Calculated structure of the nickel-reconstituted heme pentapeptide model.

• The nickel derivative of MP-11 has unbound fingerprint histidine residue, rendering one of the H-bonds in the polypeptide backbone ineffective in causing the ruffling of the porphyrin macrocycle.  Thus, the macrocycle is not all nonplanar as indicated by the resonance Raman spectra of the structure-sensitive lines.

• The porphyrin of NiMP-11 is more nonplanar when deep within the CTAB micelles.  

• More of the peptide is in the interior of the micelle at pH extremes where either the C- or N-terminal ends are uncharged.

• The fraction of the nonplanar form of NiMP-11 in CTAB solution is much greater at pH 1.0 and 13.0 than at the other investigated pH values.

• The hydrophobic micellar environment of the polypeptide segment is required to shift the equilibrium strongly in favor of the nonplanar form.

• Heterogeneity in the out-of-plane distortion also shows up in the shape of the macrocycle-breathing mode n₈. 

• n₈ of NiMP-11 is broad and asymmetric and can be resolved to two sublines, whereas NiMesoP exhibits a narrow line at nearly the same place as the low-frequency component of n₈ of NiMP-11, suggesting that NiMP-11 has more of the nonplanar form.

The major conclusion is that heme pentapeptide interacts with and induces the nonplanarity of the porphyrin, but only when the polypeptide itself is in a hydrophobic environment--either the interior of a micelle or within the rest of the cytochrome c protein.  This provides a means by which protein-protein binding can influence the structure of the heme and its functional properties such as redox potential.   In addition, the Fe(III)/Fe(II) reduction can influence the conformation of the pentapeptide and influence the interaction with a protein bound at the surface of cytochrome c near the pentapeptide. 

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