Proton pumping electron transfer with

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). 

V F rster, Y-Q Hong and W Junge (1981) Electron transfer and proton pumping under excitation of dark-adapted chloroplasts with flashes of light. Biochim Biophys Acta 638 141-152... 

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. 

In Photosystem II, chlorophyll absorbs a photon of fight, with maximum absorption occurring at 680 nm. The photon excites an electron in the chlorophyll, and this excited electron moves through the chlorophyll to chlorophylls reaction center. Here the photons energy is used by electron-transfer proteins to pump protons (hydrogen ions, H+) into the thylakoid. 

The state of the catalytic site formed by the reduction of Pm is denoted F. Transfer of an additional electron to the catalytic site is again coupled to the uptake of two protons and the release of one pumped proton. This process results in the formation of the oxidized catalytic site, denoted OH, where the subscript denotes an activated high-potential state in which it is postulated that Cub has a very high electrochemical potential. The next two electron transfer reactions convert the Oh state to the E state (Cub reduced) and, further to the R state (heme and Cub both reduced). Each of these steps is also thought to be coupled to the uptake of two protons and release of one proton. However, little is known about the nature of the activated Oh state, or the On tE and E R steps of the reaction. For the purposes of this discussion it is assumed that each of the electron transfer steps to the catalytic site. Oh t E, E —> R, Pm F, and F —> Oh, is associated with proton pumping by the same mechanism, but much more needs to be done experimentally to test this assumption. 

Rotenone interferes with the electron/proton transfer reaction at complex I, thns rendering all the subseqnent complexes fnlly oxidized becanse they have nothing from which to accept electrons. This prevents proton pumping and therefore stops oxygen consumption. The lack of proton pumping leads to a collapse of the proton gradient and an inability of the system to synthesize ATP. As a consequence, ATP concentrations fall, and the ceU dies. This is why rotenone is such an effective poison. 

The resting to pulsed state kinetic transition involves an increase in the rate of intramolecular electron transfer through the oxidase (i.e., from Cu to Cu,-Heme Oj) [63], Unlike the conformations involved with proton pumping, the resting and pulsed conformational states persist over minutes. Observation of the resting to pulsed state kinetic transition indicates that electfon ttansfer through the... 

The principles of electric potential generation and proton pumping in thylakoids are understood. There is one electrogenic reaction in each of the two photosystems and a third one associated with cyclic electron transfer through the cytochrcme-b -segment of the electron transfer chain. Proton deposition into thylakoids occurs at the level of water oxidation and of plastohydroquinone oxidation. Proton uptake from outside is initiated idien plastoquinone is reduced by photosystem II (or during cyclic electron transfer) and upon reduction of the terminal electron acceptor. Broadly... 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

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. 

Figure 9.8 Simple diagram of mitochondrial H -ion movement and axonal K -ion movement to establish membrane potentials across membranes. Note that H movement from the mitochondrial matrix to the outer surface of the inner membrane requires a specific proton pump that requires energy, which is transferred from electron transfer, whereas the K ion movement occurs via an ion channel with energy provided from the concentration difference of K ions on either side of the membrane (approximately 100-fold). The movement of both the protons and K ions generates a membrane potential. The potential across the membrane of the nerve axon provides the basis for nervous activity (see Chapter 14).

Chemiosmotic theory readily explains the dependence of electron transfer on ATP synthesis in mitochondria. When the flow of protons into the matrix through the proton channel of ATP synthase is blocked (with oligomycin, for example), no path exists for the return of protons to the matrix, and the continued extrusion of protons driven by the activity of the respiratory chain generates a large proton gradient. The proton-motive force builds up until the cost (free energy) of pumping... 

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