Silicomolybdate substitutes for the function of a primary electron acceptor and stabilizes charge separation in the photosystem II reaction center complex

Low-temperature (77 K) phosphorescence of triplet chlorophyll in isolated reaction centers of photosystem II

Phosphorescence characterized by the main emission band at 952 ± 1 nm (1.30 eV), the lifetime of 1.5 ± 0.1 ms and the quantum yield nearly equal to that for monomeric chlorophyll a in aqueous detergent dispersions, has been detected in isolated reaction centers (RCs) of spinach photosystem II at 77 K. The excitation spectrum shows maxima corresponding to absorption bands of chlorophyll a, pheophytin a, and β-carotene. The phosphorescence intensity strongly depends upon the redox state of RCs. The data suggest that the phosphorescence signal originates from the chlorophyll triplet state populated via charge recombination in the radical pair [Formula: see text].

Circular dichroism of the peripheral chlorophylls in photosystem II reaction centers revealed by electrochemical oxidation

Visible absorption spectra and circular dichroism (CD) of the red absorption band of isolated photosystem II reaction centers were measured at room temperature during progressive bleaching by electrochemical oxidation, in comparison with aerobic photochemical destruction, and with anaerobic photooxidation in the presence of the artificial electron acceptor silicomolybdate. Initially, selective bleaching of peripheral chlorophylls absorbing at 672 nm was obtained by electrochemical oxidation at +0.9 V, whereas little selectivity was observed at higher potentials. Illumination in the presence of silicomolybdate did not cause a bleaching but a spectral broadening of the 672-nm band was observed, apparently in response to the oxidation of carotene. The 672-nm absorption band is shown to exhibit a positive CD, which accounts for the 674-nm shoulder in CD spectra at low temperature. The origin of this CD is discussed in view of the observation that all CD disappears with the 680-nm absorption band during aerobic photodestruction.

What is beta-carotene doing in the photosystem II reaction centre?

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two beta-carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron-donor chlorophylls, which are known as P(680). The beta-carotenes cannot be close enough to P(680) for triplet quenching because that would also allow extremely fast electron transfer from beta-carotene to P(+)(680), preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the beta-carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P(680). Only when electron donation from water is disturbed does beta-carotene become oxidized. One beta-carotene can mediate cyclic electron transfer via cytochrome b559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.

Bicarbonate requirement for the water-oxidizing complex of photosystem II

It is well established that bicarbonate stimulates electron transfer between the primary and secondary electron acceptors, Q(A) and Q(B), in formate-inhibited photosystem II; the non-heme Fe between Q(A) and Q(B) plays an essential role in the bicarbonate binding. Strong evidence of a bicarbonate requirement for the water-oxidizing complex (WOC), both O2 evolving and assembling from apo-WOC and Mn2+, of photosystem II (PSII) preparations has been presented in a number of publications during the last 5 years. The following explanations for the involvement of bicarbonate in the events on the donor side of PSII are considered: (1) bicarbonate serves as an electron donor (alternative to water or as a way of involvement of water molecules in the oxidative reactions) to the Mn-containing O2 center; (2) bicarbonate facilitates reassembly of the WOC from apo-WOC and Mn2+ due to formation of the complexes MnHCO3+ and Mn(HCO3)2 leading to an easier oxidation of Mn2+ with PSII; (3) bicarbonate is an integral component of the WOC essential for its function and stability; it may be considered a direct ligand to the Mn cluster; (4) the WOC is stabilized by bicarbonate through its binding to other components of PSII.

Photosystem II of green plants: on the possible role of retarded protonic relaxation in water oxidation1

Photosystem II (PSII) of green plants and cyanobacteria uses energy of light to oxidize water and to produce oxygen. The available estimates of the oxidizing potential of P680+, the primary donor of PSII, yield value of about 1.15 V. Two main factors are suggested to add up and engender this high oxidizing potential, namely: (1) the electrostatic influence dominated by Arg-181 of the D2 subunit which elevates the oxidizing potential of P680+ up to 1 V, some 0.1 V above the Em value of a hydrogen-bonded chlorophyll a; and (2) the dynamic component of 0.10-0.15 V due to the experimentally demonstrated retarded protonic relaxation at the P680 site.

Bicarbonate is an essential constituent of the water-oxidizing complex of photosystem II

It is shown that restoration of photoinduced electron flow and O2 evolution with Mn2+ in Mn-depleted photosystem II (PSII) membrane fragments isolated from spinach chloroplasts is considerably increased with bicarbonate in the region pH 5.0-8.0 in bicarbonate-depleted medium. In buffered solutions equilibrated with the atmosphere (nondepleted of bicarbonate), the bicarbonate effect is observed only at pH lower than the pK of H2CO3 dissociation (6.4), which indicates that HCO3- is the essential species for the restoration effect. The addition of just 2 Mn2+ atoms per one PSII reaction center is enough for the maximal reactivation when bicarbonate is present in the medium. Analysis of bicarbonate concentration dependence of the restoration effect reveals two binding sites for bicarbonate with apparent dissociation constant (Kd) of approximately 2.5 microM and 20-34 microM when 2,6-dichloro-p-benzoquinone is used as electron acceptor, while in the presence of silicomolybdate only the latter one remains. Similar bicarbonate concentration dependence of O2 evolution was obtained in untreated Mn-containing PSII membrane fragments. It is suggested that the Kd of 20-34 microM is associated with the donor side of PSII while the location of the lower Kd binding site is not quite clear. The conclusion is made that bicarbonate is an essential constituent of the water-oxidizing complex of PSII, important for its assembly and maintenance in the functionally active state.

Purification of a Photosystem II reaction center from a thermophilic cyanobacterium using immobilized metal affinity chromatography

Oxygen-evolving PS II particles from the thermophilic cyanobacterium Synechococcus elongatus are partially purified by centrifugation on a sucrose gradient and are bound to a Chelating Sepharose column loaded with Cu(2+) ions. Bound particles are then transformed into PS II RC complexes by two washing steps. First, washing with a phosphate buffer (pH=6.5) containing 0.02% of SB 12 removes the rest of phycobilins and leaves pure PS II core particles on the column. Second, washing with a phosphate buffer (pH=6.2) containing 0.2 M LiClO4 and 0.05% of DM removes CP 47 and CP 43 and leaves bare PS II RC complexes on the column. These are then eluted with a phosphate buffer containing 1% of dodecylmaltoside (DM). The molar ratio of pigments in the eluate changes with the progress of elution but around the middle of the elution period a nearly stable ratio is maintained of Chl a: Pheo a: Car: Cyt b 559 equal to 2.9: 1: 0.9: 0.8. In these fractions the photochemical separation of charges could be demonstrated by accumulation of reduced pheophytin (ΔA of 430-440 nm) and by the flash induced formation of P680(+) (ΔA at 820 nm). The relatively slow relaxation kinetics of the latter signal (t1/2 ≈ 1 ms) may suggest that in a substantial fraction of the RCs QA remains bound to the complex.

Irreversible light-induced formation of P680+ and reduced cytochrome b559 in the D1-D2-Cyt b-559 complex at low temperature

Cytochrome b559 in D1-D2-Cyt b-559 complexes from spinach can be photoreduced in the presence of DBMIB at a temperature of 180-240 K upon continuous illumination. The reduction of Cyt b-559 is accompanied by oxidation of P680. At 240 K recombination of P680+ and reduced Cyt b-559 is complete in several seconds. At 220 K and below, the state P680+Cyt b-559red can be trapped for a long time. This indicates that the photoreduced heme is incapable of electron transfer to P680+ at 220 K and below. On the other hand, the chemically reduced heme of Cyt b-559 is oxidized by P680+ at 77 K. These results are consistent with the presence of two kinds of Cyt b-559 hemes in D1-D2-Cyt b-559 complexes which participate in different ways in the photochemical reactions.

Effect of trypsin on D1/D 2-cytochrom b 559 Photosystem 2 reaction center complex and reaction center from Rhodopseudomonas viridis

Proteolytic enzyme (trypsin) was used to structurally alter the RCs isolated from plant and bacterium as a way of probing the relation between structure (chromophore-apoprotein interactions) and function (photochemical activity). It was found that neither spectral characteristics (absorption spectrum, the 4th derivative of absorption spectrum) nor photochemical activity (pheophytine photoreduction, P680 photooxidation, etc.) were changed dramatically in D1/D2/cytochrom b 559 PS 2 reaction center complex digested with trypsin. The PS 2 RC treated with trypsin migrates by one green band during electrophoresis with dodecylmaltoside. The peptides with a molecular mass higher than 3-4 kDa were not separated from PS 2 RC. These data indicate that digestion of D1 and D2 proteins does not disturb yet the conformation of peptides or their interactions in so-called 'core' of RC and the native state of pigments. In contrast to that, the RC from Rhodopseudomonas viridis treated with enzyme has changed absorption spectrum and lost photochemical activity. The stability of the bacterial RC increased after exchange of LDAO by dodecylmaltoside.