Redox study of electron donation to P-680 in Photosystem II

Kinetics of photoinhibition in hydroxylamine-extracted photosystem II membranes: relevance to photoactivation and sites of electron donation

Kinetic analyses were made of the effects of weak-light photoinhibition on the capacity of NH2OH-extracted photosystem II membranes to photooxidize the exogenous electron donors Mn2+, diphenylcarbazide, and I- or to assemble functional water-oxidizing complexes during photoactivation. The loss of capacity for photooxidation of the donors showed two first-order components (half-times of 2-3 min and 1-4 h) with relative amplitudes dependent on the donor, suggesting two photodamageable sites of electron donation (sites 1 and 2, respectively), a conclusion confirmed by analyses of velocity curves of electron donation by each donor. All of the donors appear to be oxidized preferentially by site 1 both at saturating and at limiting light intensity; however, the contribution by site 2 was nearly comparable in saturating light. Loss of photoactivation also exhibited biphasic kinetics, with components having half-times of approximately 0.8 and 3.2 min. The major component (t1/2 = 3.2 min) corresponded to loss of site 1; essentially no photoactivation was observed after its loss. From these and other analyses, we conclude (1) the relative contributions of site 1 and site 2 to the photooxidation of various exogenous electron donors is determined largely by the rates of equilibration of the donors with the two sites, and (2) only site 1 contributes to photoactivation of the water-oxidizing complex. Site 1 is attributed to tyrosine Z of the reaction center's D1 polypeptide. The molecular identity of site 2 is unknown but may be tyrosine D of the D2 polypeptide.

Electron-spin resonance studies of the bound iron-sulfur centers in Photosystem I. II. Correlation of P-700 triplet production with urea/ferricyanide inactivation of the iron-sulfur clusters

Photosystem I charge separation in a subchloroplast particle isolated from spinach was investigated by electron spin resonance (ESR) spectroscopy following graduated inactivation of the bound iron-sulfur centers by urea-ferricyanide treatment. Previous work demonstrated a differential decrease in iron-sulfur centers A, B and X which indicated that center X serves as a branch point for parallel electron flow through centers A and B (Golbeck, J.H. and Warden, J.T. (1982) Biochim. Biophys. Acta 681, 77-84). We now show that during inactivation the disappearance of iron-sulfur centers A, B, and X correlates with the appearance of a spin-polarized triplet ESR signal with [D] = 279 X 10(-4) cm-1 and [E] = 39 X 10(-4) cm-1. The triplet resonances titrate with a midpoint potential of +380 +/- 10 mV. Illumination of the inactivated particles results in the generation of an asymmetric ESR signal with g = 2.0031 and delta Hpp = 1.0 mT. Deconvolution of the P-700+ contribution to this composite resonance reveals the spectrum of the putative primary acceptor species A0, which is characterized by g = 2.0033 +/- 0.0004 and delta Hpp = 1.0 +/- 0.2 mT. The data presented in this report do not substantiate the participation of the electron acceptor A1 in PS I electron transport, following destruction of the iron-sulfur cluster corresponding to center X. We suggest that A1 is closely associated with center X and that this component is decoupled from the electron-transport path upon destruction of center X. The inability to photoreduce A1 in reaction centers lacking a functional center X may result from alteration of the reaction center tertiary structure by the urea-ferricyanide treatment or from displacement of A1 from its binding site.

Phenylglyoxal modification of the Photosystem II reaction center

Time-resolved spectroscopic techniques, including optical flash photolysis and electron spin resonance (ESR), have been used in conjunction with fluorescence-induction and dye-reduction assays to monitor electron transport in Photosystem II (PS II) subchloroplast particles incubated with the covalent modifier, phenylglyoxal. Phenylglyoxal-modified digitonin (D-10) particles from spinach are characterized by a high initial fluorescence yield (Fi) and an abolition of the variable component of fluorescence (Fv); an inhibition of PS-II-mediated reduction of dichlorophenol indophenol (DPIP) by sym-diphenylcarbazide; an abolition of flash-induced absorption transients (t1/2 greater than 2 microseconds) at 820 nm attributed to the primary electron donor, P-680+; the inhibition of photoreduction of the acceptor Qa; and the elimination of the ESR Signal 2s and Signal 2f. These observations suggest the critical participation of specific arginine residues on both the oxidizing and reducing sides of Photosystem II and also implicate phenylglyoxal as a quinone-binding site inhibitor (Golbeck, J.H. and Warden, J.T. (1984) Biochim. Biophys. Acta 767, 263-271).

On the mechanism of linolenic acid inhibition in Photosystem II

Recent studies in our laboratory have reexamined the interaction of the unsaturated fatty acid, linolenic acid, with Photosystem II and have documented two principal regions of inhibition: one associated with the donor complex (Signal 2f or D1) to the reaction center, and the other located on the reducing side between pheophytin and Qa (Golbeck, J.H. and Warden, J.T. (1984) Biochim. Biophys. Acta 767, 263-271). A further characterization of fatty acid inhibition of secondary electron transport in Photosystem II at room and cryogenic temperatures is presented in this paper. These studies demonstrate that linolenic acid, and related fatty acid analogs, eliminate the transient absorption increase at 320 nm, attributed to Qa-; abolish the production, either chemically or photochemically, of the ESR signal (Q-Fe) associated with the bound quinone acceptor, Qa-; and prevent the photooxidation of Signal 2(1t)(D1) at cryogenic temperature. Linolenic-acid-treated samples are characterized by a high initial fluorescence yield (Fi) equivalent to the maximum level of fluorescence (Fmax); however, the spin-polarized triplet, associated with the reaction-center electron donor, P-680, is observed only in inhibited samples that have been prereduced with sodium dithionite. These results suggest the presence of an additional acceptor intermediate between pheophytin and Qa. The donor-assisted photoaccumulation of pheophytin anion in Photosystem II particles, as monitored by the decline of fluorescence yield, is inhibited by linolenic acid. Redox titrations of the fluorescence yield in control and inhibited preparations demonstrate that the midpoint potential for the primary acceptor for Photosystem II is insensitive to the fatty acid (Em approximately -583 mV) and thus indicate that primary photochemistry is functional during linolenic-acid inhibition. These data are consistent with the hypothesis that unsaturated fatty acids inhibit secondary electron transport in Photosystem II via displacement of endogenous quinone from quinone-binding peptides.

Modification of oxygen evolving center by Tris-washing

Tris-washing inhibits the O2-evolving center of chloroplasts and their particles specifically and reversibly, and it was applied to many investigations on O2-evolving center and PS II reaction center. In this review are introduced the various photosynthetic investigations in which Tris-washing was applied and are also discussed briefly on the site and the mechanism of Tris-inactivation, properties of P680 and Z, characteristic change in fluorescence and delayed light emission, and reactivation of O2-evolving center by DCPIP.H2-treatment and photo-reactivation of Tris-washed chloroplasts and their particles.