An EPR analysis of the partially purified cytochrome bf complex of higher-plant chloroplasts

Inhibitor binding to isolated chloroplast cytochrome bf complex

Effects of three inhibitors of quinol oxidation in the chloroplast cytochrome bf complex (stigmatellin, tridecylstigmatellin and dibromothymoquinone) were studied in an isolated system comprising Photosystem I (PS I) particles, plastocyanin (PC) and cytochrome bf complex, in the absence of quinol or quinone. Addition of these inhibitors increased the extent of cytochrome f oxidation after a laser flash created oxidised PS I reaction centre (P700) and PC, and decreased somewhat the extent of PC oxidation. The re-reduction of oxidised P700 was more complete than when inhibitor was absent. The data were simulated with reactions which included the putative reduction of cytochrome f by the Rieske centre (FeS) and different rate-coefficients according as to whether inhibitor was bound to the bf complex or not. It was concluded that under the conditions studied the Rieske centre donated electrons to oxidised cytochrome f and plastocyanin with an average rate coefficient of 35 s(-1). This electron transfer was prevented by any of the three inhibitors, which also increased the equilibrium coefficient for the cytochrome f/PC reaction by a maximum factor of two. This increase corresponded to a decrease in the back reaction coefficient and an increase in the forward rate. The equilibrium coefficient for the reduction of oxidised P700 by PC was about 2 in the absence of inhibitor but increased to about 20 in their presence, but only if cytochrome bf complex was additionally present. This was attributed to the transient formation of complexes between P700 with bound plastocyanin, and bf complex. The operative mid-point potential of FeS, if that of cytochrome f is 370 mV, was 390 mV. Deviations in midpoint potentials (P700/plastocyanin) from solution values were attributed to the bound state of the reactants. Estimates were made of the binding coefficient of each of the three inhibitors to p-sites in the cytochrome bf complex in the absence of competing quinol. A stoichiometry of two inhibitors per bf dimer was necessary to cause the above changes in reduction potential of cyt f and PC. A result of one inhibitor per dimer was statistically unlikely, particularly in the case of tridecylstigmatellin.

A guide to electron paramagnetic resonance spectroscopy of Photosystem II membranes

This guide is intended to aid in the detection and identification of paramagnetic species in Photosystem II membranes, by electron paramagnetic resonance spectroscopy. The spectral features and occurrence of each of the electron paramagnetic resonance signals from Photosystem II are discussed, in relation to the nature of the moiety giving rise to the signal and the role of that species in photosynthetic electron transport. Examples of most of the signals discussed are shown. The electron paramagnetic resonance signals produced by the cytochrome b6f and Photosystem I complexes, as well as the signals from other common contaminants, are also reviewed. Furthermore, references to seminal experiments on bacterial reaction centers are included. By reviewing both the spectroscopic and biochemical bases for the electron paramagnetic resonance signals of the cofactors that mediate photosynthetic electron transport, this paper provides an introduction to the use and interpretation of electron paramagnetic resonance spectroscopy in the study of Photosystem II.

Structure and function of the chloroplast cytochrome bf complex

The chloroplast cytochrome bf complex is an intrinsic multisubunit protein from the thylakoid membrane consisting of four polypeptides: cytochrome f, a two heme containing cytochrome b 6, the Rieske iron-sulfur protein, and a 17 kD polypeptide of undefined function. The complex functions in electron transfer between PSII and PSI, where most mechanisms suggest that the transfer of a single reducing equivalent from plastoquinol to plastocyanin results in the translocation of two protons across the membrane. Primary sequence analyses, dichroism studies, and functional considerations allow the construction of an approximate structural model of a monomeric complex, although some evidence exists for a dimeric structure. Resolution of the properties of the two cytochrome b 6 hemes has relied upon the availability of purified solubilized complex, while evidence in the thylakoid suggests the difference between the two hemes are not as great in situ. Such variability in the spectroscopic and electrochemical properties of the cytochrome b 6 is a major concern during the experimental use of the purified complex. There is a general consensus that the complex contains a plastoquinol oxidizing (Qz) site, although the evidence for a plastoquinone reduction (Qc) site, called for in most mechanistic hypotheses, is less substantive. Probably the most severe challenge to the so called Q-cycle mechanism comes from experimental observations made with cytochrome b 6 initially reduced, where proposed interpretations more closely resemble a b-cycle than a Q-cycle. Although functional during cyclic electron transfer, the role of the complex and its possible interaction with other proteins, has not been completely resolved.

The EPR spectra of the cytochrome b-c1 complex of Rhodopseudomonas sphaeroides

The purified cytochrome b-c1 complex of Rhodopseudomonas sphaeroides has two b cytochromes distinguishable by optical, thermodynamic and electron paramagnetic resonance criteria (gz values are approximately equal to 3.75 and approximately equal to 3.4). EPR features typical of a Rieske iron sulfur cluster (g values of 2.03 1.90 and 1.81) and a c1 type cytochrome (g approximately equal to 3.4) were also observed. The b and c1 cytochromes were individually purified from the complex. The cytochrome c1 retained its native EPR spectrum. The b cytochrome lost over 90% of the intensity from the 'b566 type' heme site (g approximately equal to 3.75), while the 'b561 type' heme site (g approximately equal to 3.4) retained its native EPR spectrum.

Mechanisms of quinol oxidation in photosynthesis

The mechanisms by which para-benzoquinols can be oxidized is reviewed. Emphasis is placed on the information available from chemical and electrochemical studies which may provide insight into the biochemical mechanisms of plastoquinol oxidation in the chloroplast. Three mechanisms of quinol oxidation are possible: (1) The removal of an electron from the quinol, QH inf2 (sup·t) , directly to produce the radical cation, QH 2 (·+) . This may be achieved electrochemically only at very high potential in acidic media. The reaction may be of relevance to D1, the donor to P-680. (2) The removal of an electron from the anionic quinol. QH(-), formed by quinol deprotonation. It is likely that the catalytic mechanism of the cytochrome bf complex involves this mechanism. (3) The removal of an electron from the dianionic quinol, Q(2-). This route will be dominant only under basic or aprotic conditions and at very low potentials.

Localization of photosynthetic electron transport components in mesophyll and bundle sheath chloroplasts of Zea mays

The organization of the electron transport components in mesophyll and bundle sheath chloroplasts of Zea mays was investigated. Grana-containing mesophyll chloroplasts (chlorophyll a to chlorophyll b ratio of about 3.0) possessed the full complement of the various electron transport components, comparable to chloroplasts from C3 plants. Agranal bundle sheath chloroplasts (Chl a/Chl b greater than 5.0) contained the full complement of photosystem (PS) I and of cytochrome (cyt) f but lacked a major portion of PS II and its associated Chl a/b light-harvesting complex (LHC), and most of the cyt b559. The kinetic analysis of system I photoactivity revealed that the functional photosynthetic unit size of PS I was unchanged and identical in mesophyll and bundle sheath chloroplasts. The results suggest that PS I is contained in stroma-exposed thylakoids and that it does not receive excitation energy from the Chl a/b LHC present in the grana. A stoichiometric parity between PS I and cyt f in mesophyll and bundle sheath chloroplasts indicates that biosynthetic and functional properties of cyt f and P700 are closely coordinated. Thus, it is likely that both cyt f and P700 are located in the membrane of the intergrana thylakoids only. The kinetic analysis of PS II photoactivity revealed the absence of PS II alpha from the bundle sheath chloroplasts and helped identify the small complement of system II in bundle sheath chloroplasts as PS II beta. The distribution of the main electron transport components in grana and stroma thylakoids is presented in a model of the higher plant chloroplast membrane system.

A cytochrome f/b6 complex of five polypeptides with plastoquinol-plastocyanin-oxidoreductase activity from spinach chloroplasts

The isolation of a cytochrome f/b6 complex from spinach chloroplasts, with high yield and purity is reported. The complex consists of five polypeptides with a molecular mass of 34, 33, 23.5, 20 and 17.5 kDa, and contains one cytochrome f, two cytochromes b6 and the Rieske Fe-S center with two non-heme irons. It does not contain plastocyanin and is almost completely devoid of chlorophyll and carotenoids, but lipid and detergent are present. It is lacking cytochrome b-559, although three of the five polypeptides seem to carry heme groups. The preparation has plastoquinol-plastocyanin oxidoreductase activity with plastoquinol-1 and plastoquinol-9, which is sensitive to 2,5-dibromomethylisopropyl-p-benzoquinone, to 2-iodo-6-isopropyl-3-methyl-2',4',4'-trinitrodiphenyl ether, to 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole, and to bathophenanthroline. Characteristics of this activity with respect to substrate concentrations, pH, detergent effect and other parameters are described.

EPR characteristics of oxygen-evolving photosytem II particles from Phormidium laminosum

The EPR characteristics of oxygen evolving particles prepared from Phormidium laminosum are described. These particles are enriched in Photosystem II allowing EPR investigation of signals which were previously small or masked by those from Photosystem I in other preparations. EPR signals from a Signal II species and high potential cytochrome beta-559 appear as they are photooxidised at cryogenic temperatures by Photosystem II. The Signal II species is a donor close to the Photosystem II reaction centre and may represent part of the charge accumulation system of water oxidation. An EPR signal from an iron-sulphur centre which may represent an unidentified component of photosynthetic electron transport is also described. The properties of the oxygen evolving particles show that the preparation is superior to chloroplasts or unfractionated alga membranes for the study of Photosystem II with a functional water oxidation system.

Factors regulating the slow electrogenic phase in green algae and higher plants

The electrochromic rise in the millisecond range, corrected for the subsequent decay, has been studied in Chlorella cells and spinach chloroplasts. The half-time of the electrochromic rise in the millisecond range is independent of the redox states of the components in the electrogenic loop. It is also independent of pH in the absence of a pH gradient, but it is dependent upon the transmembrane electric field and the pH gradient. The limiting step of the overall process is thus the electrogenic reaction itself. The amplitude of the electrochromic rise in the millisecond range is controlled by two classes of factors: the redox state of its reactants and the free energy available in the overall process including the electrogenic reaction. This free energy depends upon delta pH and transmembrane electric field. A fast reduction of cytochrome f+ (and thus probably of the Rieske protein) by Photosystem II is still possible when the electrogenic reaction is impeded for energetic reasons. The kinetic behavior of the electrogenic reaction could be interpreted by the following reaction: UH2int + FeS+ + X2 + Hext in equilibrium U + 2H+int + FeS + X2H where FeS is the Rieske protein; X2 would be a new protein (observed in Bouges-Bocquet, B. (1980) FEBS Lett. 117, 54-58); UH2 which is likely a special quinone, would release its two protons inside the thylakoids and X2 would use protons from the external medium.