Solution H-1 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 H-1 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). Compar ison of the NOESY cross-peak pattern and paramagnetic relaxation properties of the cyanomet complexes confirm a heme pocket that is essentially the sa me as Mb with either native protoheme or etioheme-I. Dipolar contacts betwe en etioheme and the conserved heme pocket residues establish a unique seati ng 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 eti oheme-reconstituted horse Mb in all accessible oxidation/spin states reveal s rotational hopping rates that decrease dramatically with either loss of l igands or reduction of the heme. and correlate qualitatively with expectati ons based on the Fe-His bond strength and the rate of heme dissociation fro m Mb. The rate of hopping for etioheme in metMbCN, in contrast to hemes wit h propionates, is the same in the sperm whale and horse proteins.