding of n-propyl and n-butyl isocyanide produced marked downfield shifts of the valine resonance, indicating that the valine residue was forced away from the center of the heme ring. In beta subunits only tert-butyl isocyanide produced a marked decrease in the ring current shift of the valine methyl protons. New peaks observed in the isonitrile protein spectra were identified as ligand proton resonances by comparing the spectra of normal and deuterated isonitrile complexes. The magnitudes of the ring current shifts for the terminal methyl protons of ethyl isocyanide suggest a linear geometry for the Fe = C = N - Cl bonds in beta chains and a bent geometry for alpha chains. Myoglobin ethyl isocyanide complexes exhibit ligand ring current shifts intermediate to those observed for the hemoglobin subunits. Kinetic and equilibrium constants for the reactions of CO and the isonitriles with a large group of proteins and a model heme compound have been determined. Results were analyzed in terms of a 3 barrier model for ligand binding. The observed rate and equilibrium constants for the protein reactions showed a complex dependence on each individual step in the binding process. Pentacoordinate heme dissolved in soap served as a simple model for the proteins. When the protein equilibrium data are converted into free energy terms and the heme-soap results subtracted, the resulting value, (DELTA)G(,prot), describes the distal protein steric barrier. The results indicate that CO binds easily to all of the heme proteins while methyl isocyanide shows a large increase in (DELTA)G(,prot). This increase in steric hindrance has been attributed to the distal histidine residue. In most of the proteins, bound ethyl isocyanide exhibits about the same steric barrier as methyl isocyanide and the longer isonitriles show increasingly larger steric hindrance.