Accelerated Degradation of the D2 Protein of Photosystem II Under Ultraviolet Radiation

The D2 protein of photosystem II is relatively stable in vivo under photosynthetic active radiation, but its degradation accelerates under UVB radiation. Little is known about accelerated D2 protein degradation. We characterized wavelength dependence and sensitivity toward photosystem II inhibitors. The in vivo D2 degradation spectrum resembles the pattern for the rapidly turning over D1 protein of photosystem II, with rates being maximal in the UVB region. We propose that D2 degradation, like D1 degradation, is activated by distinct photosensitizers in the UVB and visible regions of the spectrum. In both wavelength regions, photosystem II inhibitors that are known to be targeted to the D1 protein affect D2 degradation. This suggests that degradation of the two proteins is coupled, D2 degradation being influenced by events occurring at the Q_B niche on the D1 protein.     

Effect of Supplementary UVB Radiation on Chlorophyll Synthesis and Accumulation of Photosystems during Chloroplast Development in Spirodela oligorrhiza

Abstract— Although the effects of ultraviolet B (UVB, 290–320 nm) radiation have been studied in plants extensively, little is known about the potential impacts on maturation of chloroplasts. To address this problem, the effects of supplementary UVB on chloroplast development were examined in the aquatic higher plant Spirodela oligorrhiza. Dark-grown Spirodela-containing proplastids were exposed to photosynthetically active radiation (PAR) and ultraviolet A (UVA, 320–400 nm), plus supplementary UVB equivalent to 1% of PAR on a photon basis. The biosynthesis and assembly of chlorophyll (Chi) into reaction centers was followed for 4 days in situ by low temperature (77 K) Chi fluorescence. Impacts on chloroplast development were detected after only 1 h incubation in light with supplementary UVB. Fluorescence emission signals from Chi associated with the photosystem (PS) II antenna, PSII reaction centers and PSI reaction centers were detected at the same time with or without UVB, but the magnitude of PS fluorescence was diminished up to 60% in plants incubated in UVB. The Chi content was also lower in UVB-treated plants, but to a lesser degree than anticipated by low temperature fluorescence, suggesting lack of organization and/or association of Chi with PS. Electron transport, measured with room temperature fluorescence induction, was not consistently different in plants exposed to UVB. These results suggest that with UVB, fewer and/or smaller PS form during chloroplast development, but there is not a large inhibition of Chi synthesis or PSII activity.     

INACTIVATION OF THE ACCEPTOR SIDE AND DEGRADATION OF THE D1 PROTEIN OF PHOTOSYSTEM II BY SINGLET OXYGEN PHOTOGENERATED FROM THE OUTSIDE

Abstract— The possible association of photodynamic sensitization with photoinhibition damage to the photosystem II complex (PS II) has been investigated using isolated intact thylakoids from pea leaves. For this study singlet oxygen (¹O₂), photoproduced by endogenous chromophores that are independent of the function of PS II, was assumed to be the major reactive intermediate involved in the photoinhibition process. When thylakoid samples preincubated with rose bengal were subjected to exposure to relatively weak green light (500–600 nm) under aerobic conditions, PS II was severely damaged. The pattern of the rose bengal-sensitized inhibition of PS II was similar to that of high light-induced damage to PS II: (1) the secondary quinone (Q_B)-dependent electron transfer through PS II is inactivated much faster than the Q_B-independent electron flow, (2) PS II activity is lost prior to degradation of the D1 protein, (3) diuron, an herbicide that binds to the Q_B domain on the D1 protein, prevents D1 degradation, and (4) PS II is damaged to a greater extent by the deuteration of thylakoid suspensions but to a lesser extent by the presence of histidine. Furthermore, it was observed that destroying thylakoid Fe-S centers resulted in a marked reduction of high light-induced PS II damage. These results may suggest that the primary processes of photoinhibition are mediated by ¹O₂ and that Fe-S centers, which are located in some membrane components, but not in PS II, play an important role in photogenerating the activated oxygen immediately responsible for the initiation of photodamage to PS II.     

KINETIC STUDIES ON THE STABILIZATION OF THE PRIMARY RADICAL PAIR P680+ Pheo- IN DIFFERENT PHOTOSYSTEM II PREPARATIONS FROM HIGHER PLANTS*

Abstract— The stabilization of the primary radical pair P680+ pheophytin (Pheo)^- through rapid electron transfer from Pheo to the special plastoquinone of photosystem II (PS II), Q_A, was analyzed on the basis of time-resolved (40 ps) UV-absorption changes detected in different PS II preparations from higher plants. Lifetime measurements of¹Chi* fluorescence by single photon counting and a numerical analysis of the redox reactions revealed (1) at exciton densities required for light saturation of the stable charge separation, annihilation processes dominate the excited state decay leading to very similar lifetimes of ¹Chi* in systems with open and closed reaction centers and (2) the difference of absorption changes induced by actinic flashes of comparatively high photon density in samples with open and photochemically closed reaction centers, respectively, provides a suitable measure of the rate constant of QA formation. Conclusion 2 was confirmed in PS II membrane fragments by measurements at three wavelengths (280 nm, 292 nm and 325 nm) where the difference spectrum of Q^-_A formation exhibits characteristic features. The numerical evaluation of the experimental data led to the following results: (1) the rate constant of Q^-_A formation was found to be (300 ± 100 ps)^-1 in PS II membrane fragments and PS II core complexes deprived of the distal and proximal antenna and (2) an iron depletion treatment of membrane fragments does not affect these kinetics. The implications of these results are briefly discussed in terms of the PS II reaction pattern.     

Alternation in Photosynthetic Characteristics of Anabaena doliolum Following Exposure to UVB and Pb

Abstract— Anabaena doliolum , when exposed to either ultraviolet-B (UVB) radiation or Pb, showed reduced growth rate, carbon fixation, O₂-evolution, photosynthetic electron transport activity and ATP pool size. The rate of respiration was found to increase in UVB-treated cells; this increase was more pronounced in the cells exposed to UVB and Pb simultaneously. The UVB-induced inhibition of 2,6-dichlorophenol indophenol (DCPIP) photoreduction and lowering of chlorophyll a fluorescence could not be reversed by artificial electron donors (diphenyl carbazide, NH₂OH and MnCl₂). These electron donors, however, substantially reversed the inhibition caused by Pb, thereby suggesting that UVB primarily inhibits the photosys-tem II (PS II) reaction center and Pb arrests the electron flow at the water splitting site. Nevertheless, the suppressed fluorescence intensity and the reduced emission and excitation peaks of phycobilisomes indicate the involvement of Pb in inhibition of PS II. All combinations of UVB and Pb inhibited the different metabolic processes in a synergistic manner.     

THE PHOTOSYNTHETIC APPARATUS OF Acetabularia mediterranea GROWN UNDER RED OR BLUE LIGHT. BIOPHYSICAL QUANTIFICATION and CHARACTERIZATION OF PHOTOSYSTEM II and ITS CORE COMPONENTS

Abstract— The photosynthetic activity of white light-grown Acetabularia mediterranea Lamouroux (= A. acetabulum (L.) Silva) decreases under continuous red light to less than 20% within 3 weeks. Subsequent blue light reactivates photosynthesis within a relatively short period of 3 days. In a former publication (Wennicke and Schmid, Plant Physiol. 84 ,1252–1256, 1987) we have shown that the regulated rate limiting step, which is an immediate light driven reaction, is part of photosystem II (PS II). The following biophysical properties of PS II were analyzed in thylakoids isolated from algae grown 3 weeks under either blue or red light with or without subsequent 3 days of blue light illumination: (a) fluorescence induction in the short time domain dominated by Q_A reduction, (b) the slow fluorescence decline reflecting pheophytin photoaccumulation, (c) absorption changes at 320 and 830 nm under repetitive flash excitation as indicator for the turnover of Q_A and P₆₈₀, respectively, (d) oscillation pattern of the oxygen yield by a flash train in dark adapted samples and (e) the binding capacity for atrazine. None of these PS II functions were severely affected, but a minor impairment of20–30% was observed in the thylakoids from algae grown for 3 weeks in red irradiation. The changes do not fully account for the drastic reduction of the electron transport through PS II which was 80% after red light treatment. Therefore, the regulated rate-limiting step appears to not be mainly located in the PS II core complex itself. It seems likely that the regulation process predominantly comprises the antenna system.     

Why nature chose Mn for the water oxidase in Photosystem II

Nature performs a vital but uniquely energetic reaction within Photosystem II (PS II), resulting in the oxidation of two water molecules to yield O(2) and bio-energetic electrons, as reducing equivalents. Almost all life on earth ultimately depends on this chemistry, which occurs with remarkable efficiency within a tetramanganese and calcium cluster in the photosystem. The thermodynamic constraints for the operation of this water oxidising Mn(4)/Ca cluster within PS II are discussed. These are then examined in the light of the known redox chemistry of hydrated Mn-oxo systems and relevant model compounds. It is shown that the latest high resolution crystal structure of cyanobacterial PS II suggests an organization of the tetra-nuclear Mn cluster that naturally accommodates the stringent requirements for successive redox potential constancy with increasing total oxidation state, which the enzyme function imposes. This involves one region of the Mn(4)/Ca cluster being dominantly involved with substrate water binding, while a separate, single Mn is principally responsible for the redox accumulation function. Recent high level computational chemical investigations by the authors strongly support this, with a computed pattern of Mn oxidation states throughout the catalytic cycle being completely consistent with this interpretation. Strategies to design synthetic, bio-mimetic constructs utilising this approach for efficient electrolytic generation of hydrogen fuel within Artificial Photosynthesis are briefly discussed.     

Ultraviolet Radiation Effects on a Microscopic Green Alga and the Protective Effects of Natural Dissolved Organic Matter

Abstract— The population and photosynthetic responses of a microscopic green alga ( Selenastrum capricornutum ) to realistic levels of UV radiation (UVA and UVB) were assessed in natural lake waters of different dissolved organic carbon (DOC) concentration. Specific growth rates and photosynthetic competence (as reflected by F_v/F_m [measure of maximal quantum efficiency of photosystem II] and t_1/2 [estimate of electrons transported to the plastoquinone pool] measured by in vivo variable chlorophyll a fluorescence) were compared between two exposure levels of UVR and two concentrations of DOC (2.5 mg C L⁻¹ 7.7 mg C L⁻¹). Exposure periods of 6–9 days (five to nine generations) were used. Exposure to UVA primarily affected the efficiency of photosystem II, as evidenced by significant decreases of F_v/F_m but not growth rates or t_1/2 Exposure to UVB, in the presence of UVA, did not cause significant additional decreases of F_v/F_m but did diminish growth rates. In the low DOC water, t_1/2 was also diminished, suggesting different proximate sites of action from those for UVA. The high DOC water decreased the effective exposure to both UVA and UVB and diminished the negative impact of UV radiation on the cells, but the apparent protection was not explicable solely by the shading action of the DOC. Control values for F_v/F_m, growth rates and t_1/2 were all lower in the high DOC water, suggesting a negative side effect to the apparent protective action of the DOC against UVB.     

The Influence of Photosynthetically Active Radiation on the Effects of Ultraviolet-B Radiation on Arabidopsis thaliana

Ultraviolet-B (UVB;280–320 nm) radiation is a small but biologically significant portion of the solar spectrum reaching the earth's surface. Research interests have been fostered because UVB has been increasing in recent years due to depletion of stratospheric ozone. Ultraviolet-B that penetrates into plant tissue may damage important cellular macromolecules. Although there has been considerable research on the effects of UVB on plants, the influence of the level of photosynthetically active radiation (PAR;400–700 nm) on effects of UVB requires further definition as a prelude to studies of UVB sensitivity and defense mechanisms. Arabidopsis thaliana wildtype ecotype Landsberg erecta (LER), which is relatively insensitive to UVB, and the relatively sensitive LER-based mutant transparent testa-5 (tt5), were grown under 100 or 250 μmol m^?2 s^?1 PAR and then exposed to O or 7 kJ m^?2 day ^?1 UVB_BE under these PAR levels. Plants exposed to UVB had reduced dry weight and leaf area and higher levels of UV-absorbing compounds in leaf tissue. The level of PAR did influence the effects of UVB, with the higher level of PAR prior to UVB exposure reducing sensitivity of LER to UVB. In contrast to other studies, higher PAR supplied simultaneously with UVB increased rather than decreased sensitivity of both genotypes to UVB. These results demonstrate the importance of controlling and comparing PAR levels when undertaking studies of UVB sensitivity, as effects of UVB on plants are influenced by the PAR levels plants are growing under prior to and during exposure to UVB.     

Biologically Active Radiation in the Vicinity of a Single Tree

The horizontal photon flux density of photosynthetically active radiation (PAR) and flux density of ultraviolet A (UVA) and ultraviolet B (UVB) radiation were measured in the vicinity of isolated single trees during the summer of 1996. Measurements were made under shade and sunlit conditions along a transect aligned with the solar disk and the tree trunk. Flux density measurements were normalized by the flux density at a reference location away from the tree. Results showed (1) a more rapid decline in the flux density of UVA and UVB radiation than PAR with decreasing distance to the tree trunk on both the sunlit and shaded side of a tree and (2) more rapid changes in the flux density of UVB radiation UVA radiation, and PAR with distance from the tree on the sunlit side of the tree than the shaded side of the tree. The UVB/PAR ratio was found to increase in the shadow of a tree with increasing distance from the tree to between 4 and 6 for the conditions of the study. The potential for detrimental effects by UVB flux density under conditions of the high ratio may be mitigated by sunflecks at a given location over the course of a day.     

阴离子表面活性剂存在下光系统II中酪氨酸残基的性能研究

通过荧光光谱和富立叶变换红外(FT-IR)光谱研究了阴离子型表面活性剂－十 二烷基硫酸钠（SDS）与光系统II（PSII）的相互作用。结果表明,PSII表现出酪 氨酸荧光的特性。在PSII蛋白质内部,存在着232 nm处的组分与酪氨酸之间以及这 两种氨基酸列基与叶绿素a之间的能量传递。SDS的存在会使这些能量传递以及 PSII中蛋白的骨架结构和酪氨酸残基的结构发生改变,而变化方式又明显受SDS在 溶液中聚集状态的影响。低于其临界胶束浓度(cmc)时,SDS会促进蛋白质中232 nm外的组分与酪氨酸之间的能量传递,并且使酪氨酸残基外于极性更小的环境; 而大于cmc时,SDS却产生相反的效应。但不同浓度的SDS均会抑制酪氨酸残基至叶 绿素a的能量传递。     

UVB effects on the photosystem II-D1 protein of phytoplankton and natural phytoplankton communities

The reaction center of photosystem II is susceptible to photodamage. In particular the D1 protein located in the photosystem II core has a rapid, light-dependent turnover termed the photosystem II repair cycle that, under illumination, degrades and resynthesizes D1 protein to limit accumulation of photodamaged photosystem II. Most studies concerning the effects of UVB (280-320 nm) on this cycle have been on cyanobacteria or specific phytoplankton species rather than on natural communities of phytoplankton. During a 5-year multidisciplinary project on the effects of UV radiation (200-400 nm) on natural systems, the effects of UVB on the D1 protein of natural phytoplankton communities were assessed. This review provides an overview of photoinhibitory effects of light on cultured and natural phytoplankton, with an emphasis on the interrelation of UVB exposure, D1 protein degradation and the repair of photosystem II through D1 resynthesis. Although the UVB component of the solar spectrum contributes to the primary photoinactivation of photosystem II, we conclude that, in natural communities, inhibition of the rate of the photosystem II repair cycle is a more important influence of UVB on primary productivity. Indeed, exposing tropical and temperate phytoplankton communities to supplemented UVB had more inhibitory effect on D1 synthesis than on the D1 degradation process itself. However, the rate of net D1 damage was faster for the tropical communities, likely because of the effects of high ambient light and water temperature on mechanisms of protein degradation and synthesis.     

IMPACT OF UVB and 03 ON THE OXYGEN FREE RADICAL SCAVENGING SYSTEM IN Arabidopsis thaliana GENOTYPES DIFFERING IN FLAVONOID BIOSYNTHESIS

Abstract— The effect of UVB and ozone (O₃) on growth and ascorbate-glutathione cycle were investigated in Arabidopsis thaliana wild-type Landsberg erecta (LER) and its transparent testa ( tt 5) mutant differing in UVB sensitivity. Ultraviolet-B radiation decreased dry matter production of tt5, while the dry weight of LER remained unaltered. Ozone exposure decreased dry weight of both genotypes. Ultraviolet-B radiation decreased the F_v/F_m ratio in tt5 but not in LER plants, while O₃ exposure decreased the F_v/F_m ratio in both genotypes. Ultraviolet-B radiation enhanced total ascorbic acid, total glutathione and their redox state and superoxide dismutase and glutathione reductase activities in both genotypes and the increases were greater in tt5 compared to UVB-irradiated LER. Although O₃ exposure enhanced total ascorbic acid and total glutathione in both genotypes, the redox state was significantly higher in tt5. Ozone exposure enhanced superoxide dismutase and glutathione reductase activities in tt5 while there were no major changes in LER. These results suggested that (1) plants blocked in flavonoid biosynthesis are sensitive to UVB in spite of their ability to maintain efficient oxygen free radical scavenging systems and (2) plants sensitive to UVB are comparatively tolerant of O₃ compared to UVB-insensitive plants. The differential responses of plants are discussed with reference to their ability to maintain high redox states of ascorbate and glutathione.     

Hydrogen peroxide formation and decay in iron-rich geothermal waters: the relative roles of abiotic and biotic mechanisms

Hydrogen peroxide (H2O2) is widely distributed in surface waters where the primary photochemical formation pathway involves the interaction between dissolved organic carbon (DOC) and ultraviolet radiation (UVR). In laboratory studies using iron-rich water from Yellow-stone's Chocolate Pots spring, H2O2 formation depended on sample treatment (unfiltered, < 0.2 micron filtered, autoclaved) prior to irradiation, suggesting several formation pathways. Similar H2O2 formation in filtered and unfiltered water indicates that it is primarily soluble material that is responsible for H2O2 formation. H2O2 formation with soluble material probably includes only photochemical reactions with DOC and/or metals. Greater H2O2 formation in unfiltered and filtered water than in autoclaved water suggests that the agent(s) involved in H2O2 formation is (are) not stable at high temperatures and pressures and degrade to nonphotoreactive species. Such unstable agents may include DOC and/or dissolved complexes of iron or other metals. UVR absorbance occurs across the UV spectrum and, though slightly greater in the UVA range (320-400 nm), is similar to that of other surface waters. Increased UVR absorbance after autoclaving suggested degradation or alteration of some components, which in turn affected H2O2 formation. The spectral region used for irradiation affected net formation and yield. H2O2 formation in water irradiated with UVA radiation was 2.5-3 times that formed in water irradiated with UVB radiation (280-320 nm) in experiments using artificial light sources. Apparent quantum yields comparable to those reported by others could not be calculated because the instrumental designs are not the same. However, approximate quantum yields were calculated for these experiments but should be viewed with caution. Quantum yields were higher in these experiments (0.0040 mol H2O2 per mol photon at 310 nm and 0.0012 mol H2O2 per mol photon at 350 nm) than values reported by other researchers (< 0.0007 mol H2O2 per mol photon at 300 nm and < 0.0005 H2O2 per mol photon at 340 nm; [Scully, N. M., D. R. S. Lean, D. J. McQueen and W. J. Cooper (1996) Limnol. Oceanogr. 41, 540-548]). In natural solar source experiments, H2O2 formation was greater in experiments with UVA and photosynthetically active radiation (PAR; 400-700 nm) than with PAR alone or with UVB, UVA and PAR. However, H2O2 capacity (nM H2O2 W-1 h-1 m2) was greatest with UVB radiation and lowest with PAR radiation. Source regions could not be studied separately. Dark decay of H2O2 occurred via two mechanisms. The main mechanism responsible for H2O2 decay involved particulate matter (probably microorganisms), whereas a secondary mechanism involved soluble matter (i.e. DOC, metal ions and other dissolved species involved in Fenton reactions).     

Effect of UVB Radiation on Utilization of Inorganic Nitrogen by Antarctic Microalgae

Abstract— The impact of UVB (280-315 nm) radiation (WG 305) on uptake of ¹⁵N-ammonium and ¹⁵N-nitrate of marine phytoplankton from station 219 (47°W, 61.5°S) and sea ice-algae from station 265 (22.6°W, 73.29°S) was studied during the Polarstern Cruise (EPOS III, Leg 3) to the Weddell Sea, Antarctica 1989. Uptake rates of ¹⁵NH₄⁺ were higher and more affected by UVB radiation than those of ¹⁵N0₃-. Pool sizes of the main amino acids changed in response to the used inorganic nitrogen source and UV exposure. Pools of glutamine, serine and glycine decreased, whereas those of alanine, asparagine and glutamate increased after UVB irradiation. The ¹⁵N-incorporation into the amino acids was reduced as a result of UVB exposure of phytoplankton and ice algae. Results are discussed with reference to an inhibitory effect on the enzymes of both carbon and nitrogen metabolism as well as to adaptation strategies.     

UV effects on invertebrate and diatom assemblages of Greece

The effects of solar radiation (PAR, UVA, UVB) on the productivity and structure of diatom and invertebrate assemblages were assessed during primary succession on artificial substrate near a rocky shore of the Saronikos Gulf, Greece. Three light treatments were performed (PAR, PAR+UVA, and PAR+UVA+UVB) at 0.5, 1.0 and 1.5 m of depth. Pennate diatoms were the major component of the developing periphytic communities during the study period. Exposure to solar UVB initially reduced the biomass and altered the structure of the diatom assemblages. The highest biomass of diatom assemblages was observed under PAR (49.2 g/m2). This value was significantly higher than the biomass of assemblages growing under PAR+UVA+UVB, but not significantly different compared to the biomass of assemblages exposed to PAR+UVA. These differences, however, did not persist at later stages. The most abundant invertebrate groups present were Polychaetes and Crustaceans. Solar UVB did not have significant effects on invertebrate biomass. Analysis of the invertebrate assemblage structure revealed time-course differences but no clear trends among the different treatments.     

In situ SUSCEPTIBILITIES OF PHOTOSYSTEMS I AND II TO PHOTOSENSITIZED DEACTIVATION via SINGLET OXYGEN MECHANISM

Abstract— A comparative study was carried out on the in situ susceptibilities to photoinactivation of the photosystem I (PS I) and II (PS II) complexes of spinach thylakoids treated with efficient type II sensitizers. While the presence of the exogenous sensitizers caused a substantial increase in the extent of photoinactivation of whole chain electron transport, it did not affect PS I activity of thylakoids in light but exerted an enhanced photoinactivating effect only on PS II. The measurements of the action spectrum for the inhibition of PS II activity of the sensitizer-incorporated thylakoids and that for the generation of singlet oxygen (¹O₂) from them revealed that photosensitized inactivation of PS II is directly related to the photoproduction of ¹O₂ in thylakoid membranes. The results obtained in the present work clearly demonstrate an exceptional sensitivity of PS II to ¹O₂, providing circumstantial evidence that high light-induced damage to PS II may result from photosensitization reactions mediated by ¹O₂, which is not necessarily produced within the PS II complex.     

Radiation sources providing increased UVA/UVB ratios induce photoprotection dependent on the UVA dose in hairless mice

In studies involving mice in which doses of UVA (320-400 nm) and UVB (290-320 nm) radiation were administered alone or combined sequentially, we observed a protective effect of UVA against UVB-induced erythema/edema and systemic suppression of contact hypersensitivity. The UVA immunoprotection was mediated by the induction of the stress enzyme heme oxygenase-1 (HO-1) in the skin, protection of the cutaneous Th1 cytokines interferon-gamma (IFN-gamma) and IL-12 and inhibition of the UVB-induced expression of the Th2 cytokine IL-10. In this study, we seek evidence for an immunological waveband interaction when UVA and UVB are administered concurrently to hairless mice as occurs during sunlight exposure in humans. A series of spectra providing varying ratios of UVA/UVB were developed, with the UVA ratio increased to approximately 3.5 times the UVA component in solar simulated UV (SSUV). We report that progressively increasing the UVA component of the radiation while maintaining a constant UVB dose resulted in a reduction of both the erythema/edema reaction and the degree of systemic immunosuppression, as measured as contact hypersensitivity. The UVA-enhanced immunoprotection was abrogated in mice treated with a specific HO enzyme inhibitor. UVA-enhanced radiation also upregulated the expression of cutaneous IFN-gamma and IL-12 and inhibited expression of both IL-6 and IL-10, compared with the activity of SSUV. The results were consistent with the previously characterized mechanisms of photoprotection by the UVA waveband alone and suggest that the UVA component of solar UV may have beneficial properties for humans.     

Proton coupled electron transfer and redox active tyrosines in Photosystem II

In this article, progress in understanding proton coupled electron transfer (PCET) in Photosystem II is reviewed. Changes in acidity/basicity may accompany oxidation/reduction reactions in biological catalysis. Alterations in the proton transfer pathway can then be used to alter the rates of the electron transfer reactions. Studies of the bioenergetic complexes have played a central role in advancing our understanding of PCET. Because oxidation of the tyrosine results in deprotonation of the phenolic oxygen, redox active tyrosines are involved in PCET reactions in several enzymes. This review focuses on PCET involving the redox active tyrosines in Photosystem II. Photosystem II catalyzes the light-driven oxidation of water and reduction of plastoquinone. Photosystem II provides a paradigm for the study of redox active tyrosines, because this photosynthetic reaction center contains two tyrosines with different roles in catalysis. The tyrosines, YZ and YD, exhibit differences in kinetics and midpoint potentials, and these differences may be due to noncovalent interactions with the protein environment. Here, studies of YD and YZ and relevant model compounds are described.