Resonance Raman investigation of lysine and N-acetylmethionine complexes of ferric and ferrous microperoxidase - Influences of the axial ligation on the heme c structure

In order to evaluate the steric and electronic effects of mixed axial ligat ions on the heme c structure, lysine (Lys) and N-acetylmethionine (AcMet) c omplexes of ferric and ferrous microperoxidase-8 (MP8(III) and MP8(II), res pectively) are characterized by absorption and resonance Raman (RR) spectro scopies, Spectrophotometric titrations establish that MP8(III) binds one mo lecule of exogenous ligand while MP8(II) forms mono(ligated) and bis(ligate d) compounds. The Soret-excited RR spectra of the six-coordinated low-spin MP8(III) complexes show that the macrocycle can adopt different structures between planar and ruffled conformations. The ferriheme c conformation is p rimarily determined by the ionization state of the His side chain of MP8(II I) and, secondarily, by the bonding and nonbonding heme-ligand interactions . As far as the RR spectra of the MP8(II) complexes are concerned, they per mit us to conclude that the mixed His/Lys and His/AcMet coordinations induc e a nonplanar heme conformation, the extent of deformation again depending on the ionization state of the endogenous His ligand. In contrast, the RR s pectra of the bis(lys) and bis(AcMet) compounds are associated with a plana r heme structure. When the His of MP8 is bound to heme c, the stabilization of distorted heme conformations is thus associated with constraints exerte d by the Cys-Ala-Gln-Cys-His-peptide on the porphyrin macrocycle. More gene rally, the spectroscopic data obtained in this study can be used to predict both the axial coordination and the structure of heme in c-type cytochrome s.