The influence of heme ruffling on spin densities in ferricytochromes c probed by heme core 13C NMR.

The heme in cytochromes c undergoes a conserved out-of-plane distortion known as ruffling. For cytochromes c from the bacteria Hydrogenobacter thermophilus and Pseudomonas aeruginosa , NMR and EPR spectra have been shown to be sensitive to the extent of heme ruffling and to provide insights into the effect of ruffling on the electronic structure. Through the use of mutants of each of these cytochromes that differ in the amount of heme ruffling, NMR characterization of the low-spin (S = ½) ferric proteins has confirmed and refined the developing understanding of how ruffling influences the spin distribution on heme. The chemical shifts of the core heme carbons were obtained through site-specific labeling of the heme via biosynthetic incorporation of (13)C-labeled 5-aminolevulinic acid derivatives. Analysis of the contact shifts of these core heme carbons allowed Fermi contact spin densities to be estimated and changes upon ruffling to be evaluated. The results allow a deconvolution of the contributions to heme hyperfine shifts and a test of the influence of heme ruffling on the electronic structure and hyperfine shifts. The data indicate that as heme ruffling increases, the spin densities on the β-pyrrole carbons decrease while the spin densities on the α-pyrrole carbons and meso carbons increase. Furthermore, increased ruffling is associated with stronger bonding to the heme axial His ligand.