Transmembrane Oxidation-Reduction Reactions

Based upon a detailed analysis of reaction transients, a mechanism was proposed for chlorophyll a-photosensitized transmembrane oxidation-reduction of aqueous phase donors and acceptors that included electron transfer between juxtaposed Chi a+ r-cations and Chi a molecules as the transmembrane charge-transfer step [112]. The maximum apparent first-order rate constant for this step was 10 s , which seems large for thermal electron transfer between chlorophyll molecules located at the opposite membrane interfaces, even considering that nuclear activation barriers may be relatively small for this reaction. Transverse flip-flop diffusion of Chi b across the membrane is 10 -fold slower than transmembrane redox under these conditions, so this alternative mechanism is almost certainly unimportant. Kinetic mapping studies have shown that some of the Chi a becomes localized within the membrane at sites that are inaccessible to aqueous phase electron acceptors, presumably within the membrane interior [114]. This suggests the possibility of a transverse hopping mechanism involving electron transfer over relatively short distances from buried Chi a to interfacial Chi a+, followed by electron transfer from Chi a at the opposite interface to the buried Chi a" ". 

Before proceeding to the next topic, we look at another version of artificial phosphorylation by chloroplasts in the dark, i.e., not driven hyphotoinducedelectron transfer. This new type oftwo stage phosphorylation, called dark oxidation-reduction coupled phosphorylation, was reported by Selman and Psczolla and may be considered as a variant of the postillumination or the acid-base transition types discussed above. The authors found that ATP formation in chloroplasts in the dark can be achieved by an artificial, transmembrane redox reaction using ascorbate as the reductant for ferricyanide trapped inside the chloroplasts, provided it is mediated by a redox carrier such as DAD, DCIP or PMS that liberates protons during its oxidation, as illustrated in the scheme in Figure 13. 

There have been proposals that the membrane potential is produced mainly as a result of oxidation-reduction potentials at the membrane surfaces. The consequences of such proposals, if proven, would completely revolutionize present concepts concerning membrane biology. Thus, for example, ion diffusion and transport across membranes would be conceived to result as a consequence of the transmembrane potential and not determine the potential as described by the Nernstian concepts already outlined (Figure 8). One side of the membrane is conceived as supporting an anodic reaction, the other side a cathodic reaction. In the concepts outlined by Habib and Bockris the principal anodic reaction is envisaged to be of the type... 

Figure 18.6 Energetics of the ORR at the heme/Cu site of CcO the enzyme couples oxidation of ferroc3ftochrome c (standard potential about —250 mV all potentials are listed with respect to a normal hydrogen electrode) to reduction of O2 (standard potential at pH 7 800 mV). Of the 550 mV difference, only 100 mV is dissipated to drive the reaction 220 mV is expanded to translocate four protons from the basic matrix compartment to the acidic IMS (inter-membrane space). In addition 200 mV is converted into transmembrane electrostatic potential as ferroc3ftochrome is oxidized in the IMS, but the charge-compensating protons are taken from the matrix. The potentials are approximate.

Figure 1. The major transmembrane photosynthetic reaction centers (RC) (top) and respiratory complexes (bottom) are composed of light (zigzag) activated chains (dark gray) of redox centers (open polygons) that create a transmembrane electric field and move protons (double arrows) to create a transmembrane proton gradient, fulfilling the requirements of Mitchell s chemiosmotic hypothesis. Diffusing substrates include ubiquinone (hexagon) and other sources of oxidants and reductants. PSI and PSII, photosystems I and II, respectively.

Photosynthetic reaction centers plug into the chemiosmotic scheme by using light-excited states to create both an oxidant and a reductant. For the purple bacterial reaction centers, these oxidants and reductants are the redox carriers already described, oxidized cytochrome c and reduced ubiquinone QH2. Thus, in combination with Complex III, light drives a relatively straightforward cyclic electron transfer that generates a transmembrane electric field and proton gradient. 

Figure 12. General reaction scheme for photoinduced transmembrane reduction of Co(bpy)3 by dithiothreitol (DTT) across DHP bilayer membranes mediated by 2,4,6-triphenylpyrylium ion (TPP ). The mediator acts both as an oxidative quencher of ZnyPPS" and a cyclic electroneutral transmembrane e /OH antiporter (from [120]).

The cytochrome c oxidase reaction encompasses the so-called third site of oxidative phosphorylation. There is no doubt that oxidation of cytochrome c by dioxygen results in generation of pmf. Cytochrome oxidase was long believed to do so simply by catalysing transmembranous electron transfer, with uptake of the protons required in reduction of Oj to water from the M phase. Such a function is thermodynamically equivalent to translocation of one proton per transferred electron, although no protons appear on the C side [8]. 

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