Document Detail

Iron coordination in photosystem II: interaction between bicarbonate and the QB pocket studied by Fourier transform infrared spectroscopy.
MedLine Citation:
PMID:  11300785     Owner:  NLM     Status:  MEDLINE    
The non heme iron environment of photosystem II is studied by light-induced infrared spectroscopy. A conclusion of previous work [Hienerwadel, R., and Berthomieu, C. (1995) Biochemistry 34, 16288-16297] is that bicarbonate is a bidendate ligand of the reduced iron and a monodentate ligand in the Fe(3+) state. In this work, the effects of bicarbonate replacement with lactate, glycolate, and glyoxylate, and of o-phenanthroline binding are investigated to determine the specific interactions of bicarbonate with the protein. Fe(2+)/Fe(3+) FTIR spectra recorded with (12)C- and (13)C(1)-labeled lactate indicate that lactate displaces bicarbonate by direct binding to the iron through one carboxylate oxygen and the hydroxyl group in both the Fe(2+) and Fe(3+) states. This different binding mode with respect to bicarbonate could explain the lower midpoint of the iron couple observed in the presence of this anion [Deligiannakis, Y., Petrouleas, V., and Diner, B. A. (1994) Biochim. Biophys. Acta 1188, 260-270]. In agreement with the -60 mV/pH unit dependence of the iron midpoint potential in the presence of bicarbonate, the proton release upon iron oxidation by photosystem II is directly measured to 0.95 +/- 0.05 by the comparison of infrared signals of phosphate buffer and ferrocyanide modes. This accurate method may be applied to the study of other redox reactions in proteins. The pH dependence of the iron couple is proposed to reflect the deprotonation of D1His215, a putative iron ligand located at the Q(B) pocket, since the signal at 1094 cm(-1) assigned to the nu(C-N) mode of a histidinate ligand in the Fe(3+) state is not observed in the presence of o-phenanthroline. Specific regulation of the pK(a) of D1His215 by bicarbonate is inferred from the absence of the band at 1094 cm(-1) in Fe(2+)/Fe(3+) spectra recorded with glycolate, glyoxylate, or lactate. A broad positive continuum, maximum at approximately 2550 cm(-1), observed in the presence of bicarbonate, but absent with o-phenanthroline or lactate, glycolate, and glyoxylate, indicates a hydrogen bond network from the non heme iron toward the Q(B) pocket involving bicarbonate and His D1-215. Proton release of about 1, measured upon iron oxidation at pH 6 with the latter anions, points to a proton release mechanism different from that involved in the presence of bicarbonate.
C Berthomieu; R Hienerwadel
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't    
Journal Detail:
Title:  Biochemistry     Volume:  40     ISSN:  0006-2960     ISO Abbreviation:  Biochemistry     Publication Date:  2001 Apr 
Date Detail:
Created Date:  2001-04-13     Completed Date:  2001-06-21     Revised Date:  2006-11-15    
Medline Journal Info:
Nlm Unique ID:  0370623     Medline TA:  Biochemistry     Country:  United States    
Other Details:
Languages:  eng     Pagination:  4044-52     Citation Subset:  IM    
CEA/Saclay, Section de Bioénergétique Bat. 532, 91 191 Gif-sur-Yvette, Cedex, France.
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MeSH Terms
Benzoquinones / chemistry*
Bicarbonates / chemistry*
Binding Sites
Ferric Compounds / chemistry
Histidine / chemistry
Iron / chemistry*
Nonheme Iron Proteins / chemistry
Organometallic Compounds / chemistry
Phenanthrolines / chemistry
Photosynthetic Reaction Center Complex Proteins / chemistry*
Photosystem II Protein Complex
Potassium Compounds
Spectroscopy, Fourier Transform Infrared
Spinacia oleracea
Reg. No./Substance:
0/Benzoquinones; 0/Bicarbonates; 0/Buffers; 0/Ferric Compounds; 0/Nonheme Iron Proteins; 0/Organometallic Compounds; 0/Phenanthrolines; 0/Phosphates; 0/Photosynthetic Reaction Center Complex Proteins; 0/Photosystem II Protein Complex; 0/Potassium Compounds; 0/Protons; 106-51-4/benzoquinone; 16068-46-5/potassium phosphate; 23661-48-5/ferrous histidinate; 66-71-7/1,10-phenanthroline; 71-00-1/Histidine; 7439-89-6/Iron

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