Tyrosine Photosynthesis

In photosynthesis, water oxidation is accomplished by photosystem II (PSII), which is a large membrane-bound protein complex (158-161). To the central core proteins D1 and D2 are attached different cofactors, including a redox-active tyro-syl residue, tyrosine Z (Yz) (158-162), which is associated with a tetranuclear manganese complex (163). These components constitute the water oxidizing complex (WOC), the site in which the oxidation of water to molecular oxygen occurs (159, 160, 164). The organization is schematically shown in Fig. 18. 

Plastocyanin from parsley, a copper protein of the chloroplast involved in electron transport during photosynthesis, has been reported to have a fluorescence emission maximum at 315 nm on excitation at 275 nm at pH 7 6 (2°8) gjncc the protein does not contain tryptophan, but does have three tyrosines, and since the maximum wavelength shifts back to 304 nm on lowering the pH to below 2, the fluorescence was attributed to the emission of the phenolate anion in a low-polarity environment. From this, one would have to assume that all three tyrosines are ionized. A closer examination of the reported emission spectrum, however, indicates that two emission bands seem to be present. If a difference emission spectrum is estimated (spectrum at neutral pH minus that at pH 2 in Figure 5 of Ref. 207), a tyrosinate-like emission should be obtained. 

Fig. 3.2a Electron transport in (natural) photosynthesis. P = chlorophyll that acts as a light sensitizer, from which a photogenerated electron travels to Q = Plastquinone that in combination with CO2 forms a carbohydrate. The photo-ejected electron from Peso is replenished by taking one from the Mn cluster through the redox active tyrosine linkage (or mediator), which in turn extracts an electron from water.

Hammarstrom L, Sun L, Aakermark B, Styring S. A biomimetic approach to artificial photosynthesis Ru(II)-polypyridine photosensitizers linked tyrosine and manganese electron donors. Spectrochim Acta 2001 37 2145-60. 

Hammarstrom L, Styring S. Proton-coupled electron transfer of tyrosines in Photosystem II and model systems for artificial photosynthesis the role of a redox-active link between catalyst and photosensitizer. Energy Environ Sci. 2011 4 2379-88. 

P-A Siegenthaler and N Murata (199 ) Lipids in Photosynthesis Structure, Function and Genetics. Kluwer BA Barry and GT Babcock (1987) Tyrosine radicals are involved in the photosynthetic oxygen-evolving sys-tem. Proc Nat Acad Sci, USA 84 7099-7103... 

In plant photosynthesis molecular oxygen is produced from water. In this process tyrosine molecules carry electron holes from oxidized chlorophyll molecules to manganese-protein complexes. Short-lived tyrosine radicals keep the photoexcited chlorophyll molecules apart from reactive oxygen intermediates, such as peroxides and oxygen atoms (Calvin, 1978 Tommos et al, 1995, 1998)... 

Collective name for polyprenylated 2,3-dimethylben-zoquinone derivatives, [e.g., PQ-9 (n=9), C5jH ,02, Mr 749.22, bright yellow platelets, mp. 48-49 C, u max 314 nm (petroleum ether)], isolated from chloro-plasts. The P. play a role as redox substrates in photosynthesis for cyclic and non-cyclic electron transport where they are converted reversibly into the corresponding hydroquinones (plastoquinols). The biosynthesis of P. proceeds from homogentisic acid, a product of L-tyrosine degradation, through prenylation and methylation (methyl group from L-methionine) to the plastoquinols which are dehydrated to the P.. ... 

Ubiquinones (UQ), often called coenzyme Qio, are electron carriers in oxidative phosphorylation and photosynthesis, respectively. Ubiquinones consist of quinoid nucleus (derived from the shikimate pathway), 4-hydroxybenzoate (derived from chorismate or tyrosine), and terpenoid moiety. Zeatin, a phytohormone, is a member of the cytokinin family involved in various processes of growth and development in plants. Most cytokinins are adenine-type, where the hydrogen of amino group at Ce position of adenine is replaced with an isoprenoid. 

The criteria for EPT are that electrons and protons transfer from different orbitals in the donor to different orbitals in the acceptor, and these orbitals are electronically coupled, allowing the events to occur simultaneously. Meyer et al. propose an extension of this concept, multiple-site EPT (MS-EPT)," where an electron proton donor transfers to multiple acceptors or an acceptor receives electrons and protons from different donors. These appear to be particularly relevant in biological processes, including the oxidation of tyrosine by Pggo in oxygenic photosynthesis. 

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