Minor but key chlorophylls in Photosystem II

Abstract

A 'metal-free' chlorophyll (Chl) a, pheophytin (Phe) a, functions as the primary electron acceptor in PS II. On the basis of Phe a/PS II = 2, Phe a content is postulated as an index for estimation of the stoichiometry of pigments and photosystems. We found Phe a in a Chl d-dominant cyanobacterium Acaryochloris marina, whereas Phe d was absent. The minimum Chl a:Phe a ratio was 2:2, indicating that the primary electron donor is Chl a, accessory is Chl d, and the primary electron acceptor is Phe a in PS II of A. marina. Chl d was artificially formed by the treatment of Chl a with papain in aqueous organic solvents. Further, we will raise a key question on the mechanisms of water oxidation in PS II.

Unique photosystems in Acaryochloris marina

“…The Chl a/d heterodimer model (Fig. 4A) was first proposed on the pigment stoichiometry analysis by Kobayashi et al (2005), and was supported by the redox potential measurement of Chls a and d , the flash-induced spectral experiment , and its theoretical analysis (Renger and Schlodder 2008). The Chl d homodimer model (Fig.…”

“…P740, the primary electron donor of PS I in A. marina, was initially proposed to be a homodimer of Chl d (Hu et al 1998), then a homodimer of Chl d 0 (Akiyama et al 2001), and finally a Chl d/d 0 heterodimer (Fig. 4A) Kobayashi et al 2005Kobayashi et al , 2007, just like the Chl a/a 0 for P700 in other cyanobacteria and higher plants (Fig. 4C) (Kobayashi et al 1988;Jordan et al 2001): a dimer model for P740 was supported by FTIR spectroscopy (Sivakumar et al 2003).…”

“…1), are also present and function as key components in the reaction centers (RCs) of photosystem (PS) I and PS II (Akiyama et al 2001). The content of Chl a varies according to light conditions (Boichenko et al 2000;Akiyama et al 2004;Mimuro et al 2004), but at least one molecule of Chl a seems to be present in each RC Kobayashi et al 2005Kobayashi et al , 2007. Chl d 0 , Chl a and phylloquinone in PS I Chl d 0 , the 13 2 -epimer of Chl d (Fig.…”

“…3A, C and 4A, B) (Akiyama et al 2001Mimuro et al 2004;Kobayashi et al 2005;Razeghifard et al 2005;Chen et al 2005;Tomo et al 2007). It has not been clarified yet why A. marina uses Phe a instead of Phe d. One of the reasons might be the use of a common electron acceptor, plastoquinone, for pheophytin, because the reduction potential of Phe d (-0.63 V) was found to be significantly more positive than that of Phe a (-0.75 V) in acetonitrile ).…”

“…The Chl a homodimer model is most unlikely, because the minimum ratio of Chl a/Phe a in A. marina whole cells grown under low-light condition was 2/2 ), indicating only one Chl a molecule is located in the PS II RC (Kobayashi et al 2005) on the basis of Phe a/PS II = 2, PS I/PS II = 1 (Akiyama et al 2001), and the presence of one Chl a molecule as an active A 0 in PS I (Kumazaki et al 2002). The model was also excluded by a quantitative explanation of the experimental data (Renger and Schlodder 2008).…”