Resonance Raman investigation of nickel microperoxidase-11

Resonance Raman and UV-visible absorption spectra show that nickel(II) micr operoxidase-11 (NiMP-11) is four-coordinate in aqueous solution in the pH r ange from 1.0 to 13.0. In aqueous solutions of NiMP-11 in the absence of ce tyltrimethylammonium bromide (CTAB), NiMP-11 is aggregated. In CTAB micella r solutions, where aggregation of NiMP-11 does not occur, the Raman spectra of NiMP-11 are similar to that of nickel(II) cytochrome c (NiCyt-c). The p resence of the peptide segment shifts the equilibrium heavily in favor of t he nonplanar form, just as does the entire protein component in the case of NiCyt-c. This further elucidates the structural mechanism by which the pro tein segment ruffles the heme, most likely modulating the redox potential a s indicated for the cytochromes c(3) [Ma, J.-G., et al. (1998) Biochemistry 37, 12431-12442]. Furthermore, the hydrophobic environment that is provide d by the CTAB micelle is found to be crucial to the native folding of the p entapeptide and formation of two hydrogen bonds in the peptide backbone. Th ese two II-bonds act to contract the peptide segment exerting the force on the macrocycle that causes the ruffling and makes the redox potential more negative than if the heme were to remain planar. The structure of the heme and pentapeptide may also be associated with redox-linked triggering of the formation and release of cytochrome-protein complexes.