Proton release by the quinol oxidase site of the cytochrome b/c1 complex following single turnover flash excitation of intact cells of Rhodobacter capsulatus

Competing for the same space: protons and alkali ions at the interface of phospholipid bilayers

Maintaining gradients of solvated protons and alkali metal ions such as Na⁺ and K⁺ across membranes is critical for cellular function. Over the last few decades, both the interactions of protons and alkali metal ions with phospholipid membranes have been studied extensively and the reported interactions of these ions with phospholipid headgroups are very similar, yet few studies have investigated the potential interdependence between proton and alkali metal ion binding at the water-lipid interface. In this short review, we discuss the similarities between the proton-membrane and alkali ion-membrane interactions. Such interactions include cation attraction to the phosphate and carbonyl oxygens of the phospholipid headgroups that form strong lipid-ion and lipid-ion-water complexes. We also propose potential mechanisms that may modulate the affinities of these cationic species to the water-phospholipid interfacial oxygen moieties. This review aims to highlight the potential interdependence between protons and alkali metal ions at the membrane surface and encourage a more nuanced understanding of the complex nature of these biologically relevant processes.

Proton translocation by the cytochromebc1complexes of phototrophic bacteria: introducing the activated Q-cycle

The cytochrome bc1 complexes are proton-translocating, dimeric membrane ubiquinol:cytochrome c oxidoreductases that serve as "hubs" in the vast majority of electron transfer chains. After each ubiquinol molecule is oxidized in the catalytic center P at the positively charged membrane side, the two liberated electrons head out, according to the Mitchell's Q-cycle mechanism, to different acceptors. One is taken by the [2Fe-2S] iron-sulfur Rieske protein to be passed further to cytochrome c1. The other electron goes across the membrane, via the low- and high-potential hemes of cytochrome b, to another ubiquinone-binding site N at the opposite membrane side. It has been assumed that two ubiquinol molecules have to be oxidized by center P to yield first a semiquinone in center N and then to reduce this semiquinone to ubiquinol. This review is focused on the operation of cytochrome bc1 complexes in phototrophic purple bacteria. Their membranes provide a unique system where the generation of membrane voltage by light-driven, energy-converting enzymes can be traced via spectral shifts of native carotenoids and correlated with the electron and proton transfer reactions. An "activated Q-cycle" is proposed as a novel mechanism that is consistent with the available experimental data on the electron/proton coupling. Under physiological conditions, the dimeric cytochrome bc1 complex is suggested to be continually primed by prompt oxidation of membrane ubiquinol via center N yielding a bound semiquinone in this center and a reduced, high-potential heme b in the other monomer of the enzyme. Then the oxidation of each ubiquinol molecule in center P is followed by ubiquinol formation in center N, proton translocation and generation of membrane voltage.

Probing biological interfaces by tracing proton passage across them

The properties of water at the surface, especially at an electrically charged one, differ essentially from those in the bulk phase. Here we survey the traits of surface water as inferred from proton pulse experiments with membrane enzymes. In such experiments, protons that are ejected (or captured) by light-triggered enzymes are traced on their way between the membrane surface and the bulk aqueous phase. In several laboratories it has been shown that proton exchange between the membrane surface and the bulk aqueous phase takes as much as about 1 ms, but could be accelerated by added mobile pH-buffers. Since the accelerating capacity of the latter decreased with increase in their electric charge, it was suggested that the membrane surface is separated from the bulk aqueous phase by a barrier of electrostatic nature. In terms of ordinary electrostatics, the barrier could be ascribed to dielectric saturation of water at a charged surface. In terms of nonlocal electrostatics, the barrier could result from the dielectric overscreening in the surface water layers. It is discussed how the interfacial potential barrier can affect the reactions at interface, especially those coupled with biological energy conversion and membrane transport.

Protons @ interfaces: implications for biological energy conversion

The review focuses on the anisotropy of proton transfer at the surface of biological membranes. We consider (i) the data from "pulsed" experiments, where light-triggered enzymes capture or eject protons at the membrane surface, (ii) the electrostatic properties of water at charged interfaces, and (iii) the specific structural attributes of proton-translocating enzymes. The pulsed experiments revealed that proton exchange between the membrane surface and the bulk aqueous phase takes as much as about 1 ms, but could be accelerated by added mobile pH-buffers. Since the accelerating capacity of the latter decreased with the increase in their electric charge, it was concluded that the membrane surface is separated from the bulk aqueous phase by a barrier of electrostatic nature. The barrier could arise owing to the water polarization at the negatively charged membrane surface. The barrier height depends linearly on the charge of penetrating ions; for protons, it has been estimated as about 0.12 eV. While the proton exchange between the surface and the bulk aqueous phase is retarded by the interfacial barrier, the proton diffusion along the membrane, between neighboring enzymes, takes only microseconds. The proton spreading over the membrane is facilitated by the hydrogen-bonded networks at the surface. The membrane-buried layers of these networks can eventually serve as a storage/buffer for protons (proton sponges). As the proton equilibration between the surface and the bulk aqueous phase is slower than the lateral proton diffusion between the "sources" and "sinks", the proton activity at the membrane surface, as sensed by the energy transducing enzymes at steady state, might deviate from that measured in the adjoining water phase. This trait should increase the driving force for ATP synthesis, especially in the case of alkaliphilic bacteria.

Proton transfer dynamics at membrane/water interface and mechanism of biological energy conversion

Proton transfer between water and the interior of membrane proteins plays a key role in bioenergetics. Here we survey the mechanism of this transfer as inferred from experiments with flash-triggered enzymes capturing or ejecting protons at the membrane surface. These experiments have revealed that proton exchange between the membrane surface and the bulk water phase proceeds at > or =1 msec because of a kinetic barrier for electrically charged species. From the data analysis, the barrier height for protons could be estimated as about 0.12 eV, i.e., high enough to account for the observed retardation in proton exchange. Due to this retardation, the proton activity at the membrane surface might deviate, under steady turnover of proton pumps, from that measured in the adjoining water phase, so that the driving force for ATP synthesis might be higher than inferred from the bulk-to-bulk measurements. This is particularly relevant for alkaliphilic bacteria. The proton diffusion along the membrane surface, on the other hand, is unconstrained and fast, occurring between the neighboring enzymes at less than 1 microsec. The anisotropy of proton dynamics at the membrane surface helps prokaryotes diminish the "futile" escape of pumped protons into the external volume. In some bacteria, the inner membrane is invaginated, so that the "ejected" protons get trapped in the closed space of such intracellular membrane "sacks" which can be round or flat. The chloroplast thylakoids and the mitochondrial cristae have their origin in these intracellular structures.

Proton transfer dynamics at the membrane/water interface: dependence on the fixed and mobile pH buffers, on the size and form of membrane particles, and on the interfacial potential barrier

Crossing the membrane/water interface is an indispensable step in the transmembrane proton transfer. Elsewhere we have shown that the low dielectric permittivity of the surface water gives rise to a potential barrier for ions, so that the surface pH can deviate from that in the bulk water at steady operation of proton pumps. Here we addressed the retardation in the pulsed proton transfer across the interface as observed when light-triggered membrane proton pumps ejected or captured protons. By solving the system of diffusion equations we analyzed how the proton relaxation depends on the concentration of mobile pH buffers, on the surface buffer capacity, on the form and size of membrane particles, and on the height of the potential barrier. The fit of experimental data on proton relaxation in chromatophore vesicles from phototropic bacteria and in bacteriorhodopsin-containing membranes yielded estimates for the interfacial potential barrier for H(+)/OH(-) ions of approximately 120 meV. We analyzed published data on the acceleration of proton equilibration by anionic pH buffers and found that the height of the interfacial barrier correlated with their electric charge ranging from 90 to 120 meV for the singly charged species to >360 meV for the tetra-charged pyranine.

Low dielectric permittivity of water at the membrane interface: effect on the energy coupling mechanism in biological membranes

Protonmotive force (the transmembrane difference in electrochemical potential of protons, ) drives ATP synthesis in bacteria, mitochondria, and chloroplasts. It has remained unsettled whether the entropic (chemical) component of relates to the difference in the proton activity between two bulk water phases (deltapH(B)) or between two membrane surfaces (deltapH(S)). To scrutinize whether deltapH(S) can deviate from deltapH(B), we modeled the behavior of protons at the membrane/water interface. We made use of the surprisingly low dielectric permittivity of interfacial water as determined by O. Teschke, G. Ceotto, and E. F. de Souza (O. Teschke, G. Ceotto, and E. F. de Sousa, 2001, PHYS: Rev. E. 64:011605). Electrostatic calculations revealed a potential barrier in the water phase some 0.5-1 nm away from the membrane surface. The barrier was higher for monovalent anions moving toward the surface (0.2-0.3 eV) than for monovalent cations (0.1-0.15 eV). By solving the Smoluchowski equation for protons spreading away from proton "pumps" at the surface, we found that the barrier could cause an elevation of the proton concentration at the interface. Taking typical values for the density of proton pumps and for their turnover rate, we calculated that a potential barrier of 0.12 eV yielded a steady-state pH(S) of approximately 6.0; the value of pH(S) was independent of pH in the bulk water phase under neutral and alkaline conditions. These results provide a rationale to solve the long-lasting problem of the seemingly insufficient protonmotive force in mesophilic and alkaliphilic bacteria.

The cytochrome bc1 complex of Rhodobacter capsulatus: ubiquinol oxidation in a dimeric Q-cycle?

We studied the cytochrome bc1 complex (hereafter bc) by flash excitation of Rhodobacter capsulatis chromatophores. The reduction of the high-potential heme b(h), of cytochrome b (at 561 nm) and of cytochromes c (at 552 nm) and the electrochromic absorption transients (at 524 nm) were monitored after the first and second flashes of light, respectively. We kept the ubiquinone pool oxidized in the dark and concerned for the ubiquinol formation in the photosynthetic reaction center only after the second flash. Surprisingly, the first flash caused the oxidation of about one ubiquinol per bc dimer. Based on these and other data we propose a dimeric Q-cycle where the energetically unfavorable oxidation of the first ubiquinol molecule by one of the bc monomers is driven by the energetically favorable oxidation of the second ubiquinol by the other bc monomer resulting in a pairwise oxidation of ubiquinol molecules by the dimeric bc in the dark. The residual unpaired ubiquinol supposedly remains on the enzyme and is then oxidized after the first flash.

The ratio of protons translocated/hydride ion equivalent transferred by nicotinamide nucleotide transhydrogenase in chromatophores from Rhodospirillum rubrum

The reduction of acetylpyridine adenine dinucleotide (AcPdAD+, an NAD+ analogue) by NADPH, in chromatophores treated with valinomycin, was accompanied by alkalinisation of the external medium, as measured by the absorbance change of added cresol red, a simple, non-binding pH indicator. Experiments with a stopped-flow spectrophotometer showed that initial (linear) rates of alkalinisation persisted for 1-2s. From the results of experiments in which H+ uptake was driven by a series of short flashes of light, the dependence of the outward proton leak on the extent of H+ uptake was established. Thus, the proton leak was subtracted from the initial rate of alkalinisation during transhydrogenation to give the true proton-uptake rate. The correction factor was usually about 10%. The ratio of protons translocated/H- transferred from NADPH to AcPdAD+ (the H+/H- ratio) was 0.60 +/- 0.06. The transhydrogenation reaction between NAD+ and NADPH was measured in the presence of a regeneration system for NAD+ (pyruvate and lactate dehydrogenase). In addition to the accompanying proton-translocation reaction, scalar H+ consumption linked to the regeneration system was observed and permitted internal checks on the calibration of the cresol red absorbance changes. After correction for the proton leak and scalar proton uptake, an H+/H- ratio of 0.60 +/- 0.30 was calculated from the initial rates. The water-soluble polypeptide of transhydrogenase (Ths) was washed from a sample of chromatophores to inhibit transhydrogenation activity and the accompanying H+ uptake. Re-addition of purified Ths to depleted chromatophores led to recovery of transhydrogenation activity and of H+ uptake. In this reconstituted system the H+/H- was similar to that in the native membranes. These results make it unlikely that the H+/H- ratio is artefactually low because chromatophores have a population of transhydrogenase which is not coupled to proton translocation. Further evidence that the mechanistic H+/H- ratio of chromatophore transhydrogenase is less than 1 was provided by an analysis of the kinetics of alkalinisation of the medium during reduction of AcPdAD+ by NADPH. It was shown that the progress of the transhydrogenation-induced alkalinisation was fitted by the sum of H+ uptake (the rate of transhydrogenation multiplied by the H+/H- ratio) plus the H+ leak, when the ratio was 0.6 but not when it was 1.0. The results are discussed in terms of the possible mechanism of energy coupling by transhydrogenase.(ABSTRACT TRUNCATED AT 400 WORDS)

Flash-induced proton transfer in photosynthetic bacteria

A proton electrochemical potential across the membranes of photosynthetic purple bacteria is established by a light-driven proton pump mechanism: the absorbed light in the reaction center initiates electron transfer which is coupled to the vectorial displacement of protons from the cytoplasm to the periplasm. The stoichiometry and kinetics of proton binding and release can be tracked directly by electric (glass electrodes), spectrophotometric (pH indicator dyes) and conductimetric techniques. The primary step in the formation of the transmembrane chemiosmotic potential is the uptake of two protons by the doubly reduced secondary quinone in the reaction center and the subsequent exchange of hydroquinol for quinone from the membrane quinone-pool. However, the proton binding associated with singly reduced promary and/or secondary quinones of the reaction center is substoichiometric, pH-dependent and its rate is electrostatically enhanced but not diffusion limited. Molecular details of protonation are discussed based on the crystallographic structure of the reaction center of purple bacteriaRb. sphaeroides andRps. viridis, structure-based molecular (electrostatic) calculations and mutagenesis directed at protonatable amino acids supposed to be involved in proton conduction pathways.

Construction and characterization of a mutant of Rhodobacter sphaeroides with the reaction center as the sole pigment-protein complex

A strain of Rhodobacter sphaeroides has been constructed in which the photosynthetic reaction center is the sole pigment-protein complex. The strain, named RCO1, is capable of photoheterotrophic growth and possesses assembled and functional reaction centers which can undergo photochemical charge separation and are reduced by electrons derived from the cytochrome b/c1 complex. The circular dichroism and linear dichroism of reaction centers in membranes from strain RCO1 are similar to those described previously for reaction centers isolated in detergent solution. A second strain, named RCLH11, which is devoid of the peripheral LH2 antenna complex has also been constructed. A description of the properties of these strains is presented.