Primary photochemistry in photosystem-I

Abstract

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.

Spectroscopic characterization of PS I core complexes from thermophilic Synechococcus sp

“…Since the iron-sulfur-clusters can hardly be distinguished by optical spectroscopy, the state P700'F; could not yet been monitored as an intermediate in normal forward electron transfer. Photoreduction of F, has been observed by EPR at low temperatures, but the results more likely indicate that F, may represent an acceptor on a pathway parallel to F,, (for review see [9]). The observation, that the 200 ns forward electron transfer from A; to an iron-sulfur-center is blocked in Synechococcus after chemical pre-reduction of both FA and F,, may even indicate that F, is not involved in normal forward electron transfer [lo].…”

Optical difference spectrum of the electron acceptor Aoin photosystem I

“…During such reduction two distinct EPR signals have been detected, indicating that 'Al' consists of two components, AI and an earlier acceptor & [7,8]. The multiple component nature of the intermediary acceptor complex has also been indicated by chemically-induced dynamic electron polarisation (CIDEP) EPR signals (reviewed in [9]). A 120~s component of the low temperature decay kinetics of 820 nm absorption changes has been related to a back reaction between P700f and A; [ 101.…”

On the function of two vitamin K1molecules in the PS I electron acceptor complex

“…The PS I primary electron acceptor complex is known to contain multiple bound electron acceptors, the best studied of which are iron-sulfur centers A, B and X [1,2]. In addition to these centers, recent evidence has identified two early electron acceptors, denoted AO and Al, that precede the iron-sulfur centers [3,5].…”