Dynamics of electron transfer in photosystem II

Abstract

Photosystem II, being a constituent of light driven photosynthetic apparatus, is a highly organized pigment-protein-lipid complex. The arrangement of PSII active redox cofactors insures efficiency of electron transfer within it. Donation of electrons extracted from water by the oxygen evolving complex to plastoquinones requires an additional activation energy. In this paper we present theoretical discussion of the anharmonic fluctuations of the protein-lipid matrix of PSII and an experimental evidence showing that the fluctuations are responsible for coupling of its donor and acceptor side. We argue that the fast collective motions liberated at temperatures higher that 200 K are crucial for the two final steps of the water splitting cycle and that one can distinguish three different dynamic regimes of PSII action which are controlled by the timescales of forward electron transfer, which vary with temperature. The three regimes of the dynamical behavior are related to different spatial domains of PSII.

Influence of Cd2+on the spin state of non-heme iron and on protein local motions in reactions centers from purple photosynthetic bacteriumRhodospirilium rubrum

“…The molecular mechanisms regulating photosynthetic ET have long been a subject of intensive studies but because of their complexicity it is still unclear how exactly the protein matrix exerts its control over the ET cofactors [3][4][5][6]. In particular, the role of NHFe, a very conservative component of the Q-type RCs, in photosynthetic charge separation and the temperature effects on ET between the two external quinone acceptors are among the most challenging issues [7,8]. Intriguingly, ET from reduced Q A to Q B is inhibited at temperatures below 200±20 K whereas Q A in RCs from various organisms remains fully active at cryogenic temperatures [9,10].…”

Mössbauer spectroscopy in studies of photosynthesis

“…b 559 being the trans-membrane component was suggested to be such a link. These data give the first direct evidence that the same collective motions influence the acceptor and the donor side via cytochrome b 559 [20,26]. This finding is even more important because if it is combined with results of the earlier studies, which have shown that the occurrence of the fast collective motion is necessary for the activation of electron transfer within the Q A -Fe-Q B complex [26][27][28][29], it suggests that cytochrome b 559 can play a role of coupler between the PSII donor and acceptor side.…”

“…Clearly, cyt. b 559 seems to play an important regulatory function on the oxidizing and reducing sides of PSII in addition to its importance for the PSII assembly and stability [7,26].…”

“…Only the glutamate ligand in purpule bacteria [36] changed for bicarbonate in the PSII systems [38]. Because there are no observed changes of the valence state of the non-heme iron in natural photosynthetic systems and a certain degree of flexibility of the quinone iron complex is necessary for the activation of the electron flow through the reaction centers, the important triggering role of the non-heme iron can be suggested [26][27][28][29]. It is interesting to find the influence of the possible interchanges between the high and low spin states of the non-heme iron without changing its valence states on efficiency of PSII.…”

Chemical proprieties of the iron-quinone complex in mutated reaction centers of Rb. sphaeroides

“…The Q A ubiquinone, bound to the M subunit, is in a hydrophobic area whereas the Q B ubiquinone, bound to the L subunit, is surrounded by charged and polar residues. The molecular mechanisms of the electron transfer between Q A and Q B ubiquinones have been a subject of challenging studies but the role of the NHFe, which is a very conservative component of RCs of the type II, in this process is still unknown [1,2]. Especially the role of NHFe in stabilization of the Q A and Q B binding sites and in the primary electron transfer (ET) is not clear.…”