Cytochrome f function in photosynthetic electron transport

On rate limitations of electron transfer in the photosynthetic cytochrome b6f complex

Considering information in the crystal structures of the cytochrome b(6)f complex relevant to the rate-limiting step in oxygenic photosynthesis, it is enigmatic that electron transport in the complex is not limited by the large distance, approximately 26 Å, between the iron-sulfur cluster (ISP) and its electron acceptor, cytochrome f. This enigma has been explained for the respiratory bc(1) complex by a crystal structure with a greatly shortened cluster-heme c(1) distance, leading to a concept of ISP dynamics in which the ISP soluble domain undergoes a translation-rotation conformation change and oscillates between positions relatively close to the cyt c(1) heme and a membrane-proximal position close to the ubiquinol electron-proton donor. Comparison of cytochrome b(6)f structures shows a variation in cytochrome f heme position that suggests the possibility of flexibility and motion of the extended cytochrome f structure that could entail a transient decrease in cluster-heme f distance. The dependence of cyt f turnover on lumen viscosity is consistent with a role of ISP - cyt f dynamics in determination of the rate-limiting step under conditions of low light intensity. Under conditions of low light intensity and proton electrochemical gradient present, for example, under a leaf canopy, it is proposed that a rate limitation of electron transport in the b(6)f complex may also arise from steric constraints in the entry/exit portal for passage of the plastoquinol and -quinone to/from its oxidation site proximal to the iron-sulfur cluster.

Photooxidation of the high-potential iron-sulfur center in chloroplasts

The high-potential iron-sulfur center associated with the g = 1.89-1.90 electron paramagnetic resonance signal in spinach chloroplasts is fully reduced in the dark. It is photooxidized in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and reduced after illumination with red actinic light in the absence of inhibitors. The light-induced redox changes of the high-potential iron-sulfur center are shown to correlate with those of plastocyanin detected at g = 2.05. Demonstration of light-induced redox changes in the high-potential iron-sulfur center provides support for a functional role of this component in the chloroplast electron transport chain.

Electron transfer from the Rieske iron-sulfur protein (ISP) to cytochrome f in vitro. Is a guided trajectory of the ISP necessary for competent docking?

The time course of electron transfer in vitro between soluble domains of the Rieske iron-sulfur protein (ISP) and cytochrome f subunits of the cytochrome b(6)f complex of oxygenic photosynthesis was measured by stopped-flow mixing. The domains were derived from Chlamydomonas reinhardtii and expressed in Escherichia coli. The expressed 142-residue soluble ISP apoprotein was reconstituted with the [2Fe-2S] cluster. The second-order rate constant, k(2)((ISP-f)) = 1.5 x 10(6) m(-1) s(-1), for ISP to cytochrome f electron transfer was <10(-2) of the rate constant at low ionic strength, k(2)((f-PC))(> 200 x 10(6) m(-1) s(-1)), for the reduction of plastocyanin by cytochrome f, and approximately 1/30 of k(2)((f-PC)) at the ionic strength estimated for the thylakoid interior. In contrast to k(2)((f-PC)), k(2)((ISP-f)) was independent of pH and ionic strength, implying no significant role of electrostatic interactions. Effective pK values of 6.2 and 8.3, respectively, of oxidized and reduced ISP were derived from the pH dependence of the amplitude of cytochrome f reduction. The first-order rate constant, k(1)((ISP-f)), predicted from k(2)((ISP-f)) is approximately 10 and approximately 150 times smaller than the millisecond and microsecond phases of cytochrome f reduction observed in vivo. It is proposed that in the absence of electrostatic guidance, a productive docking geometry for fast electron transfer is imposed by the guided trajectory of the ISP extrinsic domain. The requirement of a specific electrically neutral docking configuration for ISP electron transfer is consistent with structure data for the related cytochrome bc(1) complex.

Deuterium kinetic isotope effects in the p-side pathway for quinol oxidation by the cytochrome b(6)f complex

Plastoquinol oxidation and proton transfer by the cytochrome b(6) f complex on the lumen side of the chloroplast thylakoid membrane are mediated by high and low potential electron transport chains. The rate constant for reduction, k(bred), of cytochrome b(6) in the low potential chain at ambient pH 7.5-8 was twice that, k(fred), of cytochrome f in the high potential chain, as previously reported. k(bred) and k(fred) have a similar pH dependence in the presence of nigericin/nonactin, decreasing by factors of 2.5 and 4, respectively, from pH 8 to an ambient pH = 6, close to the lumen pH under conditions of steady-state photosynthesis. A substantial kinetic isotope effect, k(H2O)/k(D2O), was found over the pH range 6-8 for the reduction of cytochromes b(6) and f, and for the electrochromic band shift associated with charge transfer across the b(6)f complex, showing that isotope exchange affects the pK values linked to rate-limiting steps of proton transfer. The kinetic isotope effect, k(bred)(H2O)/k(bred) (D2O) approximately 3, for reduction of cytochrome b in the low potential chain was approximately constant from pH 6-8. However, the isotope effect for reduction of cytochrome f in the high potential chain undergoes a pH-dependent transition below pH 6.5 and increased 2-fold in the physiological region of the lumen pH, pH 5.7-6.3, where k(fred)(H2O)/k(fred)(D2O) approximately 4. It is proposed that a rate-limiting step for proton transfer in the high potential chain resides in the conserved, buried, and extended water chain of cytochrome f, which provides the exit port for transfer of the second proton derived from p-side quinol oxidation and a "dielectric well" for charge balance.

The photosystem II subunit CP29 can be phosphorylated in both C3 and C4 plants as suggested by sequence analysis

The CP29 subunit of Photosystem II is reversibly phosphorylated in Zea mays upon exposure to high light in the cold (Bergantino et al., J Biol Chem 270 (1995) 8474-8481). This phenomenon was previously proposed to be restricted to C4 plants. We present the complete sequence of the CP29 protein, deduced from a maize Lhcb4 cDNA clone, and its comparison with the previously known Lhcb4 sequences of two C3 plants: Hordeum vulgare and Arabidopsis thaliana. Despite the relatively low degree of homology in their amino-terminal region, i.e. the part of the molecule which is phosphorylated in maize, the three polypeptides conserve consensus sequences for the site of phosphorylation. We proved by immunoblotting and 33P-labelling that the same post-translational modification occurs in barley. Being thus common to C3 and C4 plant species, the phosphorylation of this minor antenna complex of Photosystem II appears now as a widespread phenomenon, possibly part of the phosphorylation cascade which signals the redox status of the plastoquinone to the nuclear transcription apparatus. Arabidopsis plants do not show phosphorylation of CP29 in the same conditions, but other low-molecular-weight phosphoproteins, whose role need to be elucidated, become evident.

Effect of the interdomain basic region of cytochrome f on its redox reactions in vivo

The prominent interdomain basic surface region seen in the high-resolution structure of the active lumen-side C-terminal fragment of turnip cytochrome f, containing the conserved Lys58,65,66 (large domain) and Lys187 (small domain), has been inferred from in vitro studies to be responsible for docking of its physiological oxidant, plastocyanin. The effect of the putative docking region of cyt f on its reactivity in vivo was tested by site-directed mutagenesis in Chlamydomonas reinhardtii. Three charge-neutralizing mutants were constructed involving: (i)the two lysines (Lys188Asn-Lys189Gln) in the small domain, (ii) the three lysines (Lys58Gln-Lys65Ser-Lys66Glu) in the large domain, and (iii) all five of these lysines spanning both domains. All mutants grew phototrophically. The mutants displayed a 20-30% increase in average generation time, and comparable decreases in rates of steady-state oxygen evolution and the slow (millisecond) electrochromic 515 nm band shift. The magnitude of the changes was greatest in the 5-fold Lys-minus mutant (Lys58Gln-Lys65Ser-Lys66Glu-Lys188Asn-Lys189G ln). The mutants showed a small increase (approximately 25%) in the t1/2, from 0.2 to 0.25 ms, of cyt f photooxidation, far less than anticipated (ca. 100-fold) from in vitro studies of the effect of high ionic strength on the cyt f-PC interaction. The t1/2 of cyt f dark reduction via the Rieske protein increased from 5-6 ms in the wild type to 11-12 ms in the 5-fold Lys-minus mutant. Cells grown phototrophically in the absence of Cu, where cyt c6 is the electron acceptor of cyt f, displayed net rates of cytochrome photooxidation that were slightly faster than those in the presence of Cu, which also decreased by a factor of < or = 25% in the Lys-minus mutants. It was concluded that (a) the net effect of electrostatic interaction between cytochrome f and its electron acceptor in vivo is much smaller than measured in vitro and is not rate-limiting. This may be a consequence of a relatively high ionic strength environment and the small diffusional space available for collision and docking in the internal thylakoid lumen of log phase C. reinhardtii. (b) The efficiency of electron transfer to cytochrome f from the Rieske protein is slightly impaired by the neutralization of the lysine-rich domain.

Efficiency of photosynthesis in continuous and pulsed light emitting diode irradiation

The light utilization efficiency and relative photon requirement of photosynthesis in pulsed and continuous light from light emitting diodes (LEDs) has been measured. First, we chacterized the photon requirement of photosynthesis from light of LEDs that differ in spectral quality. A photon requirement of 10.3±0.4 was measured using light from a 658 nm peak wavelength (22 nm half band width) LED over the range of 0-50 μmol photons m(-2) s(-1) in 2 kPa O2 in leaves of tomato (Lycopersicon esculentum Mill., cv. VF36). Because the conversion of electrical power to photons increased with wavelength, LED lamps with peak photon output of 668 nm were most efficient for converting electricity to photosynthetically fixed carbon. The effect of pulsed irradiation on photosynthesis was then measured. When all of the light to make the equivalent of 50 μmol photons m(-2) s(-1) was provided during 1.5 μs pulses of 5000 μmol photons m(-2) s(-1) followed by 148.5 μs dark periods, photosynthesis was the same as in continuous 50 μmol photons m(-2) s(-1). When the pulse light and dark periods were lengthened to 200 μs and 19.8 ms, respectively, photosynthesis was reduced, although the averaged photon flux density was unchanged. Under these conditions, the light pulses delivered 10(17) photons m(-2), which we calculate to be equivalent to the capacitance of PS I or PS II. Data support the theory that photons in pulses of 100 μs or shorter are absorbed and stored in the reaction centers to be used in electron transport during the dark period. When light/dark pulses were lengthened to 2 ms light and 198 ms dark, net photosynthesis was reduced to half of that measured in continuous light. Pigments of the xanthophyll cycle were not affected by any of these pulsed light treatments even though zeaxanthin formation occurred when leaves were forced to dissipate an equal amount of continuous light.

Changes in P-700 Oxidation during the Early Stages of the Induction of Photosynthesis

Following dark adaptation, the response to irradiance of chlorophyll (Chl) fluorescence, the light-induced absorbance change around 820 nm (to measure reaction center Chl of photosystem I [PSI] P-700 oxidation), and CO2 fixation were examined in pea (Pisum sativum L.) leaves under a range of conditions. Initially, P-700 oxidation is restricted by a lack of regeneration of PSI electron acceptors, and the increase of oxidized P-700 (P-700+) that occurs during approximately the first 60 s of irradiation is largely independent of the resistance to electron flow between the two photosystems. Under these conditions, the quantum efficiency for linear electron flow is directly positively related to P-700+ accumulation, which is in contrast to the direct negative correlation that is the most frequently reported relationship between P-700+ accumulation and the quantum efficiency for linear electron flow.

Dissipation of the Proton Electrochemical Potential in Intact Chloroplasts (II. The pH Gradient Monitored by Cytochrome f Reduction Kinetics)

The potency of various uncouplers for collapsing the light-induced pH gradient across thylakoid membranes in intact chloroplasts was investigated by time-resolved optical spectroscopy. The thylakoid transmembrane pH gradient ([delta]pH) was monitored indirectly by measuring the rate of cytochrome (Cyt) f reduction following a light flash of sufficient duration to create a sizable [delta]pH. The results show that the rate of Cyt f reduction is controlled in part by the internal pH of the thylakoid inner aqueous space. At pH values from 6.5 to 8.0, the Cyt f reduction rate was maximal, whereas at lower pH values from 6.5 to 5.5 the reduction rate decreased to 25% of the maximal rate. The ability of three uncouplers, nigericin, carbonylcyanide m-chlorophenylhydrazone, and gramicidin, to accelerate the rate of Cyt f reduction was determined for intact chloroplasts isolated from spinach (Spinacia oleracea). The efficacy of the uncouplers for collapsing the [delta]pH was determined using the empirical relationship between the [delta]pH and the Cyt f reduction rate. For intact chloroplasts, nigericin was the most effective uncoupler, followed by carbonylcyanide m-chlorophenylhydrazone, which interacted strongly with bovine serum albumin. Gramicidin D, even at high gramicidin:chlorophyll ratios, did not completely collapse the pH gradient, probably because it partitions in the envelope membranes and does not enter the intact chloroplast.

Inactive photosystem II centers: A resolution of discrepencies in photosystem II quantitation

The abundance of photosystem II in chloroplast thylakoid membranes has been a contentious issue because different techniques give quite different estimates of photosystem II titer. This discrepancy led in turn to disagreements regarding the stoichiometry of photosystem II to photosystem I in these membranes. We believe that the discrepancy in photosystem II quantitation is resolved by evidence which shows that a large population of photosystem II centers with negligible turnover rates are present in isolated thylakoid membranes as well as in normally developed leaves of healthy plants.

Photosynthetic apparatus of pea thylakoid membranes : response to growth light intensity

We investigated the effect of growth light intensity on the photosynthetic apparatus of pea (Pisum sativum) thylakoid membranes. Plants were grown either in a growth chamber at light intensities that ranged from 8 to 1050 microeinsteins per square meter per second, or outside under natural sunlight. In thylakoid membranes we determined: the amounts of active and inactive photosystem II, photosystem I, cytochrome b/f, and high potential cytochrome b(559), the rate of uncoupled electron transport, and the ratio of chlorophyll a to b. In leaves we determined: the amounts of the photosynthetic components per leaf area, the fresh weight per leaf area, the rate of electron transport, and the light compensation point. To minimize factors other than growth light intensity that may alter the photosynthetic apparatus, we focused on peas grown above the light compensation point (20-40 microeinsteins per square meter per second), and harvested only the unshaded leaves at the top of the plant. The maximum difference in the concentrations of the photosynthetic components was about 30% in thylakoids isolated from plants grown over a 10-fold range in light intensity, 100 to 1050 microeinsteins per square meter per second. Plants grown under natural sunlight were virtually indistinguishable from plants grown in growth chambers at the higher light intensities. On a leaf area basis, over the same growth light regime, the maximum difference in the concentration of the photosynthetic components was also about 30%. For peas grown at 1050 microeinsteins per square meter per second we found the concentrations of active photosystem II, photosystem I, and cytochrome b/f were about 2.1 millimoles per mol chlorophyll. There were an additional 20 to 33% of photosystem II complexes that were inactive. Over 90% of the heme-containing cytochrome f detected in the thylakoid membranes was active in linear electron transport. Based on these data, we do not find convincing evidence that the stoichiometries of the electron transport components in the thylakoid membrane, the size of the light-harvesting system serving the reaction centers, or the concentration of the photosynthetic components per leaf area, are regulated in response to different growth light intensities. The concept that emerges from this work is of a relatively fixed photosynthetic apparatus in thylakoid membranes of peas grown above the light compensation point.

Plastocyanin stimulation of whole chain and photosystem I electron transport in inside-out thylakoid vesicles

Inside-out and right-side-out thylakoid vesicles were isolated from spinach chloroplasts by aqueous-polymer two-phase (dextran/polyethylene glycol) partitioning. Externally added plastocyanin stimulated the whole-chain and PSI electron transport rates in the inside-out thylakoid vesicles by about 500 and 350%, respectively, compared to about 50% stimulation for both assays in the fraction enriched in right-side-out vesicles. No apparent stimulation by plastocyanin was observed in unbroken Class II thylakoids. The electron transport between PSII and PSI in inside-out thylakoid vesicles appears to be interrupted due to plastocyanin release from the thylakoids by the Yeda press treatment, but it was restored by externally added plastocyanin. The P700 content of the inside-out membrane preparations, measured by chemical and photochemical methods, was 1 P700 per 1100 to 1500 chlorophylls while it was about 1 P700 per 500 chlorophylls for the right-side-out vesicles. The data presented support the concept of lateral heterogeneity of PS I and II in thylakoid membranes, but does not support a virtual or total absence of PSI in the appressed grana partitions. Further, the heterogeneity does not appear to be as extreme as suggested earlier. Although PSI is somewhat depleted in the appressed grana membrane region, there is adequate photochemically active P700, when sufficient plastocyanin is available, to effectively couple PSI electron transfer with the preponderant PSII in linear electron transport.

Lateral Distribution of the Cytochrome b(6)/f and Coupling Factor ATP Synthetase Complexes of Chloroplast Thylakoid Membranes

We have visualized directly the distribution of the cytochrome b(6)/f and coupling factor ATP synthetase complexes in thylakoid membranes of embedded, thin-sectioned, intact chloroplasts by using rabbit antibodies directed against each complex, followed by ferritin-conjugated goat anti- (rabbit immunoglobulin G) antibodies. The labeling patterns indicate that in spinach (Spinacia oleracea) chloroplasts the cytochrome b(6)/f complex is distributed laterally throughout both stacked grana and unstacked stroma membrane regions, whereas the coupling factor ATP synthetase complex is found exclusively in stroma thylakoids and in the marginal and end membranes of grana.

Stoichiometries of electron transport complexes in spinach chloroplasts

The stoichiometric relationship among photosystem II complexes, photosystem I complexes, cytochrome b/f complexes, high-potential cytochrome b-559, and chlorophyll in spinach chloroplasts has been determined. Two features of this data stand out, in contrast to currently proposed stoichiometries in which the ratio of photosystem II to photosystem I is reported to be 2:1 and the chlorophyll to reaction center ratio to be as low as 260:1. Using a variety of techniques it was found that the stoichiometry of photosystem II:photosystem I:cytochrome b/f complex was 1:1:1, within 10%, and that the ratio of total chlorophyll to these components was 600:1, also within 10%. A ratio of two high-potential cytochrome b-559 molecules per 640 chlorophyll, or two molecules per photosystem II reaction center, was found. These ratios were remarkably constant regardless of the time of year or the source of the spinach. The concentration of photosystem II complexes was determined using a pH electrode to measure the flash-induced proton release resulting from water oxidation. The photosystem I reaction center concentration was measured by two different techniques that compared favorably. In the first method a pH electrode was used to measure the amount of flash-induced proton consumption associated with the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive oxidation of N,N,N',N'- tetramethylphenylenediamine , resulting in the production of hydrogen peroxide. In the second method the amount of P700 oxidized by far-red light was determined using dual-wavelength spectroscopy. The concentration of the cytochrome b/f complex was determined assuming 1 mol of cytochrome f per complex. The concentration of cytochrome f was measured spectroscopically by its light-induced turnover and by chemical difference spectra. The concentration of high-potential cytochrome b-559 was determined by chemical difference spectra. In addition to these studies, the light-induced absorbance change exhibiting a peak at 323 nm that has been attributed to the reduction of the primary quinone acceptor of photosystem II has been investigated. This measurement frequently has been used to quantitate the photosystem II to chlorophyll ratio. However, in view of these results it is argued that this technique significantly overestimates the photosystem II concentration.

Inhibition of photosynthetic electron transport by diphenyl ether herbicides

The effects of the diphenyl ether herbicides HOE 29152 (methyl-2[4-(4-trifluoromethoxy) phenoxy] propanoate) and nitrofluorfen (2-chloro-1-[4-nitrophenoxy]-4-[trifluoromethyl]benzene) on photosynthetic electron transport have been examined with pea seedling and spinach chloroplasts. Linear electron transport (water to ferricyanide or methylviologen) is inhibited in treated chloroplasts, but neither photosystem II activity (water to dimethylquinone plus dibromothymoquinone) nor photosystem I activity (diaminodurene to methylviologen) is affected. Cyclic electron flow, cata-lyzed by either phenazine methosulfate or diaminodurene, is resistant to inhibition by nitrofluorfen. In diphenyl ether-treated chloroplasts the half-time for the dark reduction of cytochrome f is increased 5- to 15-fold. These data indicate that the site of inhibition for the diphenyl ethers is between the two photosystems in the plastoquinone-cytochrome f region.