Studies on well-coupled Photosystem I-enriched subchloroplast vesicles — characteristics and reinterpretation of single-turnover cyclic electron transfer

Spectral and kinetic resolution of the bc1 complex components in situ: a simple and robust alternative to the traditional difference wavelength approach

The kinetics of the cytochrome (cyt) components of the bc(1) complex (ubiquinol: cytochrome c oxidoreductase, Complex III) are traditionally followed by using the difference of absorbance changes at two or more different wavelengths. However, this difference-wavelength (DW) approach is of limited accuracy in the separation of absorbance changes of components with overlapping spectral bands. To resolve the kinetics of individual components in Rhodobacter sphaeroides chromatophores, we have tested a simplified version of a least squares (LS) analysis, based on measurement at a minimal number of different wavelengths. The success of the simplified LS analysis depended significantly on the wavelengths used in the set. The "traditional" set of 6 wavelengths (542, 551, 561, 566, 569 and 575 nm), normally used in the DW approach to characterize kinetics of cyt c(tot) (cyt c(1)+cyt c(2)), cyt b(L), cyt b(H), and P870 in chromatophores, could also be used to determine these components via the simplified LS analysis, with improved resolution of the individual components. However, this set is not sufficient when information about cyts c(1) and c(2) is needed. We identified multiple alternative sets of 5 and 6 wavelengths that could be used to determine the kinetics of all 5 components (P870 and cyts c(1), c(2), b(L), and b(H)) simultaneously, with an accuracy comparable to that of the LS analysis based on a full set of wavelengths (1 nm intervals). We conclude that a simplified version of LS deconvolution based on a small number of carefully selected wavelengths provides a robust and significant improvement over the traditional DW approach, since it accounts for spectral interference of the different components, and uses fewer measurements when information about all five individual components is needed. Using the simplified and complete LS analyses, we measured the simultaneous kinetics of all cytochrome components of bc(1) complex in the absence and presence of specific inhibitors and found that they correspond well to those expected from the modified Q-cycle. This is the first study in which the kinetics of all cytochrome and reaction center components of the bc(1) complex functioning in situ have been measured simultaneously, with full deconvolution over an extended time range.

Spectral analysis of the bc(1) complex components in situ: beyond the traditional difference approach

The cytochrome (cyt) bc(1) complex (ubiquinol: cytochrome c oxidoreductase) is the central enzyme of mitochondrial and bacterial electron-transport chains. It is rich in prosthetic groups, many of which have significant but overlapping absorption bands in the visible spectrum. The kinetics of the cytochrome components of the bc(1) complex are traditionally followed by using the difference of absorbance changes at two or more different wavelengths. This difference-wavelength (DW) approach has been used extensively in the development and testing of the Q-cycle mechanism of the bc(1) complex in Rhodobacter sphaeroides chromatophores. However, the DW approach does not fully compensate for spectral interference from other components, which can significantly distort both amplitudes and kinetics. Mechanistic elaboration of cyt bc(1) turnover requires an approach that overcomes this limitation. Here, we compare the traditional DW approach to a least squares (LS) analysis of electron transport, based on newly determined difference spectra of all individual components of cyclic electron transport in chromatophores. Multiple sets of kinetic traces, measured at different wavelengths in the absence and presence of specific inhibitors, were analyzed by both LS and DW approaches. Comparison of the two methods showed that the DW approach did not adequately correct for the spectral overlap among the components, and was generally unreliable when amplitude changes for a component of interest were small. In particular, it was unable to correct for extraneous contributions to the amplitudes and kinetics of cyt b(L). From LS analysis of the chromophoric components (RC, c(tot), b(H) and b(L)), we show that while the Q-cycle model remains firmly grounded, quantitative reevaluation of rates, amplitudes, delays, etc., of individual components is necessary. We conclude that further exploration of mechanisms of the bc(1) complex, will require LS deconvolution for reliable measurement of the kinetics of individual components of the complex in situ.

Kinetic modeling of the photosynthetic electron transport chain

A simulation model of the photosynthetic electron transport chain operating under steady state conditions is presented. The model enables the calculation of (1) the rates of electron transport and transmembrane proton translocation, (2) the proton/electron stoichiometry, (3) the number of electrons stored in the different redox centers and (4) the stationary transmembrane pH difference. Light intensity and proton permeability of the thylakoid membrane are varied in order to compare the predictions of the model with experimental data. The routes of electron transport and proton translocation are simulated by two coupled arithmetic loops. The first one represents the sequence of reaction steps making up the linear electron transport chain and the Q-cycle. This loop yields the electron flow rate and the proton/electron ratio. The second loop balances the H+ fluxes and yields the internal H+ concentration. The bifurcation of the electron transport pathways at the stage of plastoquinol oxidation is obligatory. The first electron enters always the linear branch and is transferred to photosystem I. The electron of the remaining semiquinone can enter the Q-cycle or, alternatively, the semiquinone can be lost from the cytochrome b6f complex. The competition between these two reactions explains the experimentally observed variability of the proton/electron ratio. We also investigated additional model variants, where the variation of the proton/electron stoichiometry is attributed to other loss reactions within the cytochrome b6f complex. However, the semiquinone detachment seems to be the best candidate for a satisfactory description of the experimental data. Additional calculations were done in order to assess the effects of the movement of the Rieske protein on linear electron transport; it was found that this conformational change does not limit the electron transport rate, if it occurs with a time constant of at least 1000 s(-1).

N-terminal mutants of chloroplast cytochrome f. Effect on redox reactions and growth in Chlamydomonas reinhardtII

The N-terminal tyrosine of cytochrome f, which provides the sixth ligand to the heme group, has been changed by site-directed mutagenesis in Chlamydomonas reinhardtii to evaluate the role of this amino acid in assembly and function. The second and third residues, proline and valine, respectively, have also been mutated. Y1P is the only strain that did not grow photoautotrophically. The other strains show cytochrome b6f complex/photosystem I reaction center chlorophyll, photosystem I unit size and chlorophyll a+b/cell ratios comparable with wild-type cells. Rates of cytochrome f photooxidation in all strains were similar (t1/2 approximately = 300 microsec), whereas the rate of re-reduction sensitive to stigmatellin (at Eh = 0 mV, (where Eh is the ambient redox potential) for wild-type, Y1W, Y1F, Y1S, P2V, and V3P had a tl/2 of 3, 4, 5, 9, 40, and 2 ms, respectively. Rates of oxygen evolution by whole cells of P2V, Y1F, and Y1S were 67, 80, and 80% of wild-type rates, respectively. At low light intensity, all competent strains had the same growth rate whereas at saturating intensities, only P2V showed a significant inhibition. These results are considered in relation to structure-function relationships in the cytochrome f molecule.