Ubiquinone proton pumps

Albracht SPJ, Hedderich R. 2000. Learning from hydrogenases location of a proton pump and of a second FMN in bovine NADH ubiquinone oxidoreductase (complex I) FEBS Lett 485 1-6. 

For reasons discussed in Chapter 20, plants must carry out this reaction even when they do not need NADH for ATP production. To regenerate NAD+ from unneeded NADH, plant mitochondria transfer electrons from NADH directly to ubiquinone and from ubiquinone directly to 02, bypassing Complexes III and IV and their proton pumps. In this process the energy in NADH is dissipated as heat, which can sometimes be of value to the plant (Box 19-1). Unlike cytochrome oxidase (Complex IV), the alternative QH2 oxidase is not inhibited by cyanide. Cyanide-resistant NADH oxidation is therefore the hallmark of this unique plant electron-transfer pathway. 

FIGURE 19-33 Bacterial respiratory chain, (a) Shown here are the respiratory carriers of the inner membrane of E. coli. Eubacteria contain a minimal form of Complex I, containing all the prosthetic groups normally associated with the mitochondrial complex but only 14 polypeptides. This plasma membrane complex transfers electrons from NADH to ubiquinone or to (b) menaquinone, the bacterial equivalent of ubiquinone, while pumping protons outward and creating an electrochemical potential that drives ATP synthesis. 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

Respiratory Chain (Complex I, II, III, and IV, Ubiquinone, Cytochrome c, Proton Pump, Membrane Potential, Proton Motive Force)... 

Step f Complex II. When FADH2 is utilized as an electron donor, the succinate-Q reductase complex (Complex II) transfers the electrons from FADH2 to ubiquinone through iron-sulfur centers. This reaction does not have a proton pumping function. 

Ubiquinone functions as a carrier in the mitochondrial electron transport chain it is responsible for the proton pumping associated with complex I (Brandt, 1999) and is directly reduced by the citric acid cycle enzyme succinate dehydrogenase (Lancaster, 2002). As shown in Figure 14.8, it undergoes two single-electron reduction reactions to form the relatively stable semiquinone radical, then the fully reduced quinol. In addition to its role in the electron transport chain, it has been implicated as a coantioxidant in membranes and plasma lipoproteins, acting together with vitamin E (Section 4.3.1 Thomas etal., 1995, 1999). 

Cytochrome oxidases are transmembrane protein complexes, which are localized at the inner mitochondrial membrane in eukaryotes or at the plasma membrane in bacteria. In addition to the reduction of oxygen, all cytochrome oxidases studied so far function as proton pumps as well, maintaining the proton gradient for the production of ATP [283 - 286]. While all cytochrome oxidases oxidize oxygen, they vary in their electron donors. Those receiving electrons from cytochrome c are called cytochrome c oxidases and those from ubiquinone ubiquinone oxidases [287]. 

Explain the roles of cytochrome Ci and the b cytochromes (bi and bu) in the oxidation of ubiquinol to ubiquinone. Are protons pumped across the inner mitochondrial membrane during these reactions ... 

Reduction of ubiquinone by addition of two electrons on the matrix side of the membrane could be the basis for the pick up of two protons from the matrix side. However, the second part of the release of two protons to the cytoplasmic side of the membrane to achieve net transport of two protons is absent. Thus, from a structural standpoint, it is understandable that Complex II does not contribute to proton pumping. Complex II does make an important contribution of diffusible ubiquinol to the membrane... 

Promper C, Schneider R, Weiss H.The role of the proton-pumping and alternative respiratory chain NADH ubiquinone oxidoreductases in overflow catabolism of Aspergillus niger. Eur J Biochem 1993 216 223-30. 

Two light-activated cyclic electron transfer systems have been reincorporated into lipid vesicles in such a way that proton pumping across the membranes may be observed under appropriate conditions. The first of these has been constructed from mammalian cytochrome bc] complex and reaction centres isolated from Rhodopseudomonas sphaeroides (RCbc vesicles), a combination used previously by Packham et al. (1980) for single turnover studies in solution. In order to maintain adequate multiple turnover electron flux under our conditions, it was necessary to add both cytochrome c and ubiquinone-2. In the presence of valinomycin, light activation caused the translocation of four protons outwards across the vesicles for each pair of electrons completing a cycle, although this ratio appeared to fall to two after a significant ApH had built up. 

Complex II contains four peptides, the two largest form succinate dehydrogenase, the largest has covalently boiuid flavin adenine dinucleotide (FAD) which reacts with succinate, and the other has three iron-sulphur centers. Smaller subunits anchor the two larger subunits to the membrane and form the UQ binding site. Ubiquinone is the electron acceptor but complex II does not pump protons (see below). 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

What is the nature of the proton-translocating pumps that link Ap with electron transport In his earliest proposals Mitchell suggested that electron carriers, such as flavins and ubiquinones, each of which accepts two protons as well as two electrons upon reduction, could serve as the proton carriers. Each pump would consist of a pair of oxidoreductases. One, on the inside (matrix side) of the coupling membrane, would deliver two electrons (but no protons) to the carrier (B in Fig. 18-13). The two protons needed for the reduction would be taken from the solvent in the matrix. The second oxidoreductase would be located on the outside of the membrane and would accept two electrons from the reduced carrier (BH2 in Fig. 18-13) leaving the two released protons on the outside of the membrane. To complete a "loop" that would allow the next carrier to be reduced, electrons would have to be transferred through fixed electron carriers embedded in the... 

---