Low-potential iron-sulfur centers in photosystem I: an x-ray absorption spectroscopy study

• Published in: Biochemistry (Easton) Vol. 27; no. 11; pp. 4013 - 4020
• Main Authors: McDermott, Ann E, Yachandra, Vittal K, Guiles, R. D, Britt, R. D, Dexheimer, S. L, Sauer, Kenneth, Klein, Melvin P
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 31-05-1988
• Subjects: azufre; Biological and medical sciences; Cell structures and functions; Chlorophyll - analysis; Chloroplast, photosynthetic membrane and photosynthesis; Cyanobacteria - analysis; cyanophyta; fer; fotosintesis; Fundamental and applied biological sciences. Psychology; hierro; iron; Iron - analysis; Light-Harvesting Protein Complexes; Molecular and cellular biology; photosynthese; photosynthesis; Photosynthetic Reaction Center Complex Proteins; Photosystem I Protein Complex; Plant Proteins - analysis; Plants - analysis; soufre; Spectrum Analysis; Spinacea oleracea; spinacia oleracea; Sulfur - analysis; sulphur; Synechococcus; X-Rays; Photosystem 1; Bacteria; EXAFS spectrometry; Cyanobacteria; Synechococcus
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00411a018

We have measured the X-ray absorption spectra of Fe in photosystem I (PS I) preparations from spinach and a thermophilic cyanobacterium, Synechococcus sp., to characterize structures of the Fe complexes that function as electron acceptors in PS I. These acceptors include centers A and B, which are probably typical [4Fe-4S] ferredoxins, and X. The structure of X is not known, but its electron paramagnetic resonance (EPR) spectrum has generated the suggestions that it is either a [2Fe-2S] or [4Fe-4S] ferredoxin or an Fe-quinone species. The iron X-ray absorption K-edge and iron extended X-ray absorption fine structure (EXAFS) spectra reveal that essentially all of the 11-14 Fe atoms present in the reaction center are present in the form of Fe-S centers and that not more than 1 atom out of 12 could be octahedral or oxygen-coordinated Fe. This suggests that, besides A and B, additional Fe-S clusters are present which are likely to be X. Our EXAFS spectra cannot be simulated adequately by a mixture of [4Fe-4S] ferredoxins with typical bond lengths and disorder parameters because the amplitude of Fe backscattering is small; however, excellent simulations of the data are consistent with a mixture of [2Fe-2S] ferredoxins and [4Fe-4S] ferredoxins, or with unusually distorted [4Fe-4S] clusters. We presume that the [2Fe-2S] or distorted [4Fe-4S] centers are X. The X-ray absorption spectra of PS I preparations from Synechococcus and spinach are essentially indistinguishable.