Figure 4.

Heme ruffling is a consequence of steric clash between the heme C(γ) and the helix i-4 side chain. (A) Space-filling model of a 17-residue idealized alanine helix with a central, flat porphyrin-bound histidine fixed at the mean χ₁ and χ₂ torsion angles for the m166 rotamer. The alanine 4 residues N-terminal from the histidine, which clashes with the porphyrin C(γ) atom, is shaded in dark grey. (B) Expansion of the Van der Walls energy surface near the m166 rotamer. The χ₁ and χ₂ angles of each heme-bound histidine in the database are overlaid in white as a function of the connectivity of the heme: ((x025CB)) b-type, noncovalently bound hemes, ((x025BD)) c-type, covalently bound hemes. (C) Van der Waals energy surface calculated using the ruffled porphyrin derived from the 0.91 Ǻ resolution structure of the D. vulgaris cytochrome C (1J0P) connected as above to the idealized 17-residue alanine helix used in part B. Histidine rotamers from the database are overlaid in white as in part B.