We have investigated the effects of heme rotational isomerism in sperm-whale carbonmonoxymyoglobin using computational techniques. Several molecular dynamics simulations have been performed
for the two rotational isomers A and B, which are related by a 180° rotation around the α−γ axis of the
heme, of sperm-whale carbonmonoxy myoglobin in water. Both neutron diffraction and NMR structures
were used as starting structures. In the absence of an experimental structure, the structure of isomer B
was generated by rotating the heme in the structure of isomer A. Distortions of the heme from planarity
were characterized by normal coordinate structural decomposition and by the angle of twist of the pyrrole
rings from the heme plane. The heme distortions of the neutron diffraction structure were conserved in the
MD trajectories, but in the NMR-based trajectories, where the heme distortions are less well defined, they
differ from the original heme deformations. The protein matrix induced similar distortions on the hemes in
orientations A and B. Our results suggest that the binding site prefers a particular macrocycle conformation,
and a 180° rotation of the heme does not significantly alter the protein's preference for this conformation.
The intrinsic rotational strengths of the two Soret transitions, separated according to their polarization in
the heme plane, show strong correlations with the ruffling deformation and the average twist angle of the
pyrrole rings. The total rotational strength, which includes contributions from the chromophores in the protein,
shows a weaker correlation with heme distortions.
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