Proton-translocating respiratory chain

An anisotropic (direction-oriented) proton-translocating respiratory chain capable of vectorial transport of protons across the membrane ... 

Mitchell s chemiosmotic theory postulates that the energy from oxidation of components in the respiratory chain is coupled to the translocation of hydrogen ions (protons, H+) from the inside to the outside of the inner mitochondrial membrane. The electrochemical potential difference resulting from the asymmetric dis-... 

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. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

The Q cycle accommodates the switch between the two-electron carrier ubiquinone and the one-electron carriers—cytochromes b562, b566, clt and c—and explains the measured stoichiometry of four protons translocated per pair of electrons passing through the Complex III to cytochrome c. Although the path of electrons through this segment of the respiratory chain is complicated, the net effect of the transfer is simple QH2 is oxidized to Q and two molecules of cytochrome c are reduced. 

This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron transfer from substrates to 02, coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19-33). They translocate protons outward across the plasma membrane as electrons are transferred to 02. Bacteria such as Escherichia coli have F0Fi complexes in their plasma membranes the F portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and P, as protons flow back into the cell through the proton channel of F0. 

Mitchell, P., and J. Moyle, Stoichiometry of proton translocation through the respiratory chain and adenosine triphosphatase systems of rat liver mitochondria. Nature 208 147, 1965. The initial observations that electron transport moves protons outward across the mitochondrial inner membrane and that ATP hydrolysis does the same. 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

Boxes indicate electron-transport chain complexes, whereas ovals represent the electron transporters UQ, RQ and cytochrome c. The open boxes represent complexes involved in the classical aerobic respiratory chain, whereas grey boxes represent complexes involved in malate dismutation. The vertical bar represents a scale for the standard redox potentials in mV. Translocation of protons by the complexes is indicated by H+ +. Abbreviations Cl, Clll and CIV, complexes I, III and IV of the respiratory chain cyt c, cytochrome c FRD, fumarate reductase Fum, fumarate SDH, succinate dehydrogenase Succ, succinate RQ, rhodoquinone UQ, ubiquinone. 

Fig. 5.3. The major components involved in mitochondrial NADH oxidation in facultative anaerobic mitochondria. In anaerobically functioning mitochondria, NADH is oxidized either by soluble enzymes (left) or by membrane-bound complexes of the electron-transport chain (middle). Under aerobic conditions, a classic respiratory chain is used to oxidize NADH (right). Proton translocation is indicated by H with arrows. Ovals represent the electron transporters RQ, UQ and cytochrome c (cyt. c), and electron transport is indicated by dashed arrows. The vertical bar represents a scale for the standard redox potentials in millivolts. Fum fumarate, NADH-DH NADH dehydrogenase, NADH-ECR soluble NADH enoyl-CoA reductase, RQH2 rhodoquinol, Succ succinate, UQH2 ubiquinol...

Ubiquinones are energy transducers that are obligatory in many respiratory and photosynthetic electron transport chains. The ubiquinone enzymes involved in these reactions usually function in a manner that couples the electron transfer by the ubiquinone to proton translocation across the membrane.The structural makeup of the ubiquinone active site permits varying functional roles that influence the electron and proton chemistry. 

Brandt U (1999) Proton translocation in the respiratory chain involving ubiquinone - a hypothetical semiquinone switch mechanism for complex I. Biofactors 9,95-101. 

FIGURE 1. Respiratory chain the enzymes of the mitochondrial iimer membrane involved in oxidative phosphorylation. From complex I to V, they are NADH-dehydrogenase, succinate dehydrogenase, cytochrome bc complex, and cytochrome c oxidase. Protons are translocated across the membrane while electrons are transferred to Oj through the chain. The proton gradient is used by ATP synthase (complex V) to make ATP. (Reprinted with permission from Saraste, 1999, American Association for the Advancement of Science.)... 

Figure 1 The mitochondrial respiratory chain. Electron transfer (brown arrows) between the three major membrane-bound complexes (I, III, and IV) is mediated by ubiquinone (Q/QH2) and the peripheral protein c)dochrome c (c). Transfer of protons hnked to the redox chemistry is shown by blue arrows red arrows denote proton translocation. NAD+ nicotinamide adenine dinucleotide, FMN flavin mononucleotide, Fe/S iron-sulfur center bH,bi, and c are the heme centers in the cytochrome bc complex (Complex III). Note the bifurcation of the electron transfer path on oxidation of QH2 by the heme bL - Fe/S center. Complex IV is the subject of this review. N and P denote the negatively and positively charged sides of the membrane, respectively...

Figure 2 The chemiosmotic theory of respiration. The mitochondrial or bacterial membrane (yellow) provides resistance to proton conduction. The respiratory chain generates a proton electrochemical gradient across the membrane by redox-coupled proton translocation (Figure 1). This gradient is used as the driving force for synthesis of ATP, as catalyzed by the H+-ATP synthase in the same membrane...

Fig. 2.2. The chemiosmotic proton circuit, a, during the synthesis of matrix ATP (State 3) b, during the synthesis of extra-mitochondrial ATP (State 3) c, during State 4 respiration d, in the presence of proton translocator. R, respiratory chain A, ATP synthase P, phosphate carrier U, uncoupler. The respiratory chain is simplified to a single proton pump.

As in the case of the ATP synthase, the most convincing evidence for the function of the respiratory chain as an autonomous proton pump comes from the ability to purify energy-conserving segments of the chain and reconstitute them into artificial bilayers with the recovery of their proton translocating capacity [21,22]. 

In the proton circuit discussed so far in this chapter, it has been assumed that the protons, after being translocated across the membrane by the respiratory chain or... 

In this account we will review the structural and functional properties of the respiratory chain components, as they are known from studies with intact mitochondria, vesicles of the inner mitochondrial membrane (submitochondrial particles), or isolated complexes. The latter may additionally be reconstituted into liposomal membranes. To some extent we will also review the knowledge on the integrated functions of the respiratory chain with main emphasis on proton translocation and essential thermodynamic and kinetic properties. 

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). 

---