Nature’s water splitting cofactor passes through a series of catalytic intermediates (S0-S4) before O-O bond formation and O2 release. In the second last transition (S2 to S3) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all MnIV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S4. How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S3') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers (>50%) in (i) chemical-modified cofactors in which Ca2+ is exchanged with Sr2+; the Mn4O5Sr cofactor remains active, but the S2-S3 and S3-S0 transitions are slower than for the Mn4O5Ca cofactor; and (ii) upon addition of 3% vol/vol methanol; methanol is thought to act as a substrate water analog. The S3' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S3 signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S2 state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S3' cofactor is similar to the high spin form of the preceding S2 state, which contains a cuboidal Mn3O4Ca unit tethered to an external, 5-coordinate Mn ion (Mn4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn4. The interaction of ammonia with the cofactor is also discussed.

Summary• Cadmium (Cd 2+ ) is an environmental pollutant that causes increased reactive oxygen species (ROS) production. To determine the site of ROS production, the effect of Cd 2+ on ROS production was studied in isolated soybean (Glycine max) plasma membranes, potato (Solanum tuberosum) tuber mitochondria and roots of intact seedlings of soybean or cucumber (Cucumis sativus).• The effects of Cd 2+ on the kinetics of superoxide ( ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical ( • OH) generation were followed using absorption, fluorescence and spin-trapping electron paramagnetic resonance spectroscopy.• In isolated plasma membranes, Cd 2+ inhibited production. This inhibition was reversed by calcium (Ca 2+ ) and magnesium (Mg 2+ ). In isolated mitochondria, Cd 2+ increased and H 2 O 2 production. In intact roots, Cd 2+ stimulated H 2 O 2 production whereas it inhibited and • OH production in a Ca 2+ -reversible manner.• Cd 2+ can be used to distinguish between ROS originating from mitochondria and from the plasma membrane. This is achieved by measuring different ROS individually. The immediate (≤ 1 h) consequence of exposure to Cd 2+ in vivo is stimulation of ROS production in the mitochondrial electron transfer chain and inhibition of NADPH oxidase activity in the plasma membrane.

Photoinhibition and production of reactive oxygen species were studied in tobacco plants overexpressing the plastid terminal oxidase (PTOX). In high light, these plants was more susceptible to photoinhibition than wild-type plants. Also oxygenevolving activity of isolated thylakoid membranes from the PTOX-overexpressing plants was more strongly inhibited in high light than in thylakoids from wild-type plants. In contrast in low light, in the PTOX overexpressor, the thylakoids were protected against photoinhibition while in wild type they were significantly damaged. The production of superoxide and hydroxyl radicals was shown by EPR spin-trapping techniques in the different samples. Superoxide and hydroxyl radical production was stimulated in the overexpressor. Two-thirds of the superoxide production was maintained in the presence of DNP-INT, an inhibitor of the cytochrome b 6 f complex. No increase of the SOD content was observed in the overexpressor compared with the wild type. We propose that superoxide is produced by PTOX in a side reaction and that PTOX can only act as a safety valve under stress conditions when the generated superoxide is detoxified by an efficient antioxidant system. The plastid terminal oxidase (PTOX 2 or IMMUTANS) is a plastid-located plastoquinol:oxygen oxidoreductase (1-3). It is distantly related to the alternative oxidase (AOX) of the mitochondrial inner membrane. The active site of both oxidases, PTOX and AOX, comprises a non-heme di-iron center (4, 5). PTOX is a minor component (ϳ1% of PSII levels in Arabidopsis thaliana) of the thylakoid membrane and is located in the stroma lamellae (6, 7). PTOX plays an important role in carotenoid biosynthesis and seems to be involved in phytoene desaturation reactions (8 -13).The physiological importance of the role of PTOX as plastoquinol oxidase in alternative photosynthetic electron transport pathways is unclear. Evidence that PTOX acts as a plastoquinol oxidase was shown in tobacco plants, which constitutively expressed the A. thaliana PTOX gene (14). It has been suggested that PTOX may serve to keep the photosynthetic electron transport chain relatively oxidized. Exposure of plants to excess light may result in over-reduction of the plastoquinol pool and may lead to photoinhibition (15). However, no major role for PTOX in oxidizing the PQ pool was found in chlorophyll fluorescence assays when thylakoids from wt A. thaliana and the immutans mutant lacking PTOX were compared (16).Recently, several groups reported that the PTOX level increased under natural stress conditions in several species specialized to harsh environmental conditions. This was the case in Ranunculus glacialis, an alpine plant, when it was acclimated to high light and low temperature (17); in the halophyte Thellungiella halophila when it was exposed to salt stress (18); and in Brassica fruticulosa when it was exposed to elevated temperature and high light (19). Although these findings support the hypothesis that PTOX may serve as a safety valve under stress conditions, t...