Superoxide Contributes to the Rapid Inactivation of Specific Secondary Donors of the Photosystem II Reaction Center during Photodamage of Manganese-Depleted Photosystem II Membranes

Abstract

The role of superoxide in the mechanism of photoinactivation of the secondary donors of the reaction center of photosystem II membranes depleted of Mn by extraction with NH2OH plus EDTA (NH2OH/EDTA-PSII) was assessed. EPR analyses (g = 2 region) in continuous light, optical kinetic spectrophotometric analyses of P680+ and Car+, and AT-band emission measurements were made after various durations of weak and strong light treatment of NH2OH/EDTA-PSII in the presence and absence of superoxide dismutase, or of PSII electron acceptors to suppress superoxide formation. Additionally, flash-induced variable fluorescence of chlorophyll a and the capabilities of the membranes of photooxidize Mn2+ (in the presence of H2O2) via a high-affinity site (Km approximately 180 nM) and to carry out the photoactivation of the Mn-cluster were determined. In the absence of any additions to the NH2OH/EDTA-PSII membranes which were highly depleted of Mn, weak light treatment caused rapid (t1/2 approximately 20 s) and parallel losses of (a) the approximately 10 microseconds phase of P680+ reduction, which reflects the TyrZ-->P680+ reaction, (b) the amplitude of chlorophyll a variable fluorescence, (c) the capability to accumulate the TyrZ(+)-radical in continuous light, and (d) the capability to photooxidize Mn2+/H2O2 in continuous light. As reported previously [Blubaugh et al. (1991) Biochemistry 30, 7586-7597], a dark-stable 12-G-wide featureless EPR signal centered at g = 2.004 was formed rapidly during illumination. This signal previously was tentatively identified as a Car+ radical and was suggested to contribute to the quenching of chlorophyll a variable fluorescence and to the slowing of the TyrZ-->P680+ reaction. However, we failed to detect Car+ formation by sensitive optical spectrophotometry and obtained no definable evidence for either a quencher of fluorescence other than P680+ itself or a slowing of the TyrZ-->P680+ reaction. Addition of a saturating concentration (96 units/mL) of superoxide dismutase diminished the rate of photodamage(s) by approximately 30-fold, but did not abolish it completely. Superoxide dismutase similarly suppressed strong light-induced photodamages, causing the loss of capability to photooxidize Mn2+/H2O2, to carry out photoactivation, and to generate the AT-band emission as well as TyrZ+ EPR signal. In contrast to others, we found no evidence that the initial target(s) of photodamage is (are) different in weak versus strong light treatment. The totality of the results suggests that the initial event in either weak light or strong light photodamage of NH2OH/EDTA-PSII is a decoupling of the redox activity of TyrZ from P680.(ABSTRACT TRUNCATED AT 400 WORDS)

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

“…Asc is not only an alternative electron donor to PSII but also a direct scavenger of reactive oxygen species (ROS) produced during donor-sideinduced photoinhibition (e.g. Chen et al, 1995;Spetea et al, 1997). To ascertain that the Asc dependence correlates with the ability of Asc to act as a PSII electron donor, we incubated intact vtc2-3 leaves in DPC solution (1 mM DPC, an artificial electron donor of PSII, with no ROS-scavenging properties) before the heat treatment.…”

“…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.…”

Photoassembly of the water-oxidizing complex in photosystem II

“…2+ is an essential ion in the photoassembly reaction in all native PSIIWOCs examined to date. If Ca 2+ is left out of the solution, binding and photooxidation of many more Mn 2+ occurs (up to 20) to the apo-WOC-PSII protein and no O 2 evolution activity is observable [45,58]. The relative binding affinities of various divalent metals (Mg 2+ , Ca 2+ , Mn 2+ , Sr 2+ ), and the oxo-cation UO 2 2+ have been characterized [11,59,60].…”

PsbP Protein, But Not PsbQ Protein, Is Essential for the Regulation and Stabilization of Photosystem II in Higher Plants

“…Previous studies suggested that H 2 O 2 is generated from the PSII acceptor side when the oxidation of Q A 2 is perturbed (Schrö der and Å kerlund, 1990), or from the PSII donor side when the function of the Mn cluster is impaired (Schrö der and Å kerlund, 1986;Fine and Frasch, 1992;Hillier and Wydrzynski, 1993;Klimov et al, 1993;Arató et al, 2004). PSII depleted of the Mn cluster has also been reported to generate O 2 2 efficiently (Chen et al, 1995). Since both the acceptor side and donor side reactions of PSII were impaired in DPsbP leaves, H 2 O 2 may present around PSII and attack the Mn cluster.…”