Heme methyl hyperfine shift pattern as a probe for determining the orientation of the functionally relevant proximal histidyl imidazole with respect to the heme in hemoproteins

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Abstract
Heme methyl 1H and 13C resonances of met-cyano form of myoglobin from the shark, Geleorhinus japonicus (GJMbCN), have been assigned via 1H-13C heteronuclear shift correlated spectroscopy (COSY) connectivities and their hyperfine shifts were compared with those of the corresponding resonances of some hemoproteins. Variation of the heme methyl 1H hyperfine shift pattern correlates well with the angle (Φ) between the projection of the proximal histidyl imidazole plane onto the heme plane and the NII-Fe-Niv vector. The alteration of the interaction of the heme peripheral side-chains and/or the iron-bound ligand with the surrounding amino acid residues cannot account for large differences in the shifts of the corresponding heme methyl resonances between GJMbCN and sperm whale MbCN. Since the heme methyl 1H shifts for GJMbCN fall in between those of the corresponding resonances for sperm whale Mb and Aplysia limacma Mb in which the Φ values have been reported to be 19° and 29°, respectively, the Φ value in GJMb is estimated to be slightly larger than 19°.