Evidence that the low-potential (−700 mV) electron acceptor (X) in Photosystem I has two iron-sulphur centres

Purification and properties of the intact P-700 and Fx-containing Photosystem I core protein

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.

Photosystem I charge separation in the absence of centers A and B. II. ESR spectral characterization of center 'X' and correlation with optical signal 'A2'

The Photosystem I electron acceptor complex was characterized by optical flash photolysis and electron spin resonance (ESR) spectroscopy after treatment of a subchloroplast particle with lithium dodecyl sulfate (LDS). The following properties were observed after 60 s of incubation with 1% LDS followed by rapid freezing. (i) ESR centers A and B were not observed during or after illumination of the sample at 19 K, although the P-700+ radical at g = 2.0026 showed a large, reversible light-minus-dark difference signal. (ii) Center 'X', characterized by g factors of 2.08, 1.88 and 1.78, exhibited reversible photoreduction at 8 K in the absence of reduced centers A and B. (iii) The backreaction kinetics at 8 K between P-700, observed at g = 2.0026, and center X, observed at g = 1.78, was 0.30 s. (iv) The amplitudes of the reversible g = 2.0026 radical observed at 19 K and the 1.2 ms optical 698 nm transient observed at 298 K were diminished to the same extent when treated with 1% LDS at room temperature for periods of 1 and 45 min. We interpret the strict correlation between the properties and lifetimes of the optical P-700+ A2 reaction pair and the ESR P-700+ center X- reaction pair to indicate that signal A2 and center X represent the same iron-sulfur center in Photosystem I.

Primary photochemistry in photosystem-I

In this review, the main research developments that have led to the current simplified picture of photosystem I are presented. This is followed by a discussion of some conflicting reports and unresolved questions in the literature. The following points are made: (1) the evidence is contradictory on whether P700, the primary donor, is a monomer or dimer of chlorophyll although at this time the balacnce of the evidence points towards a monomeric structure for P700 when in the triplet state; (2) there is little evidence that the iron sulfur centers FA and FB act in series as tertiary acceptors and it is as likely that they act in parallel under physiological conditions; (3) a role for FX, probably another iron sulfur centrer, as an obligatory electron carrier in forward electron transfer has not been proven. Some evidence indicates that its reduction could represent a pathway different to that involving FA and FB; (4) the decay of the acceptor 'A2 (-)' as defined by optical spectroscopy corresponds with 700(+) % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOramaaBa% aaleaadaqdaaqaaiaadIfaaaaabeaaaaa!37D1!\[F_{\overline X } \] recombination under some circumstances but under other conditions it probably corresponds with P700(+) A1 (-) recombination; (5) P700(+) A1 (-) recombination as originally observed by optical spectroscopy is probably due to the decay of the P700 triplet state; (6) the acceptor A1 (-) as defined by EPR may be a special semiquinone molecule; (7) A0 is probably a chlorophyll a molecule which acts as the primary acceptor. Recombination of P700(+) A0 (-) gives rise to the P700 triplet state.A working model for electron transfer in photosystem I is presented, its general features are discussed and comparisons with other photosystems are made.