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

Abstract

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.

Destruction of a single chlorophyll is correlated with the photoinhibition of photosystem II with a transiently inactive donor side.

“…Thompson and Brudvig (50) assumed that specific antenna Chl not essential for the electron transfer are destroyed first. However, light saturation measurements yielded no evidence for a destruction of antenna Chl in PSII from Chlamydomonas (this work) or in grana membranes from higher plants (7,12,51). On the other hand, Telfer et al (23) proposed that the destruction of P680 is the cause of photoinhibition in PSII-RC.…”

The Physiological Role of Ascorbate as Photosystem II Electron Donor: Protection against Photoinactivation in Heat-Stressed Leaves

“…Experiments on PSII preparations and on leaves with chemically inactivated OECs indicate that the very high sensitivity of PSII to light appears to be caused by the impaired electron donation from the OEC. This, in turn, results in strong accumulation of highly oxidizing radicals, P680 + , Tyr Z + , and superoxide (Chen et al, 1995) or hydroxyl radicals (Spetea et al, 1997), and leads to a rapid inactivation and degradation of PSII reaction centers (Callahan et al, 1986;Blubaugh and Cheniae, 1990;Jegerschöld and Styring, 1996). This type of photodamage is called weak light or donor-side-induced photoinhibition.…”

Mechanisms of Photodamage and Protein Degradation During Photoinhibition of Photosystem II

“…For example, susceptibility of PS II to photoinduced inactivation and protein degradation has been shown in hydroxylamine-treated leaf segments (Callahan et al, 1986), in leaves subjected to cold stress (Wang et al, 1992a), in Tris- (Wang et al, 1992b) and hydroxylamine-treated thylakoids (Blubaugh et al, 1990(Blubaugh et al, , 1991, and in thylakoids (Critchley et al, 1984;Jegerschöld et al, 1990). Eckert et al (1991) showed that the quantum yield of electron transport inactivation increases 1000-fold inTris-washed PS II enriched membranes inhibited reaction centers per quantum) as compared to normal oxygen evolving PS II membranes inhibited reaction centers per quantum).…”

“…This is because the LF-1 mutant does not have water splitting activity due to its inability to process the carboxy-terminus of the D1 protein (Bowyer et al, 1992), while the absence of the 33 kDa extrinsic protein of PS II in the psbO-less Synechocystis mutant perturbs its donor side activity (Vass et al, 1992a). It is generally agreed that the site of donor side inactivation is between the manganese cluster and P680 (Eckert et al, 1991;Jegerschöld et al, 1990;Blubaugh and Cheniae 1990;Blubaugh et al, 1991). coworkers (1990,1991) resolved three kinetically different phases of inactivation in hydroxylamine treated PS II membranes.…”