Protons translocation

Why has nature chosen this rather convoluted path for electrons in Complex 111 First of all. Complex 111 takes up two protons on the matrix side of the inner membrane and releases four protons on the cytoplasmic side for each pair of electrons that passes through the Q cycle. The apparent imbalance of two protons in ior four protons out is offset by proton translocations in Complex rV, the cytochrome oxidase complex. The other significant feature of this mechanism is that it offers a convenient way for a two-electron carrier, UQHg, to interact with the bj and bfj hemes, the Rieske protein Fe-S cluster, and cytochrome C, all of which are one-electron carriers. 

Fillingame, R. H., 1980. The proton-translocating pump of oxidative pho.sphorylation. Annual Review of Biochemistry 49 1079-1113. 

Mitchell, P., and Moyle, J., 1965. Stoichiometry of proton translocation through the re.spiratory chain and adeno.sine triphosphatase. systems of rat mitochondria. Nature 208 147-151. 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

Proton translocations accompany these cyclic electron transfer events, so ATP synthesis can be achieved. In cyclic photophosphorylation, ATP is the sole product of energy conversion. No NADPFI is generated, and, because PSII is not involved, no oxygen is evolved. The maximal rate of cyclic photophosphorylation is less than 5% of the rate of noncyclic photophosphorylation. Cyclic photophosphorylation depends only on PSI. 

Vacuolar-type proton translocating ATPase is a heter-eomeric protein complex, which appears to translocate two protons across the vesicle membrane for each ATP molecule that is hydrolyzed, generating chemical (ApH) and electrical (A ) gradients. Although the ATPases present on different classes of intracellular vesicle have... 

Synaptic vesicles isolated from brain exhibit four distinct vesicular neurotransmitter transport activities one for monoamines, a second for acetylcholine, a third for the inhibitory neurotransmitters GABA and glycine, and a fourth for glutamate [1], Unlike Na+-dependent plasma membrane transporters, the vesicular activities couple to a proton electrochemical gradient (A. lh+) across the vesicle membrane generated by the vacuolar H+-ATPase ( vacuolar type proton translocating ATPase). Although all of the vesicular transport systems rely on ApH+, the relative dependence on the chemical and electrical components varies (Fig. 1). The... 

Figure 7. Mechanism of the proton-translocating ubiquinol cytochrome c reductase (complex III) Q cycle. There is a potential difference of up to 150 mV across the hydrophobic core of this complex (potential barrier represented by the vertical broken line). Cytochromes hour and b N are heme groups on the same peptide subunits of complex III which can transfer electrons across the hydrophobic core. The movement of two electrons provides the driving force to transfer two protons from the matrix to the cytosol. Diffusion of UQ and UQHj, which are uncharged, in the hydrophobic core, and lipid bilayer of the inner membrane is not influenced by the membrane potential (see Nicholls and Ferguson, 1992).

Complex V (ATP Synthase, Mitochondrial Proton-Translocating ATPase)... 

Figure 9. Proposed cyclic mechanism for ATP synthesis by complex V involving all three catalytic sites of F,. In this scheme only the a and p subunits of F, are shown these are connected by a short stalk to F, in the inner membrane. Proton translocation through Fq driven by the proton motive force (AP) causes sequential conformational changes in each of the p-subunits and ATP synthesis as described in the text hexagons, high-affinity sites semicircles, low affinity sites parallelepipeds, intermediate-affinity sites (with no movement of F,).

Mitchell, P. Moyle. J. (1967). Respiration-driven proton translocation in rat liver mitochondria. Biochem. J. 105, 1147-1162. 

Vance, J.E., eds.), pp. 116-142, Benjamin/Cummings Publishing Co., Menlo Park, California. Senior, A.E. (1988). ATP synthesis by oxidative phosphorylation. Physiological Rev. 68, 177-230. Senior, A.E. (1990). The proton-translocating ATPase of Esherichia colt. Ann. Rev. Biophys. Chem. 19,7- 1. 

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.

The electrochemical potential difFetence across the membrane, once established as a tesult of proton translocation, inhibits further transport of teducing equivalents through the respiratory chain unless discharged by back-translocation of protons across the membtane through the vectorial ATP synthase. This in turn depends on availability of ADP and Pj. 

A Proton-Translocating Transhydrogenase Is a Source of Intramitochondrial NADPH... 

Osteoclasts are multinucleated cells derived from pluripotent hematopoietic stem cells. Osteoclasts possess an apical membrane domain, exhibiting a ruffled border that plays a key role in bone resorption (Figure 48-12). A proton-translocating ATPase expels protons across the ruffled border into the resorption area, which is the microenvironment of low pH shown in the figure. This lowers the local pH to 4.0 or less, thus increasing the solubility of hydroxyapatite and allowing demineralization to occur. Lysosomal acid proteases are released that digest the now accessible matrix proteins. 

A quantitative reconstitution approach was used to gain information as to the subunit composition of the -ATPase molecule [25]. Proteoliposomes prepared from asolectin and purified, radiolabeled ATPase molecules obtained by a freeze-thaw procedure similar to that of Dufour et al. [34] were shown to catalyze ATP hydrolysis-driven proton translocation, as indicated by the extensive quenching of aminochloromethoxyacridine fluorescence that occurs upon the addition of MgATP to the proteoliposomes, and the reversal of this quenching induced by... 

Taken together, these results indicate that similar to other proton-translocating membrane proteins, both types of Na /H exchangers contain critical sulfhydryl groups that are involved in the transport mechanism. These sulfhydryl groups do not appear to be present at the external transport site but may be involved in switching from an inactive to an activated state. 

Bacteria can use a range of electron acceptors in the absence of oxygen. Although they are able to reduce a number of oxyanions, only a limited number of these can support growth under anaerobic conditions by coupling reduction to the production of energy by proton translocation. A number... 

Myers CR, KH Nealson (1990) Respiration-linked proton translocation coupled to anaerobic reduction of manganese(IV) and iron (III) in Shewanella putrefaciens. J Bacteriol 172 6232-6238. 

Branden G, Gennis RB, Brzezinski P. 2006. Transmembrane proton translocation by 0340-chrome c oxidase. Biochim Biophys Acta 1757 1052. 

---