ABSTRACT
A new photosystem I core has been isolated that is devoid of the bound iron-sulfur clusters but preserves electron flow from P700 to the intermediate electron acceptor A1. The particle is prepared by incubation of a Synechococcus sp. PCC 6301 photosystem I core protein (which contains electron acceptors A0, A1, and FX) with 3 M urea and 5 mM K3Fe(CN)6 to oxidatively denature the FX iron-sulfur cluster to the level of zero-valence sulfur. In this apo-FX preparation, over 90% of the flash-induced absorption change at 820 nm decays with a 10-microseconds half-time characteristic of the decay of the P700 triplet state formed from the backreaction of P700+ with an acceptor earlier than FX. Chemical reduction at high pH values with aminoiminomethanesulfinic acid results in kinetics identical with those seen in the P700 chlorophyll a protein prepared with sodium dodecyl sulfate (SDS-CP1, which contains only electron acceptor A0); the flash-induced absorption change decays primarily with a 25-ns half-time characteristic of the backreaction between P700+ and A0-, and the magnitude of the total absorption change is larger than can be accounted for by the P700 content alone. Addition of oxygen results in a reversion to the 10-microseconds kinetic decay component attributed to the decay of the P700 triplet state. At 77 K, the optical transient in the apo-FX preparation decays with a 200-microseconds half-time characteristic of the backreaction between P700+ and A1-.(ABSTRACT TRUNCATED AT 250 WORDS)
Subject(s)
Bacterial Proteins/isolation & purification , Bacterial Proteins/metabolism , Bacterial Proteins/radiation effects , Cold Temperature , Cyanobacteria/analysis , Electron Transport , Hydrogen-Ion Concentration , Oxidation-Reduction , Oxygen/metabolism , Photosynthetic Reaction Center Complex Proteins , Spectrophotometry, InfraredABSTRACT
We report the results of a Mössbauer study of the low-potential iron-sulfur cluster FX in the Photosystem I core protein of Synechococcus 6301. The Mössbauer spectrum of FX in the oxidized state shows an isomer shift of 0.42 mm/s, which is in good agreement with the 0.43 mm/s isomer shift found in [4Fe-4S] proteins but not with the isomer shift of 0.26 mm/s found in [2Fe-2S] proteins. In the reduced state the spectrum is asymmetrically broadened at 80 K, indicating the presence of two very closely spaced doublets with an average isomer shift of 0.55 mm/s, which is also in agreement with [4Fe-4S] proteins. At 4.2 K, the spectrum exhibits broadening and magnetic splitting similar to what is observed for [4Fe-4S] proteins and quite unlike [2Fe-2S] proteins. Given the assumption that the iron atoms of FX are tetrahedrally coordinated with sulfur ligands, the data strongly support the assignment of FX as a [4Fe-4S] cluster.
Subject(s)
Chlorophyll , Iron , Plant Proteins , Spectroscopy, Mossbauer/methods , Sulfur , Cyanobacteria , Light-Harvesting Protein Complexes , Photosynthetic Reaction Center Complex Proteins , Photosystem I Protein ComplexABSTRACT
We present iron extended X-ray absorption fine structure (EXAFS) spectra of a photosystem I core preparation containing FX, the very low potential iron-sulfur cluster in photosystem I. The preparation lacks FA and FB. The amplitude of Fe-Fe backscattering in the EXAFS spectrum indicates that FX may be a [4Fe-4S] cluster and is not a [2Fe-2S] cluster or clusters.
Subject(s)
Chlorophyll/analysis , Iron/analysis , Photosynthesis , Plant Proteins/analysis , Sulfur/metabolism , Fourier Analysis , Iron/metabolism , Light-Harvesting Protein Complexes , Photosynthetic Reaction Center Complex Proteins , Photosystem I Protein Complex , Spectrometry, X-Ray EmissionABSTRACT
The intact Photosystem I core protein, containing the psaA and psaB polypeptides, and electron transfer components P-700 through FX, was isolated from cyanobacterial and higher plant Photosystem I complexes with chaotropic agents followed by sucrose density ultracentrifugation. The concentrations of NaClO4, NaSCN, NaI, NaBr or urea required for the functional removal of the 8.9 kDa, FA/FB polypeptide was shown to be inversely related to the strength of the chaotrope. The Photosystem I core protein, which was purified to homogeniety, contains 4 mol of acid-labile sulfide and has the following properties: (i) the FX-containing core consists of the 82 and 83 kDa reaction center polypeptides but is totally devoid of the low-molecular-mass polypeptides; (ii) methyl viologen and other bipyridilium dyes have the ability to accept electrons directly from FX; (iii) the difference spectrum of FX from 400 to 900 nm is characteristic of an iron-sulfur cluster; (iv) the midpoint potential of FX, determined optically at room temperature, is 60 mV more positive than in the control; (v) there is indication by ESR spectroscopy of low-temperature heterogeneity within FX; and (vi) the heterogeneity is seen by optical spectroscopy as inefficiency in low-temperature electron flow to FX. The constraints imposed by the amount of non-heme iron and labile sulfide in the Photosystem I core protein, the cysteine content of the psaA and psaB polypeptides, and the stoichiometry of high-molecular-mass polypeptides, cause us to re-examine the possibility that FX is a [4Fe-4S] rather than a [2Fe-2S] cluster ligated by homologous cysteine residues on the psaA and psaB heterodimer.
