Redox control of proton transfers in membrane b-type cytochromes: an absorption and resonance Raman study on bis(imidazole) and bis(imidazolate) model complexes of iron-protoporphyrin |
| |
Authors: | A Desbois M Lutz |
| |
Institution: | (1) Laboratoire de Biophysique, Institut de Biologie Physico-Chimique, 13 rue Pierre et Marie Curie, F-75005 Paris, France;(2) Section de Biophysique des Protéines et des Membranes, Département de Biologic Cellulaire et Moléculaire, Unité de Recherche Associée au C.N.R.S. 1290, Centre d'Etudes de Saclay, F-91191 Gif-sur-Yvette Cedex, France;(3) Present address: Section de Biophysique des Protéines et des Membranes, Département de Biologie Cellulaire et Moléculaire, Unité de Recherche Associée an C.N.R.S. 1290, Centre d'Etudes de Saclay, F-91191 Gif-sur-Yvette Cedex, France |
| |
Abstract: | Optical absorption spectra and resonance Raman (RR) spectra, obtained with Soret excitation, are reported for bis(imidazole) and bis(imidazolate) complexes of iron(II)- and iron(III)-protoporphyrin IX, prepared in aqueous conditions. Perdeuteration experiments on the axial ligands permitted the assignment of the symmetric Fe-(ligand)2 stretching mode of Fex]PP(L)2 to RR bands at 203 (x = II; L = ImH), 212 (x = II; L = Im–), 201 (x = III; L = ImH) and 226 cm–1 (x = III; L = Im–). These frequency differences indicate a strengthening of the axial bonds when the imidazole deprotonations occur. The larger difference observed for the ferric derivatives reflects the stronger -donor capability of the Im– anion for iron(III) over iron(II). For the ferrous derivatives, the frequencies of several skeletal porphyrin modes ( 4, 10, 11 and 38) are downshifted by 2–10 cm–1 upon deprotonation of the ligands. This effect corresponds to an increased back-bonding from the metal atom to the porphyrin ring when the axial ligand decreases its -acid strength. Bringing further support to this interpretation, an inverse linear relationship is established between the frequencies of (Fe(Il)-L2) and 11. This correlation is expected to monitor the overall H-bonding state of histidine ligands of reduced cytochromes b. On the other hand, absorption measurements have characterized large pKa differences for the sequential imidazole ionizations of Fex]PP(ImH)2 in aqueous cetyltrimethylammonium bromide (9.0 and 10.8 for x = 111; 13.0 and 14.1 for x = II). These titrations show that Fe(II)PP(Im–)2 and Fe(III)PP(ImH)2 are good proton-acceptor and proton-donor, respectively, and suggest a model by which heme, located in a favorable environment inside a cytochrome, could couple a cycle of electron transfer with a proton transfer. Based on sequence data and structural models, it is further proposed that, in several membrane cytochromes b (b, b
6, b
559), a positively charged amino acid residue and an imidazolate ligand of the ferriheme could form an ion pair involved in a redox control of proton transfer.Abbreviations RR
resonance Raman
- EPR
electron paramagnetic resonance
- PP
protoporphyrin IX
- ImH
imidazole
- Im–
imidazolate
- Im*
imidazole or imidazolate
- 1MeIm
1-methylimidazole
- HisH
histidine
- His–
histidinate
- CTABr
cetyltrimethylammonium bromide
- NaDS
sodium dodecylsulphate
- VLP
very low potential
- LP
low potential
- HP
high potential |
| |
Keywords: | Ligand deprotonation Iron-ligand modes Porphyrin modes Proton transfer |
本文献已被 SpringerLink 等数据库收录! |
|