Conserved nonplanar heme distortions in cytochromes c

A nonplanar distortion of the heme of c-type cytochromes is conserved in the proteins isolated from diverse species based upon a comprehensive analysis of available high-resolution X-ray crystal structures. This distortion is induced through the cysteine thioether linkages between the porphyrin pyrrole groups and the polypeptide and results in an asymmetric pyrrole distortion. This asymmetry in the heme distortion is also conserved. For other heme proteins which lack these covalent bonds, nearly planar porphyrins are observed. Resonance Raman evidence indicates that nonplanar distortion of porphyrins containing metals, like iron, with large core sizes (> or = 2.00 A) is energetically unfavorable and can occur only in the presence of significant environmental perturbations. Further, energy minimization and dynamics calculations on the ferric form of yeast iso-1-cytochrome c, starting from the crystallographic coordinates and using a molecular mechanics force field which accurately reproduces nonplanar distortions in metalloporphyrins, suggest that this distortion is indeed maintained by the protein tertiary structure. It is proposed that this protein-linked heme distortion modulates electron transfer function through modification of redox potentials of the porphyrin ring and the protein binding properties of c-type cytochromes.