Unidirectional photodamage of pheophytin in photosynthesis

The primary reactions in photosynthesis take place in the reaction centers surrounded by light harvesting complexes (Blankenship, 2002; Diner and Rappaport, 2002; Frank and Brudvig, 2004). There are two types of photosynthetic reaction centers. The type I reaction center has ironsulfur cluster as stable electron acceptor, such as photosystem I complexes, green bacterial and heliobacterial reaction centers, and the type II uses quinone as stable electron acceptor including photosystem II and purple bacterial reaction centers. The electron transfer in type II reaction center in unidirectional via the L-subunit of the reaction centers (Maroti et al., 1985; Hoerber et al., 1986; Martin et al., 1986; Michel-Beyerle et al., 1988). However, the electron transfer in the type I reaction center is different from that in the type II centers, which is bidirectionational (Guergova-Kuras et al., 2001; Li et al., 2006). The three dimensional structures of both types of reaction centers were determined at atomic resolution (Jordan et al., 2001; Ferreira et al., 2004; Loll et al., 2005; Amunts et al., 2007; Umena et al., 2011). The electron transfer pathways in both types of reaction centers are well established (Van Grondelle, 1985; Schatz et al., 1988; Fleming and van Grondelle, 1997; Dekker and Van Grondelle, 2000; Gobets and van Grondelle, 2001; Seibert and Wasielewski, 2003; Van Grondelle and Novoderezhkin, 2006). The primary electron donor in the reaction centers is a pair of chlorophyll molecules, and pheophytin is the primary electron acceptor (Klimov et al., 1977; Holzwarth et al., 2006). Light is the only source of energy for photosynthesis; it can also be harmful to plants (Powles, 1984). During the recent 10 years the molecular processes of photoinhibition had been intensively studied (Adams and Demmig-Adams, 1993; Aro et al., 1993; Baker and Bowyer, 1994; Anderson et al., 1997; Asada, 1999; Melis, 1999; Niyogi, 1999; Adir et al., 2003; Telfer, 2005; Murata et al., 2007; Tyystjarvi, 2008; Kramer, 2010; Hou and Hou, 2013). The PSII complex is composed of more than 15 polypeptides and 200 pigment molecules. Because there are many pigment and protein molecules not related directly to the photoinhibition reaction, it is difficult to identify which molecule is photodamaged. The preparation of PS II reaction center D1/D2/cytochrome b-559 complex, which contains only a few polypeptides and pigments, can be a good material to meet the difficulty (Nanba and Satoh, 1989). The PS II reaction center D1/D2/cytochrome b-559 complexes from higher plants contain five polypeptide subunits (Seibert et al., 2004). It contains six chlorophyll a (Chl a), two β-carotene (Telfer et al., 1987; Seibert et al., 1988; Gounaris et al., 1990; Kobayashi et al., 1990; Barbato et al., 1991). The PSII reaction center does not contain the quinone electron acceptors QA and QB. It photochemical reaction is restricted to radical pair formation and recombination (Takahashi et al., 1987; Crystall et al., 1989; Wasielewski et al., 1989). Addition of exogenous electron donors and acceptors allows secondary electron flow reaction to occur. Therefore the D1/D2/cytochrome b-559 complex constitutes a good simple system for probing on the mechanisms from both acceptor-side and donor-side photoinhibition (Barber and Andersson, 1992; Barber and De Las Rivas, 1993; Yu et al., 1995). It has found that primary electron donor P680, accessary chlorophyll, carotene, amino acid residues such as histidine, and pheophytin are vulnerable to excess light. The primary electron donor P680 of PSII can be damaged easily by exposure of strong light, when no additions are made (Telfer and Barber, 1989). Singlet oxygen is formed as a consequence of radical pair recombination (McTavish et al., 1989; Durrant et al., 1990). The generation of this highly toxic species causes initially a selective and irreversible bleaching of the chlorophylls that constitute P680 and a breakdown of the D1 protein to a 23-kDa fragment containing the N terminus of the complete protein (De Las Rivas et al., 1993). In the presence of a suitable electron acceptor such as silicomolybdate or decylplastoquinone, the P680+ lifetime is increased, and irreversible bleaching of β-carotene and chlorophyll are observed, which are independent of oxygen (Telfer et al., 1991). Under these conditions breakdown of the D1 protein leads to a 24-kDa fragment of C-terminal origin (Shipton and Barber, 1991). The photobleaching at 680 nm is usually attributed to the photodamage of P680 in the PS II reaction center (Telfer et al., 1990). There is the significant overlapping of absorption of P680, accessary chlorophyll, and pheophytin in the PS II reaction center (Diner and Rappaport, 2002). Using high performance liquid chromatography (HPLC) the one pheophytin in PS II reaction center is photo damaged (Hou et al., 1995; Kuang et al., 1995). The time course of pheophytin photodamage showed the content of pheophytin decreases faster than that of chlorophyll, suggesting that