Extrinsic polypeptides

One peripheral polypeptide with MW = 33 kDa is isolated with the PS II/OEC core (Table 1). Two other extrinsic peptides with molecular masses of 17 and 23 kDa have been implicated in the 02-evolving process, although they may be replaced in the core preparations by high concentrations of Ca and CE salts with preservation of 02-evolution activity. The involvement of these three polypeptides in O2 evolution was first suggested by the Tris extraction experiments on so-called inside-out chloroplast vesicles by Akerlund et al. [71] and by cholate extraction of chloroplasts by Sayre and Cheniae [72]. A flurry of activity ensued and the situation with respect to these polypeptides is now reasonably clear. The biochemical properties of these polypeptides are discussed in detail in two recent reviews [7,8]. 

While the identity of the polypeptides present in the PS II/OEC core complex is now reasonably well-established, the stoichiometries of these species remain uncertain. Little is known regarding the concentration ratios per PS II/OEC unit of the intrinsic polypeptides and controversy exists regarding the stoichiometries of the extrinsic subunits [7]. In the latter case, however, the work is reaching agreement on between 1 and 2 copies of each of the water-soluble polypeptides per P-680. More quantitative work, most profitably with C labeling, will be necessary. 

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Simpson, D.J. and Andersson B. 1986. Extrinsic polypeptides of the chloroplast oxygen evolving complex constitute the tetrameric ESs particles of higher plant thylakoids. Carlsberg Res. Commun. 51,467-474. 

Fig. 5. A possible structure of the PS II reaction centre. The model leans heavily on the analogy with the bacterial reaction centre. Discussion of the location of the chromophores within the polypeptides is given in the text. The orientation of some of the components is shown. The role of the extrinsic polypeptides and the possible structure of the manganese cluster are discussed in Chapter 6.

Also shown in Fig. 5 are the 3 extrinsic polypeptides which bind to the inner surface of the PS II reaction centre and which are involved in the chloride binding associated with the water-splitting reaction. These polypeptides may make up a pocket surrounding the manganese atoms of the 02-evolving enzyme. The four manganese atoms are probably bound to the PS II reaction centre and may be arranged as a distorted cube (see Chapter 6). 

Fig. 1. Model for the PS-II reaction center (for higher plants and green algae) showing the locations of the three extrinsic polypeptides and the Inorganic cofactors...

Chapter 21 Oxygen Evolution Extrinsic Polypeptides and Inorganic Cofactors 367... 

From what we have seen thus far, it is reasonable to conclude that one important function of the extrinsic polypeptides in photosystem II is to maintain the binding of the inorganic ionic cofactors and Cr. The other major function, particularly that of the 33-kDa polypeptide, is to protect the oxidizing state of the manganese complex by shielding it from being attacked by nearby reducing species. The possible functions of the ionic cofactors and CF will be discussed next. 

Cammarata and Cheniae found that purified photosystem II of spinach contains two per reaction center. Ono and Inoue found that treating the thylakoids with citrate at pH 3 selectively releases only one ofthe two Ca in the PS-II reaction center. The low-pH treatment also removes the 23- and 17-kDa polypeptides, but they rebind to the thylakoid when the pH is raised to 6.5, after washing to remove the free Ca. Even with all three extrinsic polypeptides intact, oxygen evolution in the citrate-treated thyla-... 

Unlike Cl , whose reactivating effect may be achieved by a number of different kinds of anions with variable effectiveness, the action of is more specific, with only having been found to be a viable substitute, and then with much lower effectiveness. Monovalent cations (Na, K, Cs ) act as inhibitors of oxygen evolution, as do Cd and La which are known inhibitors in other Ca "-binding proteins. in photosystem 11 may be quantitatively removed by La displacement, with a simultaneous release of the 23- and 17-kDa extrinsic polypeptides. Washing of La -treated thylakoids can yield... 

Homann PH. Chloride relations of photosystem II membrane preparations depleted of, and resupplied with, their 17 and 23 kDa extrinsic polypeptides Photosynth Res 1988 15 205-220. 

Seidler A. The extrinsic polypeptides of Photosystem II. Biochim Biophys Acta 1996 1277 36-60. 

Ahmed A, Tajmir-Riahi HA, Carpentier R. A quantitative secondary structure analysis of the 33 kDa extrinsic polypeptide of photosystem II by FTIR spectroscopy. FEBS Lett 1995 363 65-68. 

Rashid A, Carpentier R. The 16 and 23 kDa extrinsic polypeptides and the associated Ca and Cl modify atrazine interaction with the Photosystem II core complex. Photosynth Res 1990 24 221-227. 

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