Reaction centers of photosystems I and

If the reaction centers of photosystem I and photosystem II are segregated into separate regions of the thylakoid membrane, how can electrons move from photosystem I to photosystem II Evidently the plastoquinone that is reduced in photosystem II can diffuse rapidly in the membrane, just as ubiquinone does in the mitochondrial inner membrane. Plastoquinone thus carries electrons from photosystem II to the cytochrome b6f complex. Plastocyanin acts similarly as a mobile electron carrier from the cytochrome b f complex to the reaction center of photosystem I, just as cytochrome c carries electrons from the mitochondrial cytochrome bct complex to cytochrome oxidase and as a c-type cytochrome provides electrons to the reaction centers of purple bacteria (see fig. 15.13). 

P700 (a special chlorophyll a molecule) serves as the reaction center of photosystem I, and a bound form of ferredoxin (ferredoxin-reducing substance) may be the electron acceptor. Electrons flow subsequently to NADP through ferredoxin (a nonheme iron protein) and a flavoprotein. 

Several laboratories now make use of the ultrafast continuum pump-probe technique in the study of ultrafast processes in biological molecules or molecular complexes.Notable molecules and complexes under study are the photosynthetic reaction centers of purple bacteria, the reaction centers of photosystems I and II of green plants. 

Satoh K and Pork CD (1982) Photoinhibition of reaction centers of photosystem I and II in intact Bryopsis chloroplasts under anaerobic conditions. Plant Physiol. 70, 1004--1008. 

The reaction center of photosystem I is larger and more complex. It contains two large polypeptides and at least seven other smaller subunits. The reactive chlorophyll a dimer P700 resides on the two main polypeptides, along with about 60 additional molecules of chlorophyll a, two quinones, and an iron-sulfur center. 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

We shall now turn our attention to the specific molecules that act as electron acceptors or donors in chloroplasts. A summary of the characteristics of the most common components of this complex pathway is presented in Table 5-3. Figure 6-4 should also be consulted, if the underlying concept of redox potential is already familiar. We will begin our discussion by considering the photochemistry at the reaction center of Photosystem II and then consider the various substances in the sequence in which they are involved in electron transfer along the pathway from Photosystem II to Photosystem I. We will conclude by considering the fate of the excited electron in Photosystem I. 

In purple bacteria, the constituent bacteriochlorophyll of the primary donor is the same as the principal bacteriochlorophyll pigment, namely, BChl a. The same is true for the green bacteria and also for photosystem II of green plants. However, as discussed later in Chapter 28, the primary donor of photosystem I(PSI),P700, is now known to consist of a 13 epimer of Chi a, designated as Chi o ]. Since the reaction centers of PS I and heliobacteria are both of the FeS-type [refer to Chapter 1], it is reasonable to anticipate that the primary electron donor of heliobacteria might also be an epimer. 

D D rnemann and H Senger (1982) The structure of Chlorophyll RC I, a chromophore of the reaction center of photosystem I. Photochem Photobiol 43 573-581... 

Fig. 1. Location of the intermediary electron acceptor FeS-X (a [4Fe 4S] cluster) in the reaction center of photosystem I (A) and in the sequence of electron acceptors (with the years of their discovery shown) (B).

JM Fenton, MJ Pellin, Govindjee and KJ Kaufmann (1979) Primary photochemistry of the reaction center of photosystem I. FEBS Lett 100 1-4... 

The early pioneering work ofLichtenthaler andcoworkers established the presence as well as the specific location of phylloquinone in the thylakoid membrane. Subsequent work of Richard Malkin showed that only one ofthe two phylloquinone molecules in the reaction centers of photosystem I participates in the electron-transfer sequence. Before the various spectroscopic characterization studies described in the previous sections were undertaken, much attention was given to the establishing whether or not phylloquinone indeed serves the role of acceptor A in the PS-I electron-transfer chain. In the mid-1980s, several laboratories carried out extraction and reconstitution of phylloquinone as a means of addressing this question. The premise of all such studies may be visualized in terms of the presumed PS-I electron-carrier sequence ... 

In the photosynthesis of green plants, photosystems I and II (PS I, PS II) contain chlorophyll a, a Mg(II)-porphyrin, as an antenna system for light absorption and energy transfer to the reaction centers of PS I and PS II. PS II consists of a dimeric chlorophyll a as reaction center, pheophytin a, a metal-free chlorophyll a as electron transfer system to PS I and - on the other side - a water-oxidizing Mn cluster. The electron connection between PS II and PS I is carried out by a cyth/f complex (heme complexes and an FeS protein). The reaction center of PS I is also a dimeric chlorophyll (perhaps together with other chlorophylls), and chlorophyll and several FeS proteins for electron transfer. 

Reaction centers of photosystem I were determined by monitoring the flash-induced absorbance changes at 702 nm in the presence of 20 pJM PMS, 0.2 inM sodium ascorbate and 0.1 mM methylviologen, using the molar extinction coefficient = 64000 M cm (Hiyama, Ke 1971). 

Fig. 2 (C) shows a model representing the thylakoid membrane of a cyanobacterium or a red alga, consisting of photosystems I and II interconnected by the cytochrome- /complex, and the ATP synthase, CFo CFi. The phycobilisomes are seen as attached to the stromal surface at the PS-II reaction-center core complex.

As seen earlier in Chapter 2 on bacterial reaction centers, crystallization of the reaction-center protein of the photosynthetic h iCttn xm Rhodopseudomonas viridis by Michel in 1982 and subsequent determination ofthe three-dimensional structure ofthe reaction center by Deisenhofer, Epp, Miki, Huber and Michel in 1984 led to tremendous advances in the understanding ofthe structure-function relationship in bacterial photosynthesis. Furthermore, because of certain similarities between the photochemical behavior of the components of some photosynthetic bacteria and that of photosystem II, research in photosystem-II was greatly stimulated to its benefit by these advances. In this way, it became obvious that the ability to prepare crystals from the reaction-center complexes of photosystems I and II would be of great importance. However, it was also recognized that, compared with the bacterial reaction center, the PS-I reaction center is more complex, consisting of many more protein subunits and electron carriers, not to mention the greater number of core-antenna chlorophyll molecules. 

Fig. 7. Model for the native photosystem-l complex (PSI-200) constructed from the reaction-center core (CC I) and two copies of each ofthe four light-harvesting chlorophyll-protein complexes. Figure adapted from Boekema, Wynn and Malkin (1990) The structure of spinach photosystem I studied by eiectron microscopy. Biochim Biophys Acta 1017 55.

The relation of the structure and organization of the Photosystem II reaction centers to those from Photosystem I or from the green or purple bacteria presents an interesting example of comparative biochemistry. Similarities between PS II and purple bacterial reaction centers include aspects of the reaction center proteins, the stoichiometry of chlorophyll and pheophytin in the reaction center and the complex of iron with quinones as the primary electron acceptor. In each of these respects the reaction centers of PS I or green bacteria, however, have no obvious similarity. 

In the reaction centers of photosystem II or purple bacteria, two quinones function in series as the primary (Q ) and the secondary (Qg) electron acceptors [1], The Qg sites are known to be the herbicide binding sites [2]. On the other hand, binding of herbicide to PS I reaction center has never been reported. 

We have studied the translational control of ps1A2 (reaction center protein 2 of Photosystem I) and LS (large subunit of ribulose bisphosphete carboxylase) by chloroplast factors, using mRNAs produced in vitro from the corresponding genes cloned into vectors containing the T7 RNA polymerase promotor. 

The reaction center of photosystem II of the chlor-oplast contains 2-4 Mn atoms. The effect of Mn deficiency on photosynthesis resembles poisoning by DCMU (see), i.e. evolution of O2 by photosystem II is inhibited, while photosystem I is unaffected. It is thought that the Mn is intimately involved in the primary event of the photolysis of water, probably alternating between the III and II states. 

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