Evidence for a Proximal Histidine Interaction in the Structure of Cytochromes c' in Solution: A Resonance Raman Study
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- 作者：Samya Othman ; Pierre Richaud ; André ; Vermé ; glio ; and Alain Desbois
- 刊名：Biochemistry
- 出版年：1996
- 出版时间：July 16, 1996
- 年：1996
- 卷：35
- 期：28
- 页码：9224 - 9234
- 全文大小：668K
- 年卷期：v.35,no.28(July 16, 1996)
- ISSN：1520-4995
文摘
Soret-excited resonance Raman (RR) spectra of oxidizedand reduced cytochromes c' fromRhodospirillum molischianum and Rhodobactersphaeroides, in solution, are reported. The spectraofthe type I ferricytochromes c' in both species containdifferent extents of two forms. One of these isreadily assignable to a "normal" five-coordinated high-spin heme.The second species with v3 andv10modes at 1502 and 1635 cm-1, respectively, is attributedto a five-coordinated intermediate-spin heme.The RR data show that the equilibrium between these two forms isspecies-dependent at neutral pH and20 
C. The v(Fe-His) mode of the a form of reducedcytochromes c' is assigned to a band at228-231cm-1, indicating that the proximal His has a strongelectronegative character. X-ray crystallographicdata on R. molischianum ferricyt c'show that the proximal His has no interaction with either theproteinor 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 Hbonding at the coordinated histidine corresponds toa low frequency for the v(Fe-His) mode (195-205cm-1), the structure and/or environment of theproximalhistidine appears different for cyt c'(III) in the crystaland cyt c'(II) in aqueous solution. To accountforthe elevated frequency of the v(Fe-His) mode of cytc'(II), several possibilities have beenexamined.Among these, we propose that a conserved Lys residue, located inthe protein sequence three residuesbefore the His ligand, can form an electrostatic interaction with the(His)N1 atom, directly or through awater molecule. It is further suggested that this electrostaticinteraction could also play a role in thehigh-spin 
intermediate-spin equilibrium of oxidized cytochromesc'.
C. The v(Fe-His) mode of the a form of reducedcytochromes c' is assigned to a band at228-231cm-1, indicating that the proximal His has a strongelectronegative character. X-ray crystallographicdata on R. molischianum ferricyt c'show that the proximal His has no interaction with either theproteinor 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 Hbonding at the coordinated histidine corresponds toa low frequency for the v(Fe-His) mode (195-205cm-1), the structure and/or environment of theproximalhistidine appears different for cyt c'(III) in the crystaland cyt c'(II) in aqueous solution. To accountforthe elevated frequency of the v(Fe-His) mode of cytc'(II), several possibilities have beenexamined.Among these, we propose that a conserved Lys residue, located inthe protein sequence three residuesbefore the His ligand, can form an electrostatic interaction with the(His)N1 atom, directly or through awater molecule. It is further suggested that this electrostaticinteraction could also play a role in thehigh-spin intermediate-spin equilibrium of oxidized cytochromesc'.