Electrochemical proton gradients

Similar to C1C-5, C1C-3 is present in endosomes. It is also found in synaptic vesicles. In both instances, and similar to C1C-5, it is necessary for the efficient intravesicular acidification. The acidification of synaptic vesicles is particularly important as their uptake of neurotransmitters depends on the electrochemical proton gradient. Surprisingly, the disruption of C1C-3 in mice resulted in a drastic degeneration of the hippocampus and the retina. Much less is known about C1C-4, which, however, also appears to be present in endosomal compartments. 

Figure 12-8. Principles of the chemiosmotic theory of oxidative phosphorylation. The main proton circuit is created by the coupling of oxidation in the respiratory chain to proton translocation from the inside to the outside of the membrane, driven by the respiratory chain complexes I, III, and IV, each of which acts as a protonpump. Q, ubiquinone C, cytochrome c F Fq, protein subunits which utilize energy from the proton gradient to promote phosphorylation. Uncoupling agents such as dinitrophenol allow leakage of H" across the membrane, thus collapsing the electrochemical proton gradient. Oligomycin specifically blocks conduction of H" through Fq.

From the plastoquinone pool, the electrons pass through the cyt b6f complex, which generates much of the electrochemical proton gradient that drives the synthesis of ATP. 

Hugenholtz J, Ljungdahl LG. 1989. Electron transport and electrochemical proton gradient in membrane vesicles of Clostridium thermoautotrophicum. J Bacteriol 171 2873-5. 

The cytochrome b(6)f complex mediates electron transfer between the PSI and PSII reaction centers by oxidizing hpophUic plastoquinol (PQH2) (see Figure 7.24) and reducing the enzymes plastocyanin or cytochrome Ce. The electronic connection also generates a transmembrane electrochemical proton gradient that can support adenosine triphosphate (ATP) synthesis instead of electron transport. 

The mechanochemical rotatory motion of bacterial flagella, driven by electrochemical proton gradients across the peripheral membrane. Each complete turn requires... 

About one equivalent of protons was taken up with each equivalent of methyl a-D-glucopyranoside239 or maltose247 a-TEG was also absorbed with protons. In addition, Serrano245 and Palacios and Serrano248 found maltose transport to be coupled to the electrochemical, proton gradient, but that entry was independent of the intracellular concentration of ATP. 

The transport process is unclear. In some cases it is inhibited by uncoupling agents, suggesting that uptake is linked to the electrochemical proton gradient, while evidence has also been found for a symport mechanism involving ferrichrome and Mg2+.1201... 

Electrochemistry of respiration — The function of the enzymes in the mitochondrial respiratory chain is to transform the energy from the redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Cytochrome oxidase (EC 1.9.3.1, PDB 20CC) is the terminal electron acceptor of the mitochondrial respiratory chain. Its main function is to catalyze the reaction of oxygen reduction to water using electrons from ferrocytochrome c 4H+ + 02 + 4e 2H20. This reaction is exother-... 

Phosphorylation of ADP to ATP by mitochondria is driven by an electrochemical proton gradient established across the inner mitochondrial membrane as a consequence of vectoral transport of protons from NADH and succinate during oxidation by the respiratory chain (see Chapter 17). Hence, lipophilic weak acids or bases (such as 2,4-dinitrophenol) that can shuttle protons across membranes will dissipate the proton gradient and uncouple oxidation from ADP phosphorylation. Intrami-tochondrial ADP can be rate-limiting as demonstrated by inhibition of the mitochondrial adenosine nucleotide carrier by atractyloside. Inhibition of ATP synthesis... 

Mitchell s theory holds that an electrochemical proton gradient across the membrane (which is only slightly permeable to many ionized species and particularly to H ") is formed by the vectorial transport of into the thylakoid lumen coupled to electron transport, as a consequence of the alternate disposition across the membrane of electron carriers which can bind protons and others which cannot be protonated. 

The same treatments which elicit the ATPase reaction in the membrane-bound ATP synthase induce simultaneously a dark ATP-Pj exchange reaction. It was recently demonstrated that this exchange reaction is due to the simultaneous occurrence of phosphorylation and ATPase activity and therefore the use of the term exchange reaction may be a misnomer [37,38]. It is suggested that, as noted above, the induced ATP hydrolysis produces a transmembrane electrochemical proton gradient which in turn drives the ATP synthetic reaction. 

If ATP reversal operates via the intermediate formation of an electrochemical proton gradient, as the chemiosmatic hypothesis predicts, it should be possible to show the same reverse reaction by the direct imposition of a transmembrane electrochemical proton gradient of the proper polarity. 

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],... 

Instead relies on the vectorial transport of proteins which leads to the establishment of an electrochemical proton gradient across the cell membrane. The energy for proton translocation Is provided by light which Is absorbed by the chromophore of bacteriorhodopsin. 

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