Alkaline Conformational Transitions of FerricytochromecStudied by Resonance Raman Spectroscopy

Abstract

The pH-dependent conformational equilibria of iso-1-ferricytochrome c that occur between pH 7 and pH 12 have been studied by resonance Raman (RR) spectroscopy. Detailed analysis of the RR spectra provides the number and the spectra of the conformational species that occur over this range of pH as well as their relative concentrations at each pH. Between pH 7 and pH 12, the wild-type protein exhibits five conformers which differ with respect to the axial ligands of the heme iron. In the medium alkaline pH range, the neutral form undergoes two parallel conformational transitions with similar pKas (8.7 and 8.9) to those of the states IVa and IVb. Comparative studies with protein variants in which either Lys79 or Lys73 or both lysyl residues were replaced by alanine(s) confirm previous conclusions (Rosell et al., preceding paper in this issue) that Lys73 and Lys79 are the axial ligands replacing Met80 in the states IVa and IVb, respectively. On the basis of isotopic shifts caused by H/D exchange, the Fe−N(Lys) stretching vibrations of these species were identified at 385 cm-1. In the high alkaline pH range, the wild-type protein forms two further conformers with pKas of 10.5 and 11. In these states, Va and Vb, which are also detected in the Lys73Ala, Lys79Ala, and the Lys73Ala/Lys79Ala variant proteins, another strong-field ligand, presumably a hydroxide, occupies the sixth axial coordination site. Comparison of the RR spectra demonstrates far-reaching similarities between state Va and the conformational state B2 that cytochrome c forms in electrostatically stabilized complexes at neutral pH where, inter alia, Lys73 and Lys79 are involved in intermolecular interactions. This finding as well as the dramatically lowered pKas (<10) for formation of Va and Vb in the double variant (Lys73Ala/Lys79Ala) suggest that the residues 73 and 79 are part of a switching mechanism by which conformational changes at the heme pocket are induced to regulate the function of the protein in biological electron-transfer reactions.