Modulation of ligand-field parameters by heme ruffling in cytochromes c revealed by EPR spectroscopy

Electron paramagnetic resonance (EPR) spectra of variants of Hydrogenobacter thermophilus cytochrome c(552) (Ht c-552) and Pseudomonas aeruginosa cytochrome c(551) (Pa c-551) are analyzed to determine the effect of heme ruffling on ligand-field parameters. Mutations introduced at positions 13 and 22 in Ht c-552 were previously demonstrated to influence hydrogen bonding in the proximal heme pocket and to tune reduction potential (E(m)) over a range of 80 mV [Michel, L. V.; Ye, T.; Bowman, S. E. J.; Levin, B. D.; Hahn, M. A.; Russell, B. S.; Elliott, S. J.; Bren, K. L. Biochemistry 2007, 46, 11753-11760]. These mutations are shown here to also increase heme ruffling as E(m) decreases. The primary effect on electronic structure of increasing heme ruffling is found to be a decrease in the axial ligand-field term Δ/λ, which is proposed to arise from an increase in the energy of the d(xy) orbital. Mutations at position 7, previously demonstrated to influence heme ruffling in Pa c-551 and Ht c-552, are utilized to test this correlation between molecular and electronic structure. In conclusion, the structure of the proximal heme pocket of cytochromes c is shown to play a role in determining heme conformation and electronic structure.