Protonmotive force

This potential, or protonmotive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabohc sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

Certain chemical substances have been known for many years to uncouple oxidation firm phosphorylation and to inhibit active transport, and for this reason they are named imcoupling agerrts. They are beheved to act by rendering the membrane permeable to protons hence short-circuiting the potential gradient or protonmotive force. 

Volume 126. Biomembranes (Part N Transport in Bacteria, Mitochondria, and Chloroplasts Protonmotive Force)... 

Mitchell was struck by the parallel between the force and flow of electrons, which we call electricity, and the force and flow of protons, which he named proticity.174 This led one headline writer in Nature177 to describe Mitchell as "a man driven by proticity," but if Mitchell is right, as seems to be the case, we are all driven by proticity Mitchell also talked about protonmotive processes and referred to Ap as the protonmotive force. Although it is a potential rather than a force, this latter name is a popular designation for Ap. 

However, many authors use Ap,+ as identical to the protonmotive force Ap. 

A crucially important finding is that submitochon-drial particles or vesicles from broken chloroplasts will synthesize ATP from ADP and P , when an artificial pH gradient is imposed.172186 Isolated purified FjF0 ATPase from a thermophilic Bacillus has been coreconstituted into liposomes with the light-driven proton pump bacteiiorhodopsin (Chapter 23). Illumination induced ATP synthesis.187 These observations support Mitchell s proposal that the ATP synthase is both spatially separate from the electron carriers in the membrane and utilizes the protonmotive force to make ATP. Thus, the passage of protons from the outside of the mitochondria back in through the ATP synthase induces the formation of ATP. What is the stoichiometry of this process ... 

Boyer, P.D. (1988). Bioenergetic coupling to protonmotive force should we be considering hydronium ion coordination and not group protonation TIBS 13,5-7. 

In other membranes responsible for energy transduction, oxidation-reduction reactions are the basis for proton movement across the membrane. The classical example is found in mitochondria where electron flow through the respiratory system drives proton movement from the inner matrix to the intermembrane space. The resultant protonmotive force is used to drive ATP synthesis or to maintain mitochondrial membrane potential (negative inside). A similar association between electron transport in a membrane oxidation reduction chain and proton movement... 

Y. Ohsumi and Y. Anraku (1983). Calcium transport driven by a protonmotive force in vacuolar membrane vesicles of Saccharomyces cerevisiae. J. Biol. Chem., 258, 5614-5617. 

Larsen, R. A., Thomas, M. G., and Postle, K. (1999). Protonmotive force, ExbB and ligand-bound LepA drive conformational changes in TonB. Mol. Microbiol. 31, 1809-1824. 

The now reduced hn reduces the ubisemiquinone at the Qi site, and two protons are taken up to produce a ubiquinol on the cytoplasmic side. There is thus a net uptake of two protons from the cytoplasm, and a net release of four protons to the periplasm, resulting in an overall net pumping of two protons from the cytoplasm to the periplasm. This results in a proton gradient, a protonmotive force, across the inner membrane, which is utihzed by ATPase to synthesize ATP from ADP. 

As shown by Eq. 3, AGh (which is also called A/th+) can be separated into two components the chemical potential of protons and the electric field across the membrane, to which all charged species present are contributing. The source of free energy for ATP synthesis is therefore the protonmotive force . 

All available evidence indicates that the synthesis of ATP is not directly coupled to electron transport, but is dependent only on the protonmotive force. If an uncoupler (a substance which equilibrates H across the membrane) is added in continuous light, ATP synthesis is decreased or abolished, while electron transport is accelerated, due to the release of the control exerted by A Xh on the rate of electron transport. 

A large body of evidence indicates that the generation of the protonmotive force utilized for ATP synthesis is the cooperative result of the activity of a large num-... 

Cyt b-c complex forms the evolutionary link betw.een the respiratory and photosynthetic electron-transport pathways [3]. In both systems it oxidizes quinols and reduces metalloproteins while generating protonmotive force. In cyanobacteria, the Cyt /jg-/complex is shared by both respiration and photosynthesis [21]. 

A protonmotive force across the plasma membrane is necessary to drive the flagellar motor. Under conditions of starvation, this protonmotive force is depleted. In acidic solution, the pH difference across the membrane is sufficient to power the motor. 

Abbreviations AA, antimycin BAL, British Anti-Lewisite (2,3-dimercaptopropanol) DCCD, dicyclo-hexylcarbodiimide DTNB, 5,5 -dithiobis(2-nitrobenzoate) oxidoreduction potential relative to the Normal Hydrogen Electrode midpoint oxidoreduction potential E midpoint oxidoreduction potential at pH = x FeS, iron-sulphur (centre or protein) FMN, flavin mononucleotide HMHQQ, 7-( n-heptadecyl)mercapto-6-hydroxy-5,8-quinolinequinone HOQNO, 2-/i-heptyl-4-hydroxyquinoline N-oxide Lb, leghaemoglobin MX, myxothiazol NEM, 7V-ethylmaIeimide pmf, protonmotive force, electrochemical proton gradient Q, ubiquinone Qj i, ubiquinone bound to Complex I SQ, ubise-miquinone SQ , ubisemiquinone anion UHDBT, 5- -undecyl-6-hydroxy-4,7-dioxobenzothiazol. 

The exergonic respiratory chain activity is utilised to drive proton translocation from the matrix (M) to the cytoplasmic (C) side of the membrane with generation of an electrochemical proton gradient (protonmotive force (pmf) [13],... 

Any considered mechanism must, first of all, be consistent with the thermodynamic constraints of the system. Such limits are set by the span of oxidoreduction potentials in the respiratory chain and by the protonmotive force that opposes the proton movement. The relative magnitudes of these two forces set an absolute upper limit for H" /e stoicheiometry of proton translocation. The stoicheiometry in turn, puts limits on the underlying mechanisms. Analogous limits for the H /ATP stoicheiometry of ATP synthesis are obtained from the relative magnitudes of phosphorylation potential and pmf. An elementary thermodynamic analysis of the system can therefore be helpful in defining the degree of freedom in discussions of chemical mechanisms (see Ref. 8). 

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