The kinetics of photophosphorylation at clamped ΔpH indicate a random order of substrate binding

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Monte Carlo simulation from proton slip to "coupled" proton flow in ATP synthase based on the bi-site mechanism

ATP synthase couples proton flow to ATP synthesis, but is leaky to protons at very low nucleotide concentration. Based on the bi-site mechanism, we simulated the proton conduction from proton slip to "coupled" proton flow in ATP synthase using the Monte Carlo method. Good agreement is obtained between the simulated and available experimental results. Our model provides deeper insight into the nucleotide dependence of ATP catalysis, and the kinetic cooperativity in three catalysis subunits. The results of simulation support the bi-site mechanism in ATP synthesis.

Paracoccus denitrificans proton-translocating ATPase: kinetics of oxidative phosphorylation

The initial rates of ATP synthesis catalyzed by tightly coupled Paracoccus denitrificans plasma membrane were measured. The reaction rate was hyperbolically dependent on the substrates, ADP and inorganic phosphate (P(i)). Apparent K(m) values for ADP and P(i) were 7-11 and 60-120 µM, respectively, at saturating concentration of the second substrate (pH 8.0, saturating Mg²(+)). These values were dependent on coupling efficiency. The substrate binding in the ATP synthesis reaction proceeds randomly: K(m) value for a given substrate was independent of the concentration of the other one. A decrease of electrochemical proton gradient by the addition of malonate (when succinate served as the respiratory substrate) or by a decrease of steady-state level of NADH (when NADH served as the respiratory substrate) resulted in a proportional decrease of the maximal rates and apparent K(m) values for ADP and P(i) (double substitution, ping-pong mechanism). The kinetic scheme for ATP synthesis was compared with that described previously for the proton-translocating ATP hydrolysis catalyzed by the same enzyme preparation (T. V. Zharova and A. D. Vinogradov (2006) Biochemistry, 45, 14552-14558).

FoF1-ATPase, rotary motor and biosensor

F(o)F(1)-ATPase is an amazing molecular rotary motor at the nanoscale. Single molecule technologies have contributed much to the understanding of the motor. For example, fluorescence imaging and spectroscopy revealed the physical rotation of isolated F(1) and F(o), or F(o)F(1) holoenzyme. Magnetic tweezers were employed to manipulate the ATP synthesis/hydrolysis in F(1), and proton translation in F(o). Here, we briefly review our recent works including a systematic kinetics study of the holoenzyme, the mechanochemical coupling mechanism, reconstituting the delta-free F(o)F(1)-ATPase, direct observation of F(o) rotation at single molecule level and activity regulation through external links on the stator.

Inactivation of the chloroplast ATP synthase gamma subunit results in high non-photochemical fluorescence quenching and altered nuclear gene expression in Arabidopsis thaliana

The nuclear atpC1 gene encoding the gamma subunit of the plastid ATP synthase has been inactivated by T-DNA insertion mutagenesis in Arabidopsis thaliana. In the seedling-lethal dpa1 (deficiency of plastid ATP synthase 1) mutant, the absence of detectable amounts of the gamma subunit destabilizes the entire ATP synthase complex. The expression of a second gene copy, atpC2, is unaltered in dpa1 and is not sufficient to compensate for the lack of atpC1 expression. However, in vivo protein labeling analysis suggests that assembly of the ATP synthase alpha and beta subunits into the thylakoid membrane still occurs in dpa1. As a consequence of the destabilized ATP synthase complex, photophosphorylation is abolished even under reducing conditions. Further effects of the mutation include an increased light sensitivity of the plant and an altered photosystem II activity. At low light intensity, chlorophyll fluorescence induction kinetics is close to those found in wild type, but non-photochemical quenching strongly increases with increasing actinic light intensity resulting in steady state fluorescence levels of about 60% of the minimal dark fluorescence. Most fluorescence quenching relaxed within 3 min after dark incubation. Spectroscopic and biochemical studies have shown that a high proton gradient is responsible for most quenching. Thylakoids of illuminated dpa1 plants were swollen due to an increased proton accumulation in the lumen. Expression profiling of 3292 nuclear genes encoding mainly chloroplast proteins demonstrates that most organelle functions are down-regulated. On the contrary, the mRNA expression of some photosynthesis genes is significantly up-regulated, probably to compensate for the defect in dpa1.

The H+-ATPase from chloroplasts: effect of different reconstitution procedures on ATP synthesis activity and on phosphate dependence of ATP synthesis

The H+-ATP synthase from chloroplasts, CF0F1, was isolated, reconstituted into liposomes and ATP synthesis activity was measured after energization of the proteoliposomes with an acid-base transition. The ATP yield was measured as a function of the reaction time after energization, the data were fitted by an exponential function and the initial rate was calculated from the fit parameters. CF0F1 was reconstituted by detergent dialysis in asolectin liposomes and phosphatidylcholine/phosphatidic acid (PtdCho/PtdAc from egg yolk) liposomes. In asolectin liposomes, high initial rates of ATP synthesis (up to 400 s(-1)) were observed with a rapid decline of the rate; in PtdCho/PtdAc liposomes the initial rate is smaller (up to 200 s(-1)), but the decline of the activity is slower. CF0F1 was reconstituted into PtdCho/PtdAc liposomes either by detergent dialysis or into reverse phase liposomes. The dependence of the rate of ATP synthesis on the phosphate concentration was measured with both types of proteoliposomes. The data can be described by Michaelis-Menten kinetics with a K(M) value of 350 microM for reverse phase liposomes and a K(M) value of 970 microM for dialysis liposomes. Both K(M) values depend neither on the magnitude of DeltapH nor on the electric potential difference, whereas V(max) decreases strongly with decreasing energization. At low phosphate concentration, there are small deviations from Michaelis-Menten kinetics. The measured rates are higher than those calculated from the fitted Michaelis-Menten parameters. This effect is interpreted as evidence that more than one phosphate binding site is involved in ATP synthesis.

Diatom fucoxanthin chlorophyll a/c-binding protein (FCP) and land plant light-harvesting proteins use a similar pathway for thylakoid membrane Insertion

The light-harvesting proteins in plastids of different lineages including algae and land plants represent a superfamily of chlorophyll-binding proteins that seem to be phylogenetically related, although some of the light-harvesting complex (LHC) proteins bind different carotenoids. LHCs can be divided into chlorophyll a/b-binding proteins found in green algae, euglenoids, and higher plants and into chlorophyll a/c-binding proteins of various algal taxa. LHC proteins from diatoms are named fucoxanthin-chlorophyll a/c-binding proteins (FCP). In contrast to chlorophyll a/b-binding proteins, there is no information so far about the way FCPs integrate into thylakoid membranes. The diatom FCP preproteins have a bipartite presequence that is necessary to enable transport into the four membrane-bound diatom plastids, but similar to chlorophyll a/b-binding proteins there is apparently no presequence present for targeting to the thylakoid membrane. By establishing an in vitro import assay for diatom thylakoids, we demonstrated that thylakoid integration of diatom FCP depends on the presence of stromal factors and GTP. This indicates that a pathway involving signal recognition particles (SRP) is involved in membrane integration just as shown for LHCs in higher plants. We also demonstrate integration of diatom FCP into thylakoids of higher plants and vice versa SRP-dependent targeting of LHCs from pea and Arabidopsis into diatom thylakoids. The similar SRP-dependent modes of thylakoid integration of land plant LHCs and FCPs support recent analyses indicating a common origin of chlorophyll a/b- and a/c-binding proteins.

Proton gradient-induced changes of the interaction between CF0 and CF1 related to activation of the chloroplast ATP synthase

Thylakoid energization by light causes destabilization of CF0CF1 so that the peripheral CF1 sector is more readily detached from the membrane by intermediate concentrations of the chaotropic salt NaSCN. Here we have investigated the correlation between the proton gradient-induced change of CF0CF1 interaction and CF0CF1 activation. The results indicate a close relationship between the two phenomena. The effect is most probably due to reduction of the electrostatic interaction between the two subcomplexes CF0 and CF1 as a consequence of protonations in the interface region.

Kinetics and thioredoxin specificity of thiol modulation of the chloroplast H+-ATPase

The kinetics of thiol modulation of the chloroplast H+-ATPase (CF0CF1) in membrana were analyzed by employing thioredoxins that were kept reduced by 0.1 mM dithiothreitol. The kinetics of thiol modulation depend on the extent of the proton gradient. The process is an exponential function of the thioredoxin concentration and reaction time and can be described by an irreversible second order reaction. The results indicate that the formation of the complex between thioredoxin and CF0CF1 is slow compared with the subsequent reduction step. Furthermore we have compared the efficiencies of the Escherichia coli thioredoxin Trx and the two chloroplast thioredoxins Tr-m and Tr-f. The second order rate constants are 0.057 (Tr-f), 0.024 (Trx), and 0.010 s-1 microM-1 (Tr-m) suggesting that Tr-f rather than Tr-m is the physiological reductant for the chloroplast ATPase. The often employed artificial reductant dithiothreitol exhibits a second order rate constant in thiol modulation of 1.02.10(-6) s-1 microM-1.