Electron transfer from plastocyanin to the photosystem I reaction center in mutants with increased potential of the primary donor in Chlamydomonas reinhardtii

Electron Transfer in a Bio-Photoelectrode Based on Photosystem I Multilayer Immobilized on the Conducting Glass

A film of ~40 layers of partially oriented photosystem I (PSI) complexes isolated from the red alga Cyanidioschyzon merolae formed on the conducting glass through electrodeposition was investigated by time-resolved absorption spectroscopy and chronoamperometry. The experiments were performed at a range of electric potentials applied to the film and at different compositions of electrolyte solution being in contact with the film. The amount of immobilized proteins supporting light-induced charge separation (active PSI) ranged from ~10%, in the absence of any reducing agents (redox compounds or low potential), to ~20% when ascorbate and 2,6-dichlorophenolindophenol were added, and to ~35% when the high negative potential was additionally applied. The origin of the large fraction of permanently inactive PSI (65–90%) was unclear. Both reducing agents increased the subpopulation of active PSI complexes, with the neutral P700 primary electron donor, by reducing significant fractions of the photo-oxidized P700 species. The efficiencies of light-induced charge separation in the PSI film (10–35%) did not translate into an equally effective generation of photocurrent, whose internal quantum efficiency reached the maximal value of 0.47% at the lowest potentials. This mismatch indicates that the vast majority of the charge-separated states in multilayered PSI complexes underwent charge recombination.

Comparative kinetic and energetic modelling of phyllosemiquinone oxidation in Photosystem I

The oxidation kinetics of phyllo(semi)quinone (PhQ), which acts as an electron transfer (ET) intermediate in the Photosystem I reaction centre, are described by a minimum of two exponential phases, characterised by lifetimes in the 10-30 ns and 150-300 ns ranges. The fastest phase is considered to be dominated by the oxidation of the PhQ molecule coordinated by the PsaB reaction centre subunit (PhQB), and the slowest phase is dominated by the oxidation of the PsaA coordinated PhQ (PhQA). Testing different energetic schemes within a unified theory-based kinetic modelling approach provides reliable limit-values for some of the physical-chemical parameters controlling these ET reactions: (i) the value of ΔG(0) associated with PhQA oxidation is smaller than ∼+30 meV; (ii) the value of the total reorganisation energy (λt) likely exceeds 0.7 eV; (iii) different mean nuclear modes are coupled to PhQB and PhQA oxidation, the former being larger, and both being ≥100 cm(-1).

Excitation dynamics in Photosystem I from Chlamydomonas reinhardtii. Comparative studies of isolated complexes and whole cells

Identical time-resolved fluorescence measurements with ~3.5-ps resolution were performed for three types of PSI preparations from the green alga, Chlamydomonas reinhardtii: isolated PSI cores, isolated PSI-LHCI complexes and PSI-LHCI complexes in whole living cells. Fluorescence decay in these types of PSI preparations has been previously investigated but never under the same experimental conditions. As a result we present consistent picture of excitation dynamics in algal PSI. Temporal evolution of fluorescence spectra can be generally described by three decay components with similar lifetimes in all samples (6-8ps, 25-30ps, 166-314ps). In the PSI cores, the fluorescence decay is dominated by the two fastest components (~90%), which can be assigned to excitation energy trapping in the reaction center by reversible primary charge separation. Excitation dynamics in the PSI-LHCI preparations is more complex because of the energy transfer between the LHCI antenna system and the core. The average trapping time of excitations created in the well coupled LHCI antenna system is about 12-15ps longer than excitations formed in the PSI core antenna. Excitation dynamics in PSI-LHCI complexes in whole living cells is very similar to that observed in isolated complexes. Our data support the view that chlorophylls responsible for the long-wavelength emission are located mostly in LHCI. We also compared in detail our results with the literature data obtained for plant PSI.

Interquinone electron transfer in photosystem I as evidenced by altering the hydrogen bond strength to the phylloquinone(s)

The kinetics of electron transfer from phyllosemiquinone (PhQ(*-)) to the iron sulfur cluster F(X) in Photosystem I (PS I) are described by lifetimes of approximately 20 and approximately 250 ns. These two rates are attributed to reactions involving the quinones bound primarily by the PsaB (PhQ(B)) and PsaA (PhQ(A)) subunits, respectively. The factors leading to a approximately 10-fold difference between the observed lifetimes are not yet clear. The peptide nitrogen of conserved residues PsaA-Leu722 and PsaB-Leu706 is involved in asymmetric hydrogen-bonding to PhQ(A) and PhQ(B), respectively. Upon mutation of these residues in PS I of the green alga, Chlamydomonas reinhardtii , we observe an acceleration of the oxidation kinetics of the PhQ(*-) interacting with the targeted residue: from approximately 255 to approximately 180 ns in PsaA-L722Y/T and from approximately 24 to approximately 10 ns in PsaB-L706Y. The acceleration of the kinetics in the mutants is consistent with a perturbation of the H-bond, destabilizing the PhQ(*-) state, and increasing the driving force of its oxidation. Surprisingly, the relative amplitudes of the phases reflecting PhQ(A)(*-) and PhQ(B)(*-) oxidation were also affected by these mutations: the apparent PhQ(A)(*-)/PhQ(B)(*-) ratio is shifted from 0.65:0.35 in wild-type reaction centers to 0.5:0.5 in PsaA-L722Y/T and to 0.8:0.2 in PsaB-L706Y. The most consistent account for all these observations involves considering reversibility of oxidation of PhQ(A)(*-) and PhQ(B)(*-) by F(X), and asymmetry in the driving forces for these electron transfer reactions, which in turn leads to F(x)-mediated interquinone electron transfer.

In vivo photosystem I reduction in thermophilic and mesophilic cyanobacteria: The thermal resistance of the process is limited by factors other than the unfolding of the partners

Photosystem I reduction by plastocyanin and cytochrome c(6) in cyanobacteria has been extensively studied in vitro, but much less information is provided on this process inside the cell. Here, we report an analysis of the electron transfer from both plastocyanin and cytochrome c(6) to photosystem I in intact cells of several cyanobacterial species, including a comparative study of the temperature effect in mesophilic and thermophilic organisms. Our data show that cytochrome c(6) reduces photosystem I by following a reaction mechanism involving complex formation, whereas the copper-protein follows a simpler collisional mechanism. These results contrast with previous kinetic studies in vitro. The effect of temperature on photosystem I reduction leads us to conclude that the thermal resistance of this process is determined by factors other than the proper stability of the protein partners.

Equilibration between cytochrome f and P700 in intact leaves

Electron transport between the two photosynthetic reaction centres of high plants is mediated by plastoquinone, a rieske iron-sulfur centre, cytochrome f and plastocyanin. Measurements of redox equilibration amongst these have produced confusing results, with apparent equilibrium constants being estimated that are inconsistent with in vitro measurements of redox midpoint potentials of the components concerned. We have critically reexamined methods for deconvoluting cytochrome f absorbance signals in intact leaves. We have determined the decay of cytochrome f+ following light to dark transitions from steady state and compared this with the decay of the oxidised photosystem I primary donor, P700+. Measurements across a wide range of different irradiances and CO2 concentrations were all consistent with cyt f and P700 existing in redox equilibrium, with a potential difference of around 117 mV. These results are discussed in relation to our understanding of the organisation of the photosynthetic electron transport. They also have implications for measurements of PSI electron flux--provided more than about 20% of P700+ is oxidised in the light, then the initial decay in the concentration of P700+ following a light to dark transition provides a good estimate of electron flux through PSI. Where P700 is largely reduced in the light, net reduction of cyt f+ might need to be corrected for.

The Hydrophobic Recognition Site Formed by Residues PsaA-Trp651 and PsaB-Trp627 of Photosystem I in Chlamydomonas reinhardtii Confers Distinct Selectivity for Binding of Plastocyanin and Cytochrome c6

On the lumenal side of photosystem I (PSI), each of the two large core subunits, PsaA and PsaB, expose a conserved tryptophan residue to the surface. PsaB-Trp(627) is part of the hydrophobic recognition site that is essential for tight binding of the two electron donors plastocyanin and cytochrome c(6) to the donor side of PSI (Sommer, F., Drepper, F., and Hippler, M. (2002) J. Biol. Chem. 277, 6573-6581). To examine the function of PsaA-Trp(651) in binding and electron transfer of both donors to PSI, we generated the mutants PsaA-W651F and PsaA-W651S by site-directed mutagenesis and biolistic transformation of Chlamydomonas reinhardtii. The protein-protein interaction and the electron transfer between the donors and PSI isolated from the mutants were analyzed by flash absorption spectroscopy. The mutation PsaA-W651F completely abolished the formation of a first order electron transfer complex between plastocyanin (pc) and the altered PSI and increased the dissociation constant for binding of cytochrome (cyt) c(6) by more than a factor of 10 as compared with wild type. Mutation of PsaA-Trp(651) to Ser had an even larger impact on the dissociation constant. The K(D) value increased another 2-fold when the values obtained for the interaction and electron transfer between cyt c(6) and PSI from PsaA-W651S and PsaA-W651F are compared. In contrast, binding and electron transfer of pc to PSI from PsaA-W651S improved as compared with PSI from PsaA-W651F and admitted the formation of an inter-molecular electron transfer complex, resulting in a K(D) value of about 554 microm that is still five times higher than observed for wild type. These results demonstrate that PsaA-Trp(651) is, such as PsaB-Trp(627), crucial for high affinity binding of pc and cyt c(6) to PSI. Our results also indicate that the highly conserved structural recognition motif that is formed by PsaA-Trp(651) and PsaB-Trp(627) confers a differential selectivity in binding of both donors to PSI.