Multiple Redox-Active Chlorophylls in the Secondary Electron-Transfer Pathways of Oxygen-Evolving Photosystem II (original) (raw)
Related papers
P680+ reduction in oxygen-evolving Photosystem II core complexes
Photosynthesis Research, 1996
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 ofa 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 ps and 90 #s with the possibility of a small component with a decay time of 20-40 ps. The precise origin of the 4 #s component remains uncertain but appears to be associated with the water-splitting center or its binding site while the 90 ps 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.
Two Redox-Active β-Carotene Molecules in Photosystem II
Biochemistry, 2003
Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b 559 (Cyt b 559), chlorophyll (Chl), and-carotene (Car) that are active under conditions when oxygen evolution is blocked such as in inhibited samples or at low temperature. Intermediates of the secondary electrontransfer pathways of PS II core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS II membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redoxactive carotenoids are formed upon low-temperature illumination. The Car + near-IR absorption peak varies in wavelength and width as a function of illumination temperature. Also, the rate of decay during dark incubation of the Car + peak varies as a function of wavelength. Factor analysis indicates that there are two spectral forms of Car + (Car A + has an absorbance maximum of 982 nm, and Car B + has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS II, we observe a shift of the Car + peak to shorter wavelength when oxidized tyrosine D (Y D •) is present in the sample that is explained by an electrostatic interaction between Y D • and a nearby-carotene that disfavors oxidation of Car B. The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS II is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.
Biochemistry, 1999
The effect of global 15 N or 2 H labeling on the light-induced P700 + /P700 FTIR difference spectra has been investigated in photosystem I samples from Synechocystis at 90 K. The small isotopeinduced frequency shifts of the carbonyl modes observed in the P700 + /P700 spectra are compared to those of isolated chlorophyll a. This comparison shows that bands at 1749 and 1733 cm-1 and at 1697 and 1637 cm-1 , which upshift upon formation of P700 + , are candidates for the 10a-ester and 9-keto CdO groups of P700, respectively. A broad and relatively weak band peaking at 3300 cm-1 , which does not shift upon global labeling or 1 H-2 H exchange, is ascribed to an electronic transition of P700 + , indicating that at least two chlorophyll a molecules (denoted P 1 and P 2) participate in P700 +. Comparisons of the 3 P700/P700 FTIR difference spectrum at 90 K with spectra of triplet formation in isolated chlorophyll a or in RCs from photosystem II or purple bacteria identify the bands at 1733 and 1637 cm-1 , which downshift upon formation of 3 P700, as the 10a-ester and 9-keto CdO modes, respectively, of the half of P700 that bears the triplet (P 1). Thus, while the P 2 carbonyls are free from interaction, both the 10a-ester and the 9-keto CdO of P 1 are hydrogen bonded and the latter group is drastically perturbed compared to chlorophyll a in solution. The Mg atoms of P 1 and P 2 appear to be five-coordinated. No localization of the triplet on the P 2 half of P700 is observed in the temperature range of 90-200 K. Upon P700 photooxidation, the 9-keto CdO bands of P 1 and P 2 upshift by almost the same amount, giving rise to the 1656(+)/1637(-) and 1717(+)/1697(-) cm-1 differential signals, respectively. The relative amplitudes of these differential signals, as well as of those of the 10a-ester CdO modes, appear to be slightly dependent on sample orientation and temperature and on the organism used to generate the P700 + /P700 spectrum. If it is assumed that the charge density on ring V of chlorophyll a, as measured by the perturbation of the 10a-ester or 9-keto CdO IR vibrations, mainly reflects the spin density on the two halves of the oxidized P700 special pair, a charge distribution ranging from 1:1 to 2:1 (in favor of P 2) is deduced from the measurements presented here. The extreme downshift of the 9-keto CdO group of P 1 , indicative of an unusually strong hydrogen bond, is discussed in relation with the models previously proposed for the PSI special pair.
FEBS Letters, 2001
Kinetic analysis using pulsed electron paramagnetic resonance (EPR) of photosynthetic electron transfer in the photosystem I reaction centres of Synechocystis 6803, in wildtype Chlamydomonas reinhardtii, and in site directed mutants of the phylloquinone binding sites in C. reinhardtii, indicates that electron transfer from the reaction centre primary electron donor, P700, to the iron^sulphur centres, Fe^S XaAaB , can occur through either the PsaA or PsaB side phylloquinone. At low temperature reaction centres are frozen in states which allow electron transfer on one side of the reaction centre only. A fraction always donates electrons to the PsaA side quinone, the remainder to the PsaB side.
Characterization of Carotenoid and Chlorophyll Photooxidation in Photosystem II
Biochemistry, 2001
Photosystem II (PSII) contains two accessory chlorophylls (Chl Z , ligated to D1-His118, and Chl D , ligated to D2-His117), carotenoid (Car), and heme (cytochrome b 559) cofactors that function as alternate electron donors under conditions in which the primary electron-donation pathway from the O 2evolving complex to P680 + is inhibited. The photooxidation of the redox-active accessory chlorophylls and Car has been characterized by near-infrared (near-IR) absorbance, shifted-excitation Raman difference spectroscopy (SERDS), and electron paramagnetic resonance (EPR) spectroscopy over a range of cryogenic temperatures from 6 to 120 K in both Synechocystis PSII core complexes and spinach PSII membranes. The following key observations were made: (1) only one Chl + near-IR band is observed at 814 nm in Synechocystis PSII core complexes, which is assigned to Chl Z + based on previous spectroscopic studies of the D1-H118Q and D2-H117Q mutants [
Electrogenicity of Electron and Proton Transfer at the Oxidizing Side of Photosystem II †
Biochemistry, 1997
The electrogenicity of electron and proton transfer at the oxidizing side of PSII was monitored by transmembrane electrochromism of carotenoids in thylakoids and, independently, by electrometry in oxygen-evolving photosystem II core particles. It yielded dielectrically weighted distances between cofactors. They were related to the one between Y Z ox and Q A -()100%). The electron transfer from Y Z to P 680 + ranged over a relative distance of 15%, while the one from Mn 4 to Y Z ox ranged over less than 3.5%. The latter result placed Mn 4 and Y Z at about the same weighted depth in the membrane. The oxidation of cofactor X by Y Z ox during S 2 w S 3 ranged over 10%. We tentatively attributed 7% to proton transfer into the lumen and 3% to electron transfer, in line with our notion that one proton is liberated from X ox itself. This placed X at the same depth in the membrane as Mn. Proton release upon the final oxidation of water during the oxygen-evolving step S 4 f S 0 revealed relative electrogenic components of 5.5% in core particles and between 10.5% (pH 7.4) and 2% (pH 6.2) in thylakoids. The former likely reflected proton transfer from bound water into the lumen and the latter to intraprotein bases that were created in the foregoing transitions. A tentative scheme for the arrangement of cofactors at the oxidizing side of photosystem II is presented. † Financial support by the Deutsche Forschungsgemeinschaft (SFB 171/A2), the Fonds der Chemischen Industrie, and from INTAS (INTAS-93-2852) is gratefully acknowledged. A.M. acknowledges additional funding by the Deutsche Forschungsgemeinschaft (Mu-1285/ 1-1 and Mu7-1285/1-2). Abstract published in AdVance ACS Abstracts, July 15, 1997.
Redox Potential of the Oxygen-Evolving Complex in the Electron Transfer Cascade of Photosystem II
The Journal of Physical Chemistry Letters, 2019
In photosystem II (PSII), water oxidation occurs in the Mn 4 CaO 5 cluster with the release of electrons via the redox-active tyrosine (TyrZ) to the reaction-center chlorophylls (P D1 /P D2). Using a quantum mechanical/molecular mechanical approach, we report the redox potentials (E m) of these cofactors in the PSII protein environment. The E m values suggest that the Mn 4 CaO 5 cluster, TyrZ, and P D1 /P D2 form a downhill electron transfer pathway. E m for the first oxidation step, E m (S 0 /S 1), is uniquely low (730 mV) and is ∼100 mV lower than that for the second oxidation step, E m (S 1 /S 2) (830 mV) only when the O4 site of the Mn 4 CaO 5 cluster is protonated in S 0. The O4-water chain, which directly forms a low-barrier H-bond with the Mn 4 CaO 5 cluster and mediates protoncoupled electron transfer in the S 0 to S 1 transition, explains why the second lowest oxidation state, S 1 , is the most stable and S 0 is converted to S 1 even in the dark.
Photochemical & Photobiological Sciences, 2005
We review our recent low-temperature absorption, circular dichroism (CD), magnetic CD (MCD), fluorescence and laser-selective measurements of oxygen-evolving Photosystem II (PSII) core complexes and their constituent CP43, CP47 and D1/D2/cytb 559 sub-assemblies. Quantitative comparisons reveal that neither absorption nor fluorescence spectra of core complexes are simple additive combinations of the spectra of the sub-assemblies. The absorption spectrum of the D1/D2/cytb 559 component embedded within the core complex appears significantly better structured and red-shifted compared to that of the isolated sub-assembly. A characteristic MCD reduction or 'deficit' is a useful signature for the central chlorins in the reaction centre. We note a congruence of the MCD deficit spectra of the isolated D1/D2/cytb 559 sub-assemblies to their laser-induced transient bleaches associated with P680. A comparison of spectra of core complexes prepared from different organisms helps distinguish features due to inner light-harvesting assemblies and the central reaction-centre chlorins. Electrochromic spectral shifts in core complexes that occur following low-temperature illumination of active core complexes arise from efficient charge separation and subsequent plastoquinone anion (Q A − ) formation. Such measurements allow determinations of both charge-separation efficiencies and spectral characteristics of the primary acceptor, Pheo D1 . Efficient charge separation occurs with excitation wavelengths as long as 700 nm despite the illuminations being performed at 1.7 K and with an extremely low level of incident power density. A weak, homogeneously broadened, charge-separating state of PSII lies obscured beneath the CP47 state centered at 690 nm. We present new data in the 690-760 nm region, clearly identifying a band extending to 730 nm. Active core complexes show remarkably strong persistent spectral hole-burning activity in spectral regions attributable to CP43 and CP47. Measurements of homogeneous hole-widths have established that, at low temperatures, excitation transfer from these inner light-harvesting assemblies to the reaction centre occurs with ∼70-270 ps −1 rates, when the quinone acceptor is reduced. The rate is slower for lower-energy sub-populations of an inhomogeneously broadened antenna (trap) pigment. The complex low-temperature fluorescence behaviour seen in PSII is explicable in terms of slow excitation transfer from traps to the weak low-energy charge-separating state and transfer to the more intense reaction-centre excitations near 685 nm. The nature and origin of the charge-separating state in oxygen-evolving PSII preparations is briefly discussed. Fig. 1 Schematic representation (adapted from Ferreira et al. 5 ) of chlorins and redox cofactors in the PSII reaction centre, viewed along the membrane plane and with the protein removed.