Plastocyanin and cyt

Integration of PS II and PS I via cyt b6f in chloroplasts (Z scheme of photosynthesis). Pheo a represents pheophytin pQA and pQB represent phytoquinone A and B, respectively A, and At represent electron acceptor chlorophylls, respectively pc represents plastocyanin and Fd represents ferridoxin. 

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. 

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

Most of the thylakoid proteins are organized into four intrinsic protein complexes PS II complex, Cyt b/f complex, PS I complex and ATP synthetase (Fig. 1). The electron transport complexes are linked by mobile electron transport carriers, plastoquinone, plastocyanin and ferredoxin (see Chapter 10). Furthermore, chloroplasts that possess Chi b have the major light-harvesting Chi a/h-proteins of PS II (LHC II) that may represent over 50% of the thylakoid protein [13], as well... 

In 1965 Hill elaborated the two-photosystem scheme further as shown in Fig. 15 (B). In this Z-shaped scheme, two groups of chloroplast components with known redox potentials were placed at the bends of the Z Cyt/, plastocyanin and P700, close to -1-0.4 V, and plastoquinone and Cyt b( close to 0 V. Ferredoxin, with a potential of-0.43 V, is close to the midpoint potential ofhydrogen electrode. For oxygen production, the midpoint potential of the unknown component must exceed that of the oxygen electrode. Over the past thirty years, a variety of Z-schemes have been published in the literature to illustrate the electron-transfer processes in green-plant photosynthesis, but their basic features have not deviated from that shown in Fig. 15 (B). For instance, we show a currently accepted, concise Z-scheme in Fig. 15 (C) it includes many more individual components than were originally envisioned, plus a representation of the operation of the so-called Q-cycle in the Cyi-b(,f complex. 

The Cyt f complex lying between PS II and PS I in the electron-transport system resembles the Cyt be complex of mitochondria and photosynthetic bacteria. These cytochrome complexes possess one Rieske iron-sulfur protein R-FeS (a [2Fe-2S] protein discovered by John Rieske) and a so-called subunit IV. The two fc-hemes of Cyt b(, and the subunit IV span the thylakoid membrane, while the R-FeS and Cyt/ are located near the lumen side. As previously noted, the placement of the i>-hemes across the thylakoid membrane helps form a redox chain across the membrane. The function of the Cyt complex in green-plant thylakoids is to oxidize the plastohydroquinone formed by PS II and to transfer these electrons to plastocyanin. Accordingly, the Cyt ig/ complex has therefore also been called the plastohydroquinone-plastocyanin-oxidoreductase. ... 

In the meantime, Morand, Frame, Colvert, Johnson, Krogmann and Davis used EDC to cross-link spinach PC and turnip Cyt/to form an adduct with a 1 1 stoichiometry. The authors found that Lys-187 of Cyt/cross-linked with Asp-44 of PC, and another, but unidentified, amino-acid residue of Cyt/cross-linked with Glu-60 of PC. Interestingly, the authors found that the PC/Cyt/adduct was incapable of binding with photosystem I or donating electrons to it. This latter finding is apparently consistent with other evidence that Asp-44 is one of the group of four acidic residues (42-45) that are generally believed to bind PsaF and crucial for electron transfer between plastocyanin and photosystem I. 

Hurt and Hauska isolated from spinach aCyt- /complex that is redox active in vitro in plastoquinol-plastocyanin oxidoreduction. Their work showed Cyt/oxidation and Cyt-Z>6 reduction when an isolated PS-I reaction-center particle and reduced plastocyanin and/or plastoquinol are provided as auxiliary redox components for the Cyt-bJcomplex, as illustrated in Fig. 12 (A). Below it are panels showing absorbance changes associated with redox reactions of cytochromes/and b under different experimental conditions. Immediately above the panels are descriptions of the corresponding reaction conditions. 

The redox reactions between PSII and PSI involve the diffusion of the electron carriers plastoquinol and plastocyanin. The cyt bs/f behaves as a control gate with plastoquinol-plastocyanin oxydoreductase activity. 

Plastocyanin functions between cyt bg/f and PS I in the lumen, a continuous space inside the thylakoid membrane system. Fig. 1 illustrates the lateral differences in the membrane composition which may result in an inhomogeneous distribution of plastocyanin in stroma, grana, and exposed grana regions of the lumen. The average distance between PS I and c b5/f in non-appressed and cyt bg/f in appressed membranes is about 20 and 200 nm, respectively. The longer distance from cyt bg/f in appressed membranes may result in a... 

In these conditions, the plastoquinone pool is oxidized while the donors (cyt f, plastocyanin and P700) are fully reduced. The relationship between the redox states of P700, cyt f and plastocyanin was studied during the course of a weak continuous illumination (Fig.5). The low apparent equilibrium constant we measured suggests that... 

K1net1cs of the interaction of the modified PC s with cvt f. mammalian cytochrome c (cvt c) and postassium ferricvanide. The kinetics of the Interaction of the CDNB-modified plastocyanins with cyt f, mammalian cyt c and potassium ferricyanide were measured using the stopped-flow attachment of the Aminco-DW2a spectrophotometer. In each case, second order rate constants were determined using pseudo first order conditions. The reactions were carried out in 25 mM... 

Rate of Reaction of CDNB plastocyanins with K3Fe(CN)Q, Cyt c and Cyt f... 

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

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. 

Fig. 26. Z-Scheme of photosynthesis in plants. Chi is chlorophyll, cyt b, f is cytochrome b, / PC is plastocyanine, (Fe-S) is iron sulfer protein. ATP is adenosine triphosphate ADP is adenosine diphosphate Pj is the phosphate ion and NADP is the nicotinamide adenine dinucleotide phosphate ion [203]...

Figure 2. Schematic of photoinduced electron transport and phosphorylation reactions considered to occur in chloroplast lamellae [from Moreland and Hilton (2)]. Open arrows indicate light reactions solid arrows indicate dark reactions and the narrow dashed line represents the cyclic pathway. Abbreviations used PS I, photosystem I PS II, photosystem II Y, postulated electron donor for photosystem II Q, unknown primary electron acceptor for photosystem II PQ, plastoquinones cyt b, b-type cytochromes cyt f, cytochrome f PC, plastocyanin P700, reaction center chlorophyll of photosystem I FRS, ferredoxin-reducing substance Fd, ferredoxin Fp, ferredoxin-NADP oxidoreductase FeCy, ferricyanide asc, ascorbate and DPIP, 2,6-dichloropheno-lindophenol. The numbers la, lb, 2, 3, and 4 indicate postulated sites of action by...

Fig. 6. Sites of inhibitory action of DCMU in photosynthetic electron transport chain. The abbreviations are as follows - Cyt f cytochrome f, Fd ferredoxin, Mn water-splitting complex (manganese-containing), P680 pigment complex of photosystem II, P700 pigment complex of photosystem I, PC plastocyanin, PQ plastquinone, Q quencher, Rd NADP reductase and X direct electron acceptor complex...

Plastocyanin 11 2 1 0.37 Blue protein (reduced form is colorless) that accepts electrons from the Cyt bff complex and donates them to Photosystem I contains one Cu soluble in aqueous solutions, but occurs in the thylakoid lumen... 

---