On the effects of thiocyanate and venturicidin on respiration-driven proton translocation in Paracoccus denitrificans

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.

Effects of pathogenic mutations in membrane subunits of mitochondrial Complex I on redox activity and proton translocation studied by modeling in Escherichia coli

Effects of Complex I mutations were studied by modeling in NuoH, NuoJ or NuoK subunits of Escherichia coli NDH-1 by simultaneous optical monitoring of deamino-NADH oxidation and proton translocation and fitting to the data a model equation of transmembrane proton transport. A homolog of the ND1-E24 LHON/MELAS mutation caused 95% inhibition of d-NADH oxidation and proton translocation. The NuoJ-Y59F replacement decreased proton translocation. The NuoK-E72Q mutation lowered the enzyme activity, but proton pumping could be rescued by the double mutation NuoK-E72Q/I39D. Moving the NuoK-E72/E36 pair one helix turn towards the periplasm did not affect redox activity but decreased proton pumping.

Tracking of proton flow during transition from anaerobiosis to steady state. 1. Response of matrix pH indicators

1. The kinetics of acidification and realkalinization of the matrix after addition of nigericin to respiring and non-respiring mitochondria, recorded by intramitochondrial pH indicators such as neutral red and 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), is complementary to that recorded by extramitochondrial pH indicators. The extent of acidification decreases with the logarithm of the KCl concentration and is inhibited by Pi and ammonium ions. 2. Proton translocation during respiration has been compared with proton extraction from matrix bulk water. During oxygen pulses to EGTA-untreated mitochondria, BCECF records an extraction of protons from matrix bulk water of about 2-3 nmol H+/mg, reduced to 1-2 nmol H+/mg in EGTA-treated mitochondria. Since the amount of proton translocation required to achieve steady state is of the order of 6-7 nmol H+/mg, it appears that 75-90% of the protons are not extracted from matrix bulk water. Only a slight response is recorded by neutral red. 3. The effect of permeant cations and of uncouplers on the distribution of proton extraction between membrane and matrix bulk water has been studied in presteady state. During Sr2+ uptake, proton extrusion into cytosolic bulk water, as well as proton extraction from matrix bulk water, corresponds almost to 100% of the protons translocated by the redox proton pumps. In the absence of Sr2+, parallel to the disappearance of the proton extrusion in cytosolic bulk water, the proton extraction from matrix bulk water diminishes to about 20% of the proton translocation. 4. The mechanism by which divalent cation uptake and protonophoric uncouplers affect the distribution of proton extraction between matrix bulk water and membrane domains and the nature of the membrane domains are discussed.

Respiration-linked proton translocation coupled to anaerobic reduction of manganese(IV) and iron(III) in Shewanella putrefaciens MR-1

An oxidant pulse technique, with lactate as the electron donor, was used to study respiration-linked proton translocation in the manganese- and iron-reducing bacterium Shewanella putrefaciens MR-1. Cells grown anaerobically with fumarate or nitrate as the electron acceptor translocated protons in response to manganese (IV), fumarate, or oxygen. Cells grown anaerobically with fumarate also translocated protons in response to iron(III) and thiosulfate, whereas those grown with nitrate did not. Aerobically grown cells translocated protons only in response to oxygen. Proton translocation with all electron acceptors was abolished in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone (20 microM) and was partially to completely inhibited by the electron transport inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide (50 microM).

Tracking of proton flow during transition from anaerobiosis to steady state in rat liver mitochondria

(1) The hydrophobic pH indicator Bromthymol blue and the hydrophilic pH indicator Phenol red have been used to follow the redox-pump-linked proton flows during transition from anaerobiosis to static head. The domains monitored by the pH indicators, whether external or internal, and the localization of the dye, whether free or membrane bound, have been identified by recording the absorbance changes following addition of nigericin or valinomycin to anaerobic or aerobic mitochondria and the effects of permeant and impermeant buffers. (2) After addition of the H+/K+ exchanger, nigericin, to anaerobic mitochondria. Phenol red and Bromthymol blue record an alkalinization and an acidification, respectively, indicating that while the hydrophilic pH indicator faces an external domain, the hydrophobic pH indicator faces, at least partly, an internal domain. The latter effect is sensitive to phosphate and to phosphate carrier inhibitors. On the other hand, addition of nigericin to aerobic mitochondria leads to an increased Bromthymol blue absorbance, which reflects an alkalinization, indicating that the pH indicator faces an external domain. The reorientation of the dye from the internal to the external domain is a function of the uncoupler concentration and thus of the membrane potential (cf. Mitchell et al. (1968) Eur. J. Biochem. 4, 9-19). (3) The amount of oxygen required for the transition from anaerobiosis to static head has been determined by following in parallel the extent of oxidation of cytochrome aa3 and the rise of delta mu H+. With succinate as substrate, 50% levels of cytochrome oxidation are obtained at 0.125 ngatom oxygen/mg and 50% of Safranine response at about 0.2 ngatom oxygen/mg. These amounts of oxygen correspond to an H+ displacement of about 0.8-1.2 ngatom/mg on the basis of the H+/O stoichiometry. It is concluded that mitochondria are in presteady state below, and in static head above, displacement of 2-3 ngatom H+/mg. This figure is very close to the original calculation of Mitchell (Mitchell, P. (1966) Biol. Rev. 41, 445-502). (4) Transition, by oxygen pulses, of EGTA-supplemented mitochondria from anaerobiosis to either presteady state or static head state results in a response of the hydrophilic pH indicator, Phenol red, which is negligible in amount and/or kinetically unrelated to the delta mu H+ rise. The fact that H+ extrusion in the bulk aqueous phase is negligible also in presteady state excludes proton cycling as an explanation. Addition of oxygen pulses to Sr2(+)-supplemented anaerobic mitochondria results in an H+ extrusion whose amount and rate is proportional to the Sr2+ concentration.(ABSTRACT TRUNCATED AT 400 WORDS)

Energization-induced redistribution of charge carriers near membranes

The electric field arising from proton pumping across a topologically closed biological membrane causes accumulation close to the membrane of ionic charges equivalent to the charge of the pumped protons, positive on the side towards which protons are pumped, negative on the other side. We shall call this the 'active surface charge'. We here use the Poisson-Boltzmann equation to evaluate the effects of zwitterionic buffer molecules and uncharged proteins in the aqueous phase bordering the membrane on the magnitude and ionic composition of the active surface charge. For the positive side of the membrane, the main results are: (1) If the membrane is freely accessible to bulk phase ions, pumped protons exchange with these ions, such that the active surface charge consists of salt cations. (2) If a significant fraction of the ions in bulk solution consists of buffer molecules, then some of the pumped protons will remain close to the membrane and constitute a major fraction of the active surface charge. (3) If a protein layer borders the membrane, a significant part of the transmembrane electric potential difference exists within that protein layer and protons inside this layer dominate the active surface charge. (4) On the negative side of the membrane the corresponding phenomena would occur. (5) All these effects are strictly dependent on the transmembrane electric potential difference arising from proton pumping and would come in addition to the well known effects of buffers and electrically charged proteins on the retention of scalar protons. (6) No additional proton diffusion barrier may be required to account for a deficit in number of protons observed in the aqueous bulk phase upon aeration-induced proton pumping.