Electron transport proton pumps

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

Figure 6. Pathways of protons and electrons during mitochondrial oxidations. The diagrams show the pathways of electrons which enter the electron chain at the level ofcomplexi (a)orcomplex II (b). Complexes I, III, and IV usethefreeenergy of electron transport to pump protons out of the matrix. This diagram also distinguishes formally between protons released by dehydrogenation and those which are pumped out of the matrix, although they all enter or leave the same pool.

Complexes III and IV have Fe-porphyrin prosthetic groups (hemes), complex IV also contains copper atoms which are involved in electron transport. Complexes I, III, and IV use the energy of electron transport to pump protons out of the matrix so as to maintain a pH gradient and an electrical potential difference across the inner membrane required for ATP synthesis (see below and Appendix 3). It is important to remember that all dehydrogenations of metabolic substrates remove two protons as well as two electrons and that a corresponding number of protons are consumed in the final reduction of dioxygen (Figures 5, 6). 

Correct answer = D. Thirteen of the approximately 100 polypeptides required for oxidative phosphorylation are coded for by mitochondrial DNA, including the electron transport components cytochrome c and coenzyme Q. Oxygen directly oxidizes cytochrome oxidase. Succinate dehydrogenase directly reduces FAD. Cyanide inhibits electron flow, proton pumping, and ATP synthesis. 

According to the chemiosmotic theory, flow of electrons through the electron-transport complexes pumps protons across the inner membrane from the matrix to the intermembrane space. This raises the pH in the matrix and leaves the matrix negatively charged with respect to the intermembrane space and the cytosol. Protons flow passively back into the matrix through a channel in the ATP-synthase, and this flow drives the formation of ATP. 

Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP synthase to generate ATP in the matrix.

During electron transport, protons are pumped out of the mitochondrion at each of the major sites except for ... 

Figure 2.5 compares the orientation of the ATP synthase F0/Fj complex in mitochondria with that in chloroplasts. The lumen enclosed by the thylakoid membrane is slightly acidic it corresponds to the mitochondrial intermembrane space where electron transport first pumps protons (H+). In chloroplasts, ATP is made as protons diffuse from the thylakoid lumen through the membrane to the chloroplast stroma (Fig. 2.4). In mitochondria, ATP is made as protons diffuse from the mitochondrial intermembrane space through the inner mitochondrial membrane to the mitochondrial lumen or matrix.

CoQ and Three Electron-Transport Complexes Pump Protons Out of the Mitochondrial Matrix... 

Fig. 21.10. The concentration of ADP (or the phosphate potential -[ATP]/[ADP][PJ) controls the rate of oxygen consumption. (1) ADP is phosphorylated to ATP by ATP synthase. (2) The release of the ATP requires proton flow through ATP synthase into the matrix. (3) The use of protons from the intermembrane space for ATP synthesis decreases the proton gradient. (4) As a result, the electron transport chain pumps more protons, and oxygen is reduced to H2O. (5) As NADH donates electrons to the electron transport chain, NAD" is regenerated and returns to the TCA cycle or other NADH-producing pathways.

Step 1 Oxidation of NADH and FADH2, Via the Electron Transport Chain, Pumps Protons into the Intermembrane Space and Results in the Reduction of Oxygen to Water... 

In eukaryotes, oxidative phosphorylation occurs in mitochondria, while photophosphorylation occurs in chloroplasts to produce ATP. Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2 in all aerobic organisms. After, carbon fuels (nutrients) are oxidized in the citric acid cycle, electrons with electron-motive force is converted into a proton-motive force. Photophosphorylation involves the oxidation of H2O to O2, with NADP as electron acceptor. Therefore, the oxidation and the phosphorylation of ADP are coupled by a proton gradient across the membrane. In both organelles, mitochondria and chloroplast electron transport chains pump protons across a membrane from a low proton concentration region to one of high concentration. The protons flow back from intermembrane to the matrix in mitochondria, and from thylakoid to stroma in chloroplast through ATP synthase to drive the synthesis of adenosine triphosphate. Therefore, the adenosine triphosphate is produced within the matrix of mitochondria and within the stroma of chloroplast. 

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

The Stoichiometry of Proton Pumping by the Mitochondrial Electron-Transport Chain 129... 

Complex III 280 kDa 11 28 type hemes (b and bg) bound to same mitochondrially coded peptide 1 C heme (cytochrome c,) 1 Fe-S center Rieske factor Spans membrane, cytochrome b, and b in membrane, cytochrome c, and Fe-S center on outer face 0.25-0.53 Pumps protons out of matrix during electron transport/2e"... 

Complex IV 200 kDa (probably as a dimer) 13 2 A type hemes (a, aj) 2 or 3 Cu Spans inner membrane, cytochrome c site on outer face 0.16-1.00 Pumps protons out of matrix during electron transport/4e"... 

The AG of the redox span of segments of the electron-transport chain concerned with proton-pumping (Appendix 2) which is given by AG° = iFAE° in the standard state, or by... 

Although only two protons are pumped out of the matrix, two others from the matrix are consumed in the formation of H2O. There is therefore a net translocation of four positive charges out of the matrix which is equivalent to the extrusion of four protons. If four protons are required by the chemiosmotic mechanism to convert cytosolic ADP + Pj to ATP, then 0.5 mol ATP is made for the oxidation of one mol of ubiquinol and one mol ATP for the oxidation of 2 mols of reduced cytochrome c. These stoichiometries were obtained experimentally when ubiquinol was oxidized when complexes I, II, and IV were inhibited by rotenone, malonate, and cyanide, respectively, and when reduced cytochrome c was oxidized with complex III inhibited by antimycin (Hinkle et al., 1991). (In these experiments, of course, no protons were liberated in the matrix by substrate oxidation.) However, in the scheme illustrated in Figure 6, with the flow of two electrons through the complete electron transport chain from substrate to oxygen, it also appears valid to say that four protons are extmded by complex I, four by complex III, and two by complex 1. 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

What do I mean by a proton concentration gradient Simply, there is a higher concentration of protons in the space between the inner and outer membranes of the mitochondrion than in the mitochondrial interior. The gradient is formed from the energy released in the transfer of electrons down the electron transport chain. Put another way, the released energy is employed to pump protons across the inner mitochondrial membrane into the intermembrane space. 

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-... 

Proton transport via complexes I, III, and IV takes place vectorially from the matrix into the intermembrane space. When electrons are being transported through the respiratory chain, the concentration in this space increases—i. e., the pH value there is reduced by about one pH unit. For each H2O molecule formed, around 10 H ions are pumped into the intermembrane space. If the inner membrane is intact, then generally only ATP synthase (see p. 142) can allow protons to flow back into the matrix. This is the basis for the coupling of electron transport to ATP synthesis, which is important for regulation purposes (see p. 144). 

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