Solution 1H NMR investigation of the seating and rotational 'hopping' of centrosymmetric etioheme-I in myoglobin: effect of globin origin and its oxidation/spin state on heme dynamics

Solution 1H NMR spectroscopy was used to investigate the heme active-site structure and dynamics of rotation about the Fe-His bond of centrosymmetric etioheme-I reconstituted into sperm whale and horse myoglobin (Mb). Comparison of the NOESY cross-peak pattern and paramagnetic relaxation properties of the cyanomet complexes confirm a heme pocket that is essentially the same as Mb with either native protoheme or etioheme-I. Dipolar contacts between etioheme and the conserved heme pocket residues establish a unique seating of etioheme that conserves the orientation of the N-Fe-N vector relative to the axial His plane, with ethyl groups occupying the vinyl positions of protoheme. Saturation transfer between methyls on adjacent pyrroles in etioheme-reconstituted horse Mb in all accessible oxidation/spin states reveals rotational hopping rates that decrease dramatically with either loss of ligands or reduction of the heme, and correlate qualitatively with expectations based on the Fe-His bond strength and the rate of heme dissociation from Mb. The rate of hopping for etioheme in metMbCN, in contrast to hemes with propionates, is the same in the sperm whale and horse proteins.