**4. References**


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**3. Acknowledgment** 

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**4** 

*USA* 

**Mutations in the CP43 Protein of** 

*2Louisiana State University/Department of Biological Sciences* 

**and Cytochrome C550 Binding** 

*1East Carolina University/Biology Department* 

**Photosystem II Affect PSII Function** 

Brandon D. Burch1, Terry M. Bricker2 and Cindy Putnam-Evans1

During the process of photosynthesis, light energy is converted to chemical energy that is utilized in the biosynthesis of carbohydrate. The initial events of photosynthesis in cyanobacteria, green algae and higher plants are the photoinduced transfer of electrons from water to plastoquinone. These reactions are catalyzed by Photosystem II (PSII), a large pigment-protein complex embedded within the thylakoid membrane (Renger, et al., 2008). The reactions catalyzed by PSII proceed as follows. First, light is trapped by pigments, predominately chlorophyll, associated with the thylakoid membrane (Glazer, 1983). Excitation energy is then transferred to the photochemically active chlorophyll species P680. Excited P680 then donates an electron (charge separation) to the primary PSII electron acceptor, a protein-bound pheophytin molecule (Klimov et al., 1980). Pheophytin is then oxidized by a tightly bound plastoquinone, QA, which in turn reduces a loosely bound plastoquinone, QB. A second light-induced charge separation results in the formation of plastoquinol (QBH2). Photooxidized P680 is reduced by the primary donor of PSII, a tyrosyl radical (YZ●) that is Tyr161 of the D1 protein of PSII (Debus et al., 1988a; Debus et al., 1988b). YZ then is reduced by an electron from the oxygen-evolving complex of PSII, located on the lumenal face of the thylakoid. The catalytic site of the oxygen-evolving complex consists of a metal ion cluster of four manganese atoms and one calcium atom bound via five -oxobridges (Mn4CaO5 cluster; Umena et al., 2011). Two chloride ions bound nearby are also required for activity. The oxygen-evolving complex functions to extract electrons and protons from water, ultimately resulting in the release of molecular oxygen. The manganese atoms cycle through a series of redox states, or S states (Joliot et al., 1969; Kok et al., 1970), from S0 to S4, with each S state representing a successively more oxidized form of the cluster. One molecule of O2 is evolved when four electrons and four protons are extracted by the

PSII consists of both intrinsic thylakoid polypeptides and extrinsic polypeptides located within the thylakoid lumen. Together, these proteins ligate the Mn4CaO5 cluster, chlorophylls and other pigments, and the electron transport chain components. The intrinsic polypeptides necessary to form a PSII complex capable of evolving oxygen are CP47, CP43, D1, D2, the and subunits of cytochrome b559, and the 4kDa *psb*I gene product (Bricker et

**1. Introduction** 

Mn4CaO5 cluster.

