Photosystem II to

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). 

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. 

Because Photosystem II tends to occur in the grana and Photosystem I in the stromal lamellae, the intervening components of the electron transport chain need to diffuse in the lamellar membranes to link the two photosystems. We can examine such diffusion using the time-distance relationship derived in Chapter 1 (Eq. 1.6 x je = 4Djtife). In particular, the diffusion coefficient for plastocyanin in a membrane can be about 3 x 10 12 m2 s-1 and about the same in the lumen of the thylakoids, unless diffusion of plastocyanin is physically restricted in the lumen by the appres-sion of the membranes (Haehnel, 1984). For such a D , in 3 x 10-4 s (the time for electron transfer from the Cyt b(f complex to P ), plastocyanin could diffuse about [(4)(3 x 10-12 m2 s-1) (3 x 10-4 s)]1/2 or 60 nm, indicating that this complex in the lamellae probably occurs in relatively close proximity to its electron acceptor, Photosystem I. Plastoquinone is smaller and hence would diffuse more readily than plastocyanin, and a longer time (2 x 10-3 s) is apparently necessary to move electrons from Photosystem II to the Cyt b(f complex hence, these two components can be separated by greater distances than are the Cyt b f complex and Photosystem I. 

By transferring an electron to pheophytin (one of the two molecules shown in Fig. 2.18b), the donating chlorophyll-fl is left in a positively charged state. The pheophytin passes on the electron to a plastoquinone, from which it is passed to a second plastoquinone believed to be better able to form pcH2 and move around, after taking two protons from two of the processes shown in Fig. 2.20, and subsequently migrate from photosystem II to the cytochrome b/complex for the next step shown in Fig. 2.14. The plastoquinone structure is shown in Fig. 2.21, taken from a recent study of the cytochrome h/complex (Kurisu et al., 2003) only the ring of one of the pq s is seen in the photosystem II X-ray studies of Kamiya and Shen (2003) or Zouni et al. (2001). Considerable attention has been paid to identification of the pro-... 

ML Gilchrist, Jr., JA Ball, DW Randall and RD Britt (1995) Proximity of the manganese cluster of photosystem II to the redox-active tyrosine Yz- Proc Nat Acad Sci, USA 92 9545-9549... 

The flow of electrons from photosystem II to photosystem I drives the transport of protons into the thylakoid lumen. Electron transfer through the iron-sulfur proteins FeA and FeB is not understood. ° values are approximate. MSP contains a manganese cluster. (MSP = manganese stabilizing protein)... 

Cytochrome b6f is a is part of the electron transport chain that transfers electrons from photosystem II to photosystem I. Cytochrome b6f is a complex of proteins that includes cytochromes f, b6, and an iron sulfur protein. Cytochrome b6f accepts electrons from plastoquinone QH2 and passes them to plastocyanin (Figure 17.12). In addition to transferring electrons, the cytochrome b6f complex pumps protons into the thylakoid lumen, helping to build the proton gradient, which is used by the CFO-CFl complex to make ATP. 

The oxidation of water by photosystem II to produce oxygen is the ultimate source of electrons in photosynthesis. These electrons are subsequently passed from photosystem II to photosystem I by the electron transport chain. The electrons from water are needed to fill the hole that is left when the absorption of one photon of light leads to donation of an electron from photosystem II to the electron transport chain. 

The second part is the transfer of electrons from the excited-state chlorophyll of photosystem II to an electron transport chain consisting of accessory pigments and cytochromes, with enei provided by absorption of a photon of light. The components of this electron transport chain resemble those of the mitochondrial electron transport chain they pass the electrons to the reaction-center chlorophyll of photosystem I. 

It is well established that the path of electrons in photosynthesis goes from photosystem II to photosystem I. The reason for the nomenclature is that photosystem I is easier to isolate than photosystem II and was studied more extensively at an earlier date. 

In cyclic photophosphorylation, the excited chlorophyll of photosystem I passes electrons directly to the electron transport chain that normally links photosystem II to photosystem I. This electron transport chain is coupled to ATP production (see Figure 22.8). 

Betts SD, Ross JR, Picherslgr E et al. Mutation val35ala weakens binding of the 33-kDa manganese stabilizing protein of photosystem II to one of two sites. Biochemistry 1997 36 4047-4053. 

Giardi MT, Komenda J, Masojfdek J. Involvement of protein phosphorylation in the sensitivity of photosystem II to strong illumination. Physiol Plant 1994 92 181-187. 

Chlorophyll a fluorescence induaion is a widespread method to evaluate the photosynthetic activity. This method is noninvasive, highly sensitive, fast, and easily measured. When chlorophyll molecules in photosystem II absorb light, that light may be assimilated into the hght reactions of photosynthesis or may be released as fluorescence or heat energy. In vivo fluorescence increases when photosynthesis declines or is inhibited. Numerous environmental f ors can affect the rate of electron transport between photosystem II and photosystem I due to interference with electron carriers between the two photosystems. For example, when the diuton is added in the measured sample, electron transport from photosystem II to photosystem I is blocked resulting in maximum fluorescence. This method was often employed to detect the photosynthetic activity of immobilized photosynthetic material. ... 

This protein is found exclusively in the chloroplast where it is involved in electron transfer from Photosystem II to Photosystem I. Plastocyainin has been isolated from a number of green algae Chlorella ellipsoidea 31), and Chlamydomonas rheinhardi 32), as well as from spinach 33), French bean 34), parsley 35), and Chenopodium album 36). 

Two water molecules are oxidized by four consecutive charge-separation reactions through photosystem II to form a molecule of diatomic oxygen and four hydrogen ions. The outcoming electron in each step is transferred to a redox-active tyrosine residue followed by the reduction of a photoxidized... 

Tentatively, it seems that DCCD-binding to a LHCII-poljrpeptide is responsible for the redirection of protons from water oxidation across photosystem II to the bound quinone. Consequently, LHCII-pol3rpeptides must be involved in normal proton conduction from the Mn-centre into the lumen. 

The genes psbA-F were the first six chloroplast genes for polypeptides of photosystem II to be described. The remaining authenticated chloroplast genes for photosystem II polypeptides were all identified by V-reiminal amino acid sequ icing of snull polypeptides associated with photosystem II preparations. 

Dichlorophenyl-dimethyl urea, DCMU, Diuron, 3-(3,4-dichlorophenyl)-l,l-dimethylurea, N -(3,4-dichlorophenyl)-N,N-dimethyluTaK a systemic herbicide which blocks electron transport from photosystem II to photosystem I (see Photosynthesis). It is absorbed principally by the roots then translocated ac-ropetally in the xylem. 

Barber, J. (1980). An explanation for the relationship between salt-induced thylakoid stacking and the chlorophyll fluorescence changes associated with changes in spillover of energy from photosystem II to photosystem I. FEBS Lett., 118 1-10. 

Lichtle C,Jupin H and Duval JC (1980) Energy transfers from photosystem II to photosystem I in Cryptomonas rufescens (Cryptophyceae), Biochim.Biophys. Acta 591, 104-112. 

---