Kinetics and Pathways of Charge Recombination in Photosystem II

Abstract

The mechanism of charge recombination of the S(2)Q(A)(-) state in photosystem II was investigated by modifying the free energy gap between the quinone acceptor Q(A) and the primary pheophytin acceptor Ph. This was done either by changing the midpoint potential of Ph (using mutants of the cyanobacterium Synechocystis with a modified hydrogen bond to this cofactor), or that of Q(A) (using different inhibitors of the Q(B) pocket). The results show that the recombination rate is dependent on the free energy gap between Ph and Q(A), which confirms that the indirect recombination pathway involving formation of Ph(-) has a significant contribution. In the mutant with the largest free energy gap, direct electron transfer from Q(A)(-) to P(+) predominates. The temperature dependence of the recombination rate was investigated, showing a lower activation enthalpy in this mutant compared with the WT. The data allow the determination of the rate of the direct route and of its relative weight in the various strains. The set of currently accepted values for the midpoint potentials of the Q(A)/Q(A)(-), Ph/Ph(-), and P(+)/P* couples is not consistent with the relatively rapid rate of the indirect recombination pathway found here, nor with the 3% yield of delayed fluorescence as previously estimated by de Grooth and van Gorkom (1981, Biochim. Biophys. Acta 635, 445-456). It is argued that a likely explanation is that the midpoint potentials of the two latter couples are more positive than believed due to electrostatic interactions. If such is the case, the estimation of the midpoint potential of the P(+)/P and S(2)/S(1) couples must also be revised upward, with values of 1260 and 1020 mV, respectively.

Mechanism and energy diagram for O–O bond formation in the oxygen-evolving complex in photosystem II

“…You stated that in the S-state cycle of photosynthetic oxygenevolution transitions 30 kcal mol K1 is 'wasted' in the form of heat release. The redox potentials of the donorside redox factors are most likely more positive than previously assumed (E m of P680/P680 C of w1.25 V; see Rappaport et al (2002) and Grabolle & Dau (2005)). Nonetheless, I feel that the figure of 30 kcal mol K1 is clearly too high and in conflict with common assumptions on the PSII energetics.…”

Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

“…Chlorophyll triplets are relatively long-lived and very reactive with O 2 to generate 1 O 2 , a very toxic species. This concept of redox tuning controlling the competition between two main charge recombination routes in PSII (13) has dominated much of the subsequent thinking concerning photoinhibition and redox-based protective mechanisms (18)(19)(20).…”

Photoassembly of the water-oxidizing complex in photosystem II

“…However, oxidation of water to O 2 is a mechanistically complex and highly energy demanding chemical reaction [2]driven by photooxidation of a chlorophyll with electrochemical reduction potential of ca. 1.2-1.3 V [3]. In nature, this reaction proceeds by using the visible light energy that is trapped and made chemically available by chlorophyll photo-pigments, and it is ultimately carried out at the inorganic active site (Mn 4 O x Ca 1 Cl y ), the water-oxidizing complex (WOC), of a membrane spanning protein complex known as photosystem II (PSII) [4].…”