Photoactivation of the water-oxidizing complex of photosystem II core complex depleted of functional Mn

Coordination between manganese and nitrogen within the ligands in the manganese complexes facilitates the reconstitution of the water-oxidizing complex in manganese-depleted photosystem II preparations

The water-oxidizing complex (WOC) within photosystem II (PSII) can be reconstituted with synthetic manganese complexes by a process called photoactivation; however, the key factors affecting the efficiency of synthetic manganese complexes in reconstitution of electron transport and oxygen evolution activity in manganese-depleted PSII remain unclear. In the present study, four complexes with different manganese coordination environments were used to reconstitute the WOC, and an interesting relationship was found between the coordination environment of the manganese atom in the complexes and their efficiency in restoring electron transport and oxygen evolution. If Mn(II) is coordinated to nitrogen atoms within the ligand, it can restore significant rates of electron transport and oxygen evolution; however, if the manganese atom is coordinated only to oxygen atoms instead of nitrogen atoms, it has no capability to restore electron transport and oxygen evolution. So, our results demonstrate that the capability of manganese complexes to reconstitute the WOC is mainly determined by the coordination between nitrogen atoms from ligands and the manganese atom. It is suggested from our results that the ligation between the nitrogen atom and the manganese atom within the manganese complex facilitates the photoligation of the manganese atom to histidyl residues on the apo-protein in manganese-depleted PSII during photoactivation.

Relationship between the organization of the PS II super complex and the functions of the photosynthetic apparatus

The chlorophyll fluorescence and the photosynthetic oxygen evolution (flash-induced oxygen yield patterns and oxygen bursts under continuous irradiation) were investigated in the thylakoid membranes with different stoichiometry and organization of the chlorophyll-protein complexes. Data show that the alteration in the organization of the photosystem II (PS II) super complex, i.e. the amount and the organization of the light-harvesting chlorophyll a/b protein complex (LHCII), which strongly modifies the electric properties of the membranes, influences both the energy redistribution between the two photosystems and the oxygen production reaction. The decrease of surface electric parameters (charge density and dipole moments), associated with increased degree of LHCII oligomerization, correlates with the strong reduction of the energy transfer from PS II to PSI. In the studied pea thylakoid membranes (wild types Borec, Auralia and their mutants Coeruleovireus 2/16, Costata2/133, Chlorotica XV/1422) with enhanced degree of oligomerization of LHCII was observed: (i) an increase of the S(0) populations of PS II in darkness; (ii) an increase of the misses; (iii) an alteration of the decay kinetics of the oxygen bursts under continuous irradiation. There is a strict correlation between the degree of LHCII oligomerization in the investigated pea mutants and the ratio of functionally active PS II alpha to PS II beta centers, while in thylakoid membranes without oligomeric structure of LHCII (Chlorina f2 barley mutant) the PS II alpha centers are not registered.

The inorganic biochemistry of photosynthetic oxygen evolution/water oxidation

At the request of the organizer of this special edition, we have attempted to do several things in this manuscript: (1) we present a mini-review of recent, selected, works on the light-induced inorganic biogenesis (photoactivation), composition and structure of the inorganic core responsible for photosynthetic water oxidation; (2) we summarize a new proposal for the evolutionary origin of the water oxidation catalyst which postulates a key role for bicarbonate in formation of the inorganic core; (3) we summarize published studies and present new results on what has been learned from studies of 'inorganic mutants' in which the endogenous cofactors (Mn(n+), Ca2+, Cl-) are substituted; (4) the first DeltapH changes measured during the photoactivation process are reported and used to develop a model for the stepwise photo-assembly process; (5) a comparative analysis is given of data in the literature on the kinetics of substrate water exchange and peroxide binding/dismutation which support a mechanistic model for water oxidation in general; (6) we discuss alternative interpretations of data in the literature with a view to forecast new avenues where progress is needed.

Metallo-radical hypothesis for photoassembly of (Mn)4-cluster of photosynthetic oxygen evolving complex

A new hypothetical mechanism is proposed for photoassembly of the (Mn)4-cluster of the photosynthetic oxygen evolving complex (OEC). In this process, a neutral radical of Y(Z) tyrosine plays a role in oxidizing Mn2+ associated with an apo-OEC, and also in abstracting a proton from a water molecule bound to the Mn2+ ion, together with D1-His190. This is in a similar fashion to the metallo-radical mechanism proposed for photosynthetic water oxidation by the (Mn)4-cluster. The model insists that a common mechanism participates in the photoassembly of the (Mn)4-cluster and the photosynthetic water oxidation.

Photoassembly of the manganese cluster and oxygen evolution from monomeric and dimeric CP47 reaction center photosystem II complexes

Isolated subcomplexes of photosystem II from spinach (CP47RC), composed of D1, D2, cytochrome b(559), CP47, and a number of hydrophobic small subunits but devoid of CP43 and the extrinsic proteins of the oxygen-evolving complex, were shown to reconstitute the Mn(4)Ca(1)Cl(x) cluster of the water-splitting system and to evolve oxygen. The photoactivation process in CP47RC dimers proceeds by the same two-step mechanism as observed in PSII membranes and exhibits the same stoichiometry for Mn(2+), but with a 10-fold lower affinity for Ca(2+) and an increased susceptibility to photodamage. After the lower Ca(2+) affinity and the 10-fold smaller absorption cross-section for photons in CP47 dimers is taken into account, the intrinsic rate constant for the rate-limiting calcium-dependent dark step is indistinguishable for the two systems. The monomeric form of CP47RC also showed capacity to photoactivate and catalyze water oxidation, but with lower activity than the dimeric form and increased susceptibility to photodamage. After optimization of the various parameters affecting the photoactivation process in dimeric CP47RC subcores, 18% of the complexes were functionally reconstituted and the quantum efficiency for oxygen production by reactivated centers approached 96% of that observed for reconstituted photosystem II-enriched membranes.

Involvement of the CP47 protein in stabilization and photoactivation of a functional water-oxidizing complex in the cyanobacterium Synechocystis sp. PCC 6803

Oscillation patterns of the oxygen yield per flash induced by a train of single-turnover flashes were measured as a function of dark incubation and different pre-illumination conditions in several autotrophic mutant strains of Synechocystis sp. PCC 6803 carrying short deletions within the large, lumen-exposed hydrophilic region (loop E) of the chlorophyll a-binding photosystem II protein CP47. A physiological and biochemical characterization of these mutant strains has been presented previously [Eaton-Rye, J. J., & Vermaas, W. F. J. (1991) Plant Mol. Biol. 17, 1165-1177; Haag, E., Eaton-Rye, J. J., Renger, G., & Vermaas, W. F. J. (1993) Biochemistry 32, 4444-4454], and some functional properties were described recently [Gleiter, H. M., Haag, E., Shen, J.-R., Eaton-Rye, J. J., Inoue, Y., Vermaas, W. F. J., & Renger, G. (1994) Biochemistry 33, 12063-12071]. The present study shows that in several mutants the water-oxidizing complex (WOC) became inactivated during prolonged dark incubation, whereas the WOC of the wild-type strain remained active. The rate and extent of the inactivation in the mutants depend on the domain of loop E, where 3-8 amino acid residues were deleted. The most pronounced effects are observed in mutants delta(A373-D380) and delta(R384-V392). A competent WOC can be restored from the fully inactivated state by illumination with short saturating flashes. The number of flashes required for this process strongly depends on the site at which a deletion has been introduced into loop E. Again, the most prominent effects were found in mutants delta(A373-D380) and delta(R384-V392). Interestingly, the number of flashes required for activation was reduced by more than an order of magnitude in both mutants by the addition of 10 mM CaCl2 to the cell suspension. On the basis of a model for photoactivation proposed by Tamura and Cheniae (1987) [Biochim. Biophys. Acta 890, 179-194], a scheme is presented for the processes of dark inactivation and photoactivation in these mutants. The results presented here corroborate an important role of the large hydrophilic domain (loop E) of CP47 in a functional and stable WOC.

Reconstitution of the water-oxidizing complex in manganese-depleted photosystem II complexes by using synthetic binuclear manganese complexes

The efficiency of synthetic binuclear manganese complexes in reconstituting PS II electron flow and oxygen-evolution capacity was analyzed in PS II enriched preparations deprived of their manganese and of the extrinsic regulatory subunits. Measurements of the variable fluorescence induced by actinic illumination with continuous light led to the following results: (a) the synthetic binuclear complexes are more efficient than MnCl2 in establishing a PS II electron flow; (b) an almost complete restoration is achieved at concentrations of these complexes that correspond with an overall stoichiometry of two manganese per PS II; and (c) the electron flow restored by the binuclear manganese complexes closely resembles that of normal O2-evolving PS II preparations in its resistance to addition of 50 microM EDTA, while that supported by MnCl2 is practically completely suppressed at the same chelator concentration. The rate of O2 evolution was used as a measure of the capability to function as manganese source in reconstitution of the oxygen evolution capacity. It was found that (i) as in the case of PS II electron transport, the synthetic binuclear manganese complexes are significantly more efficient than MnCl2; (ii) with respect to the manganese concentration, the maximum effect is achieved with a mu-oxo bridged binuclear Mn(III) complex (symbolized by M-3) at concentrations corresponding to four manganese per PS II; and (iii) at all concentrations of binuclear manganese complex M-3 a significantly higher restoration of the O2 evolution rate is achieved if the reconstitution assay contains in addition the extrinsic regulatory 33 kDa protein (PS II-O protein).(ABSTRACT TRUNCATED AT 250 WORDS)