Structural study of the heme crevice in cytochrome b5 based on individual assignments of the 1H-NMR lines of the heme group and selected amino acid residues.

Measurements of saturation transfer, spin-decoupling and truncated-driven nuclear Overhauser effect difference spectra were applied to indidually assign the 1H-NMR lines of the heme group and nearby amino acid residues in reduced and oxidized cytochrome b5. These data imply that the orientation of the heme group in the major cytochrome b5 conformation in solution differs from that reported for the X-ray crystal structure by a 180 degree rotation about an axis through the meso-carbon atoms alpha and gamma. Otherwise comparison of the experimental chemical shifts with those obtained from ring current calculations using the refined X-ray atomic coordinates provide no evidence that the polypeptide conformation near the heme is different in the crystals and in solution. It seems quite likely that the previously described second solution conformation of cytochrome b5 is related to the major species through a different orientation of the heme.