Proton pump, respiratory chain complexes

Each of the respiratory chain complexes I, III, and IV (Figures 12-7 and 12-8) acts as a proton pump. The inner membrane is impermeable to ions in general but particularly to protons, which accumulate outside the membrane, creating an electrochemical potential difference across the membrane (A iH )-This consists of a chemical potential (difference in pH) and an electrical potential. 

The redox carriers are grouped into respiratory chain complexes in the inner mitochondrial membrane. These use the energy released in the redox gradient to pump protons to the outside of the membrane, creating an electrochemical potential across the membrane. 

The inner membrane itself plays an important part in oxidative phosphorylation. As it is impermeable to protons, the respiratory chain—which pumps protons from the matrix into the intermembrane space via complexes 1, 111, and IV—establishes a proton gradient across the inner membrane, in which the chemical energy released during NADH oxidation is conserved (see p. 126). ATP synthase then uses the energy stored in the gradient to form ATP from ADP and inorganic phosphate. Several of the transport systems are also dependent on the H"" gradient. 

Complex IV Cytochrome c to 02 In the final step of the respiratory chain, Complex IV, also called cytochrome oxidase, carries electrons from cytochrome c to molecular oxygen, reducing it to H20. Complex IV is a large enzyme (13 subunits Mr 204,000) of the inner mitochondrial membrane. Bacteria contain a form that is much simpler, with only three or four subunits, but still capable of catalyzing both electron transfer and proton pumping. Comparison of the mitochondrial and bacterial complexes suggests that three subunits are critical to the function (Fig. 19-13). 

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

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). 

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. 

Figure 18-19 The ammonia oxidation system of the bacterium Nitrosomonas. Oxidation of ammonium ion (as free NH3) according to Eq. 18-17 is catalyzed hy two enzymes. The location of ammonia monooxygenase (step a) is uncertain but hydroxylamine oxidoreductase (step b) is periplas-mic. The membrane components resemble complexes I, III, and IV of the mitochondrial respiratory chain (Fig. 18-5) and are assumed to have similar proton pumps. Solid green lines trace the flow of electrons in the energy-producing reactions. This includes flow of electrons to the ammonia monoxygenase. Complexes HI and IV pump protons out but complex I catalyzes reverse electron transport for a fraction of the electrons from hydroxylamine oxidoreductase to NAD+. Modified from Blaut and Gottschalk.315...

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. 

Figure 17.2. Respiratory chain. Electrons flow through Complex I (I)/Complex II (II), ubiquinone (Q), Complex III (III), cytochrome c (CytC), and Complex IV (IV) and protons are pumped from the matrix to the intermembrane space.

The Respiratory Chain Consists of Four Complexes Three Proton Pumps and a Physical Link to the Citric Acid Cycle... 

The second of the three proton pumps in the respiratory chain is Q-cytochrome c oxidoreductase (also known as Complex III and cytochrome reductase). A cytochrome is an electron-transferring protein that contains a heme prosthetic group. The iron ion of a cytochrome alternates between a reduced ferrous (+2) state and an oxidized ferric (+3) state during electron transport. The function of Q-cytochrome c oxidoreductase is to catalyze the transfer of electrons from QH2 to oxidized cytochrome c (cyt c), a water-soluble protein, and concomitantly pump protons out of the mitochondrial matrix. 

The nature of the second proton-pumping complex in each membrane is dependent on the primary energy source utilized by the organelle. In the case of mitochondria or respiring bacteria a respiratory chain (also called an electron-transfer chain) transfers electrons from a donor substrate to an acceptor, often oxygen, at... 

Since the ATP synthase is reversible, any displacement from this equilibrium which increases A/Hh or lowers AG jp would cause the complex to reverse, allowing protons to flow back down their electrochemical potential and re-synthesize ATP. In energy-transducing organelles the function of the second proton pump in the membrane (respiratory chain or photosynthetic electron transfer chain) is to create the conditions for such a reversal by continuously replenishing In this way... 

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