1. The electron is transferred to PheoA on a timescale of tens of picoseconds (Holzwarth et al. 2006), and then to QA
with a timescale of 200–500 picoseconds (ps) (Rappaport and Diner 2008). The electron–hole pair on P680 + and Q A − is stable for close to 1 ms in cyanobacteria (Reinman et al. 1981; Gerken et al. 1989; Metz et al. 1989), during which time, under catalytic conditions, the oxygen-evolving complex (OEC) donates an electron to P680 + via a redox-active tyrosine, YZ. Once the OEC, which consists of a Mn4CaO5 cluster (Umena et al. 2011), has been oxidized four times via sequential charge separations to reach a high-valent state, probably Mn(IV)Mn(IV)Mn(IV)Mn(IV)-O∙ (Siegbahn 2006; Sproviero et al. 2008), it is capable GW786034 cost of oxidizing water to dioxygen. Meanwhile, the electron on QA is transferred to QB, which dissociates away from PSII after two reductions and subsequent protonations, carrying Lazertinib supplier reducing equivalents to the next step in photosynthesis and ultimately resulting in the storage of energy in the chemical bonds of sugars. Fig. 1 The arrangement of cofactors in the D1/D2/Cyt
b 559 sub-complex of cyanobacterial PSII, viewed along the membrane plane (PDB ID: 3ARC). Black arrows represent electron transfer. The oxygen-evolving complex (OEC) is shown with manganese ions in purple, oxygen in red, and calcium in green; tyrosine Z (YZ) and tyrosine D (YD) are shown in yellow; chlorophylls (Chl) are shown in green; β-carotenes (Car) are shown in orange; pheophytins (PheoA and PheoB) are shown in magenta; quinones (QA and QB) are shown in blue; and cytochrome b 559 (Cyt b 559) and the nonheme iron are shown Arachidonate 15-lipoxygenase in red. The surface of the protein is shown in the background and colored according to atom identity with C in
green, N in blue, and O in red However, the intermediates associated with water splitting are very oxidizing, and cause damage to the protein over time. The D1 subunit of PSII, which contains most of the cofactors involved in water oxidation, turns over every 30 min, in a process that involves disassembly of the PSII complex, membrane diffusion, and protein synthesis (Nixon et al. 2010). In order to minimize damage, PSII has evolved multiple mechanisms of photoprotection to prolong the lifetime of its subunits and minimize energy expenditure for protein synthesis. One mechanism involves adjusting the size of the light-harvesting antenna; other mechanisms involve dissipating excess solar energy as heat, as in the xanthophyll cycle in plants (Niyogi 1999) or via the orange carotenoid protein in cyanobacteria (Kirilovsky and Kerfeld 2012). In addition, when water-oxidation catalysis is impaired, oxidation of secondary donors, including carotenoids (Car), chlorophylls (Chl), and cytochrome b 559 (Cyt b 559), may serve to GM6001 nmr remove excess oxidizing equivalents from PSII (Thompson and Brudvig 1988; Buser et al. 1992) or to quench chlorophyll excited states (Schweitzer and Brudvig 1997).