Effects of trypsin and bivalent cations on P-680+-reduction, fluorescence induction and oxygen evolution in Photosystem II membrane fragments from spinach

Coupling of electron and proton transfer in oxidative water cleavage in photosynthesis

This minireview addresses questions on the mechanism of oxidative water cleavage with special emphasis on the coupling of electron (ET) and proton transfer (PT) of each individual redox step of the reaction sequence and on the mode of O-O bond formation. The following topics are discussed: (1) the multiphasic kinetics of Y(Z)(ox) formation by P680(+*) originate from three different types of rate limitations: (i) nonadiabatic electron transfer for the "fast" ns reaction, (ii) local "dielectric" relaxation for the "slow" ns reaction, and (iii) "large-scale" proton shift for the micros kinetics; (2) the ET/PT-coupling mode of the individual redox transitions within the water oxidizing complex (WOC) driven by Y(Z)(ox) is assumed to depend on the redox state S(i): the oxidation steps of S(0) and S(1) comprise separate ET and PT pathways while those of S(2) and S(3) take place via proton-coupled electron transfer (PCET) analogous to Jerry Babcock's hydrogen atom abstractor model [Biochim. Biophys. Acta, 1458 (2000) 199]; (3) S(3) is postulated to be a multistate redox level of the WOC with fast dynamic equilibria of both redox isomerism and proton tautomerism. The primary event in the essential O-O bond formation is the population of a state S(3)(P) characterized by an electronic configuration and nuclear geometry that corresponds with a complexed hydrogen peroxide; (4) the peroxidic type S(3)(P) is the entatic state for formation of complexed molecular oxygen through S(3) oxidation by Y(Z)(ox); and (5) the protein matrix itself is proposed to exert catalytic activity by functioning as "PCET director". The WOC is envisaged as a supermolecule that is especially tailored for oxidative water cleavage and acts as a molecular machine.

Biosynthetic Ca2+/Sr2+ Exchange in the Photosystem II Oxygen-evolving Enzyme of Thermosynechococcus elongatus

The thermophilic cyanobacterium, Thermosynechococcus elongatus, has been grown in the presence of Sr2+ instead of Ca2+ with the aim of biosynthetically replacing the Ca2+ of the oxygen-evolving enzyme with Sr2+. Not only were the cells able to grow normally with Sr2+, they actively accumulated the ion to levels higher than those of Ca2+ in the normal cultures. A protocol was developed to purify a fully active Sr(2+)-containing photosystem II (PSII). The modified enzyme contained a normal polypeptide profile and 1 strontium/4 manganese, indicating that the normal enzyme contains 1 calcium/4 manganese. The Sr(2+)- and Ca(2+)-containing enzymes were compared using EPR spectroscopy, UV-visible absorption spectroscopy, and O2 polarography. The Ca2+/Sr2+ exchange resulted in the modification of the EPR spectrum of the manganese cluster and a slower turnover of the redox cycle (the so-called S-state cycle), resulting in diminished O2 evolution activity under continuous saturating light: all features reported previously by biochemical Ca2+/Sr2+ exchange in plant PSII. This allays doubts that these changes could be because of secondary effects induced by the biochemical treatments themselves. In addition, the Sr(2+)-containing PSII has other kinetics modifications: 1) it has an increased stability of the S3 redox state; 2) it shows an increase in the rate of electron donation from TyrD, the redox-active tyrosine of the D2 protein, to the oxygen-evolving complex in the S3-state forming S2; 3) the rate of oxidation of the S0-state to the S1-state by TyrD* is increased; and 4) the release of O2 is slowed down to an extent similar to that seen for the slowdown of the S3TyrZ* to S0TyrZ transition, consistent with the latter constituting the limiting step of the water oxidation mechanism in Sr(2+)-substituted enzyme as well as in the normal enzyme. The replacement of Ca2+ by Sr2+ appears to have multiple effects on kinetics properties of the enzyme that may be explained by S-state-dependent shifts in the redox properties of both the manganese complex and TyrZ as well as structural effects.

Effects of Ca2+ and EGTA on P680*+ reduction kinetics and O2 evolution of Photosystem II

We report for the first time significant changes in the P680*+ reduction kinetics of Photosystem II (PS II) in which the 17 and 23 kDa extrinsic polypeptides are intact, in the presence of Ca(2+) or ethylene glycol bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) which were added to vary the Ca(2+) concentration from 5 microM to 30 mM. The decrease in the extent of normal P680*+ reduction decay with lifetimes of 40-370 ns and a corresponding increase in the extent of kinetics with lifetimes of 20-220 micros was interpreted as being due to electron transfer from Y(Z) to P680*+ being replaced by slow forward conduction and by processes including P680*+/Q(A)(-) recombination. The question of whether changes in P680*+ reduction kinetics were caused by loss of Ca(2+) from PS II or by direct interaction of EGTA with PS II was addressed by lowering the free-Ca(2+) concentration of suspensions of PS II core complexes by serial dilution in the absence of EGTA. Despite a significant decrease in the rate of O(2) evolution after this treatment, only small changes in the P680*+ reduction kinetics were observed. Loss of Ca(2+) did not affect P680*+ reduction associated with electron transfer from Y(Z). Since much larger changes in the P680*+ reduction kinetics of intact PS II occurred at comparable free-Ca(2+) concentrations in the presence of EGTA, we conclude that EGTA influenced the P680*+ reduction kinetics by directly interacting with PS II rather than by lowering the free Ca(2+) concentration of the surrounding media. Notwithstanding these effects, we show that useful information about Ca(2+) binding to PS II can be obtained when direct interaction of EGTA is taken into account.

Photosystem II and photosynthetic oxidation of water: an overview

Conceptually, photosystem II, the oxygen-evolving enzyme, can be divided into two parts: the photochemical part and the catalytic part. The photochemical part contains the ultra-fast and ultra-efficient light-induced charge separation and stabilization steps that occur when light is absorbed by chlorophyll. The catalytic part, where water is oxidized, involves a cluster of Mn ions close to a redox-active tyrosine residue. Our current understanding of the catalytic mechanism is mainly based on spectroscopic studies. Here, we present an overview of the current state of knowledge of photosystem II, attempting to delineate the open questions and the directions of current research.

P680(+) reduction in oxygen-evolving Photosystem II core complexes

The kinetics of P680(+) reduction in oxygen-evolving spinach Photosystem II (PS II) core particles were studied using both repetitive and single-flash 830 nm transient absorption. From measurements on samples in which PS II turnover is blocked, we estimate radical-pair lifetimes of 2 ns and 19 ns. Nanosecond single-flash measurements indicate decay times of 7 ns, 40 ns and 95 ns. Both the longer 40 ns and 95 ns components relate to the normal S-state controlled Yz → P680(+) electron transfer dynamics. Our analysis indicates the existence of a 7 ns component which provides evidence for an additional process associated with modified interactions involving the water-splitting catalytic site. Corresponding microsecond measurements show decay times of 4 μs and 90 μs with the possibility of a small component with a decay time of 20-40 μs. The precise origin of the 4 μs component remains uncertain but appears to be associated with the water-splitting center or its binding site while the 90 μs component is assigned to P680(+)-QA (-) recombination. An amplitude and kinetic analysis of the flash dependence data gives results that are consistent with the current model of the oxygen-evolving complex.

The effects of ultraviolet irradiation on P680(+) reduction in PS II core complexes measured for individual S-states and during repetitive cycling of the oxygen-evolving complex

Flash-induced absorbance measurements at 830 nm on both nanosecond and microsecond timescales have been used to characterise the effect of ultraviolet light on Photosystem II core particles. A combination of UV-A and UV-B, closely simulating the spectrum of sunlight below 350 nm, was found to have a primary effect on the donor side of P680. Repetitive measurements indicated reductions in the nanosecond components of the absorbance decay with a concomitant appearance and increase in the amplitude of a component with a 10 μs time constant attributed to slow reduction of P680(+) by Tyrz when the function of the oxygen evolving complex is inhibited. Single-flash measurements show that the nanosecond components have amplitudes which vary with S-state. Increasing UV irradiation inhibited the amplitude of these components without changing their S-state dependence. In addition, UV irradiation resulted in a reduction in the total amplitude, with no change in the proportion of the 10 μs contribution.

Isolation and properties of PS II membrane fragments depleted of the non heme iron center

The functional properties and the content of non heme iron and cytochrome b559 were investigated by measuring flash induced transient changes of the relative fluorescence quantum yield and applying Mössbauer spectroscopy. It was found that untreated PS II membrane fragments contain a heterogeneous population of two types of non heme iron centers and about 2 cytochrome b559 per PS II. Twofold treatment of these samples with a recently described 'iron depletion' procedure (MacMillan, F., Lendzian, F., Renger, G. and Lubitz, W. (1995) Biochemistry 34, 3144-3156) leads to a complete loss (below the detection limit of Mössbauer spectroscopy) of the non heme iron center while more than 50% of the PS II complexes retain the functional integrity for light induced formation of the 'stable' radical pair Y(OX)(Z) P680Pheo Q(-.)(A). This sample type deprived of virtually all non heme iron in PS II provides a most suitable material for magnetic resonance studies that require an elimination of the interaction between Fe2+ and nearby radicals.

A possible calcium binding site in D1 protein: A fluorescence and FTIR study of the interaction between lanthanides and a synthetic peptide

A peptide ranging from residue 229 to 240 of the D1 protein of Photosystem (PS) II was synthesized and lanthanides were used as candidates of calcium. Fluorescence and FTIR spectroscopy were used to test the conformational adaptation after lanthanide additions. Fluorescence spectroscopy showed that the synthetic peptide provides lanthanide binding site, and that glutamic acids are involved in lanthanide binding. Resolution enhancement techniques were combined with band curve-fitting procedures to quantitate the FTIR spectral information from the amide 1 bands. The relative areas of these component bands indicate that lanthanide induced a substantial decrease in the amount of unordered structure and turns, while a corresponding increase in the amount of α-helix and 'open loop' was also observed. This indicates that a relatively compact structure of the synthetic peptide is formed if lanthanides are applied. The results may reflect on the physiological and biochemical function of calcium in PS II, including preventing D1 from trypsin digestion.

Inhibition of tyrosine Z photooxidation after formation of the S3 state in Ca(2+)-depleted and Cl(-)-depleted photosystem II

Ca2+ and Cl- are obligatory cofactors in photosystem II (PS-II), the oxygen-evolving enzyme of plants. The sites of inhibition in both Ca(2+)- and Cl(-)-depleted PS-II were compared using EPR and flash absorption spectroscopies to follow the extent of the photooxidation of the redox-active tyrosine (TyrZ) and of the primary electron donor chlorophyll (P680) and their subsequent reduction in the dark. The inhibition occurred after formation of the S3 state in Ca(2+)-depleted PS-II. In Cl(-)-depleted photosystem II, the inhibition occurred after formation of the S3 state in about half of the centers and probably after S2TyrZ+ formation in the remaining centers. After the S3 state was formed in Ca(2+)- and Cl(-)-depleted photosystem II, electron transfer from TyrZ to P680 was inhibited. This inhibition is discussed in terms of electrostatic constraints resulting from S3 formation in the absence of Ca2+ and Cl-.

Isolation, purification and partial characterization of a 30-kDa chlorophyll-a/b-binding protein from spinach

A 30-kDa chlorophyll-a/b-binding protein was purified from photosystem II membrane fragments using Ca(2+)-chelating Sepharose 6B chromatography. The protein binds approximately four chlorophyll a molecules, one chlorophyll b molecule and carotenoids. Its 77-K fluorescence-emission spectrum exhibits a maximum at 680 +/- 1 nm. The protein has a high tendency to form a dimer in the presence of Ca2+.Ca2+ binding affects the low-temperature fluorescence-emission maximum, leading to a decrease in its intensity and a blue shift of 1 nm. Similar spectral changes were obtained in the presence of Mg2+, possibly indicating a common binding domain for both cations. We interpret these observations as cation-induced conformational changes of the protein, which were reversible upon subsequent incubation in EDTA. Evidence is presented for the involvement of carboxyl groups in the coordination sphere of the bivalent cations. The possible structural and functional role of the protein is discussed.

Studies on the electron transfer from Tyr-161 of polypeptide D-1 to P680(+) in PS II membrane fragments from spinach

The functional connection between redox component Y z identified as Tyr-161 of polypeptide D-1 (Debus et al. 1988) and P680(+) was analyzed by measurements of laser flash induced absorption changes at 830 nm in PS II membrane fragments from spinach. It was found that neither DCMU nor the ADRY agent 2-(3-chloro-4-trifluoromethyl) anilino-3,5-dinitrothiophene (ANT 2p) affects the rate of P680(+) reduction by Y z under conditions where the catalytic site of water oxidation stays in the redox state S1. In contrast to that, a drastic retardation is observed after mild trypsin treatment at pH=6.0. This effect which is stimualted by flash illumination can be largely reversed by Ca(2+). The above mentioned data lead to the following conclusions: (a) the segment of polypeptide D-1 containing Tyr-161 and coordination sites of P680 is not allosterically affected by structural changes due to DCMU binding at the QB-site which is also located in D-1. (b) ANT 2p as a strong protonophoric uncoupler and ADRY agent does not modify the reaction coordinate of P680(+) reduction by Y z , and (c) Ca(2+) could play a functional role for the electronic and vibrational coupling between the redox groups Y z and P680. The electron transport from Y z to P680(+) is discussed within the framework of a nonadiabatic process. Based on thermodynamic considerations the reorganization energy is estimated to be in the order of 0.5 V.

Studies on the deconvolution of flash-induced absorption changes into the difference spectra of individual redox steps within the water-oxidizing enzyme system

The possibility to determine the difference spectra Δεi+1jλ of each univalent redox step Si→Si+1(i=0,...3) of the water-oxidizing enzyme system was analyzed by theoretical calculations and by measurements of 320 nm absorption changes induced by a train of saturating laser flashes (FWHM:7 ns) in PS II membrane fragments. It was found: a) Lipophilic quinones complicate the experimental determination of optical changes due the Si-state transitions because they lead to an additional binary oscillation probably caused by a reductant-induced oxidation of the Fe(2+) at the PS II acceptor side. b) In principle, a proper separation can be achieved at sufficiently high K3[Fe(CN)6] concentrations. c) An unequivocal deconvolution into the difference spectra Δεi+1jλ of flash train-induced optical changes which are exclusively due to Si-state transitions is impossible unless the Kok parameters α, β and [Si]0 can be determined by an independent method.Measurements of the oxygen yield induced by a flash train reveals, that in thylakoids and PS II membrane fragments Si is the stable state of dark adapted samples even at alkaline pH (up to pH=9). However, in PS II membrane fragments at pH>7.7 the misses probability α markedly increases, in contrast to the properties of intact thylakoids. Based on these data the possibility is discussed that an equilibrium exists of two types of S2-states with different properties.