Evidence for a proximal histidine interaction in the structure of cytochromes c in solution: a resonance Raman study.
Soret-excited resonance Raman (RR) spectra of oxidized and reduced cytochromes ć from Rhodospirillum molischianum and Rhodobacter sphaeroides, in solution, are reported. The spectra of the type I ferricytochromes ć in both species contain different extents of two forms. One of these is readily assignable to a "normal" five-coordinated high-spin heme. The second species with v3 and v10 modes at 1502 and 1635 cm-1, respectively, is attributed to a five-coordinated intermediate-spin heme. The RR data show that the equilibrium between these two forms is species-dependent at neutral pH and 20 degrees C. The v(Fe-His) mode of the a form of reduced cytochromes ć is assigned to a band at 228-231 cm-1, indicating that the proximal His has a strong electronegative character. X-ray crystallographic data on R. molischianum ferricyt ć show that the proximal His has no interaction with either the protein or water molecules [Finzel, B.C., Weber, P.C., Hardman, K.D., & Salemme, F.R.(1985) J. Mol. Biol. 186, 627-643]. Considering that the absence of H bonding at the coordinated histidine corresponds to a low frequency for the v(Fe-His) mode (195-205 cm-1), the structure and/or environment of the proximal histidine appears different for cyt ć (III) in the crystal and cyt ć (II) in aqueous solution. To account for the elevated frequency of the v(Fe-His) mode of cyt ć (II), several possibilities have been examined. Among these, we propose that a conserved Lys residue, located in the protein sequence three residues before the His ligand, can form an electrostatic interaction with the (His)N1 atom, directly or through a water molecule. It is further suggested that this electrostatic interaction could also play a role in the high-spin <--> intermediate-spin equilibrium of oxidized cytochromes ć.
Othman
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Département de Biologie Cellulaire et Moléculaire, CEA et CNRS-URA 1290, Gij-sur-Yvette, France.
Richaud
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P
Verméglio
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Desbois
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A
D000595:Amino Acid Sequence; D001665:Binding Sites; D003574:Cytochrome c Group / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D006639:Histidine / Q000737:chemistry*; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D012242:Rhodobacter sphaeroides / Q000737:chemistry*; D012246:Rhodospirillum / Q000737:chemistry*; D013059:Spectrum Analysis, Raman
D003574:Cytochrome c Group; D006418:Heme; D006639:Histidine