Two Redox-Active β-Carotene Molecules in Photosystem II (original) (raw)
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Photosynthesis Research, 1991
Photoinhibition was analyzed in O2-evolving and in Tris-treated PS II membrane fragments by measuring flash-induced absorption changes at 830nm reflecting the transient P680 ÷ formation and oxygen evolution. Irradiation by visible light affects the PS II electron transfer at two different sites: a) photoinhibition of site I eliminates the capability to perform a 'stable' charge separation between P680 ÷ and QA within the reaction center (RC) and b) photoinhibition of site II blocks the electron transfer from Yz to P680 ÷. The quantum yield of site I photoinhibition (2-3 x 10 -7 inhibited RC/quantum) is independent of the functional integrity of the water oxidizing system. In contrast, the quantum yield of photoinhibition at site II depends strongly on the oxygen evolution capacity. In O2-evolving samples, the quantum yield of site II photoinhibition is about 10 -7 inhibited RC/quantum. After selective elimination of the O2-evolving capacity by Tris-treatment, the 2quantum yield of photoinhibi~ion at site II depends on the light intensity. At low intensity (<3 W/m ), the quantum yield is 10-inhibited RC/quantum (about 1000 times higher than in oxygen evolving samples). Based on these results it is inferred that the dominating deleterious effect of photoinhibition cannot be ascribed to an unique target site or a single mechanism because it depends on different experimental conditions (e.g., light intensity) and the functional status of the PS II complex.
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.
β-Carotene Redox Reactions in Photosystem II: Electron Transfer Pathway †
Biochemistry, 2001
Car, redox-active carotenoid; Chl, chlorophyll; ChlZ, redox-active chorophyll in PSII; Cyt b559, cytochrome b559; DM,-dodecyl maltoside; EPR, electron paramagnetic resonance; HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; MES, 2-(N-morpholino)ethanesulfonic acid; P680, photooxidizable chlorophyll in PSII; PSII, photosystem II; QA and QB, two quinones acting in series as electron acceptors in PSII; TyrZ, tyrosine acting as the electron donor to P680; TyrD, tyrosine acting as a side path donor to P680.
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.
Electron transfer in photosystem I
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2001
This mini-review focuses on recent experimental results and questions, which came up since the last more comprehensive reviews on the subject. We include a brief discussion of the different techniques used for time-resolved studies of electron transfer in photosystem I (PS I) and relate the kinetic results to new structural data of the PS I reaction centre.
Proceedings of the National Academy of Sciences, 2010
Photosystem I (PSI) is a large pigment-protein complex that unites a reaction center (RC) at the core with ∼100 core antenna chlorophylls surrounding it. The RC is composed of two cofactor branches related by a pseudo-C2 symmetry axis. The ultimate electron donor, P 700 (a pair of chlorophylls), and the tertiary acceptor, F X (a Fe 4 S 4 cluster), are both located on this axis, while each of the two branches is made up of a pair of chlorophylls (ec2 and ec3) and a phylloquinone (PhQ). Based on the observed biphasic reduction of F X , it has been suggested that both branches in PSI are competent for electron transfer (ET), but the nature and rate of the initial electron transfer steps have not been established. We report an ultrafast transient absorption study of Chlamydomonas reinhardtii mutants in which specific amino acids donating H-bonds to the 13 1 -keto oxygen of either ec3 A (PsaA-Tyr696) or ec3 B (PsaB-Tyr676) are converted to Phe, thus breaking the H-bond to a specific ec3 cofactor. We find that the rate of primary charge separation (CS) is lowered in both mutants, providing direct evidence that the primary ET event can be initiated independently in each branch. Furthermore, the data provide further support for the previously published model in which the initial CS event occurs within an ec2/ec3 pair, generating a primary ec2 þ ec3 − radical pair, followed by rapid reduction by P 700 in the second ET step. A unique kinetic modeling approach allows estimation of the individual ET rates within the two cofactor branches.
Species dependent differences in the secondaryelectron-donation reactions of photosystem II
Science Access, 2001
Under conditions when the primary electron-donation pathway from the O2-evolving complex in photosystem II (PSII) is inhibited, several alternate electron donors can be photooxidized. These include a monomeric chlorophyll (ChlZ), b-carotene (Car) and cytochrome b559. The involvement of these alternate electron donors and the redox role of a carotenoid in PSII are unique among photosynthetic reaction centers and stem from the very highly oxidizing intermediates that form in the process of water oxidation. ChlZ and Car photooxidation have been characterized by near-infrared absorbance, shifted-excitation Raman difference (SERDS) and EPR spectroscopies over a range of cryogenic temperatures from 30 to 120 K in both cyanobacterial PSII core complexes and spinach PSII membranes. The EPR signals of the individual species, previously not resolved at X-band frequency (9 GHz), are resolved at higher D-band frequency (130 GHz) in deuterated Synechococcus lividus PSII (Lakshmi, K.V., Reifler, ...