Influence of propionate side chains on the equilibrium heme orientation in sperm whale myoglobin. Heme resonance assignments and structure determination by nuclear Overhauser effect measurements

Sperm whale myoglobin was reconstituted with hemins methylated at the 2-, 4-, and 6- or 7-positions, and the corresponding metcyano complexes were studied by lH NMR spectroscopy. Nuclear Overhauser effects (NOEs) were observed between heme methyls attached to the same pyrrole ring and between heme methyls adjacent to a common meso position. The relative magnitudes of these effects could be relied on to identify heme methyl resonances and are proposed to be used as a simple method to reach spectral assignments. Along with selected and characteristic dipolar contacts with clearly identified peripheral protein side chains, the inter-methyl NOE allows direct determination of the heme orientation in the holoprotein. Thus, it was found that the replacement of either propionate side chain with a methyl does not affect the nature of the thermodynamically preferred isomer and does not perturb the equilibrium proportion of the two species.