Carrier-mediated electron transfer

Such a mechanism appears to be adequate enough for the description of the carrier-mediated electron transfer across membranes. It allows, for example, one to describe quantitatively the influence of electrical polarization on the transmembrane electron transfer. 

The electrical contacting of redox enzymes that defy direct electrical communication with electrodes can be established by mediated electron transfer using synthetic or biologically active charge carriers. Mediated electron transfer (MET) can be effected by a diffusional mechanism (Figure 2), where the electron relay is either oxidized or reduced at the electrode surface. Diffusional penetration of the oxidized or... 

Reconstitution Experiments Demonstrate the Need for Carriers to Mediate Electron Transfer between Complexes Oxidative Phosphorylation... 

The oxidation potential of the substrate S in Figure 2 is beyond the range accessible by the electrochemical method so that direct electron transfer from S to the anode hardly occurs, and also the high oxidation potential necessary for the direct oxidation of S causes unexpected side reactions involving oxidation of the solvent or supporting electrolyte. However, when a compound Mrxi (a reduced form of M) which may be oxidized at a sufficiently lower potential than S is added to the reaction system, the oxidation of Mied to Mox (an oxidized form of M) will take place prior to the oxidation of S. Provided that Mox is able to oxidize S to product P, the oxi tion of S will be achieved at a potential lower than that necessary for its direct oxidation. Oxidation of S with Mox may be effected in two ways, namely by direct electron transfer (homogeneous electron transfer) from S to Mox in solution or by chemical oxidation of S with Mox. The former system is called a homomediatory system and the latter a heteromediatory (or chemomediatory) system. The compound M is called a mediator or an electron carrier, since M mediates electron transfer between S and the anode. When Mox oxidizes S in solution, Mox is reduced to Mrxi... 

Menaquinone (vitamin K2) mediates electron transfer between dehydrogenases and the cytochromes and is the major electron carrier cofactor during anaerobic growth of E. coli. Vitamin K is also an essential cofactor in the post-translational carboxylation of glutamic acid residues in several protein systems [200]. 

It was found that some small redox-active chemicals (termed electron shuttles) can diffuse across the cell membrane and serve as electron carriers to assist the electron transfer from the bacteria to the electrode. The electron shuttle mediated electron transfer process usually contains three steps, i.e., being reduced by cells (the electron shuttle is converted to a reductive state). 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

During the last two decades, more studies have been conducted to explore the catalytic effects of different redox mediators on the bio-transformation processes. Redox mediators, also referred to as electron shuttles, have been shown to play an important role not only as final electron acceptor for many recalcitrant organic compounds, but also facilitating electron transfer from an electron donor to an electron acceptor, for example, azo dyes [8, 11, 12], Redox mediators accelerate reactions by lowering the activation energy of the total reaction, and are organic molecules that can reversibly be oxidized and reduced, thereby conferring the capacity to serve as an electron carrier in multiple redox reactions. 

Fe(II)/Fe(in) as a Mediator in Electron Transfer. The Fe(III)-Fe(II) system often acts as an electron carrier. A possible schematic example is given by... 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

Iron-sulfur clusters are important co-factors in electron-transfer. Type I reaction centres contain [4Fe-4S] clusters as final electron acceptors mediating ET to soluble electron carriers like ferredoxin or flavodoxin (reviewed in references 188, 224, 314, 315) In PS I three clusters (F FA and FB) have been clearly identified and spectroscopically characterized. The PsaA and PsaB subunits carry most of the ET cofactors in PS I.18178-316 Each of them provides two Cys ligands to the binding site of the interpolypeptide [4Fe-4S] cluster Fx. This binding site is identical on both core PS I subunits.317 Both [4Fe-4S] clusters FA and Fb are bound to the PS I stromal subunit PsaC. It contains two identical [4Fe-4S] consensus binding sites CxxCxxCxxxCP (C = cysteine, P = proline). 

Fig. 21. Transmembrane electron transfer processes carrier mediated via a redox carrier (left) or channel mediated via a molecular wire (right). Both processes may be coupled to light by introduction of photoactive groups in the carrier or in the wire.

The other way of PET across membrane is carrier-mediated transfer as shown in Fig. 3b. This way suggests the use of an intermediate acceptor, Al5 embedded into the membrane and capable in the reduced form, Af, to carry electron across the membrane to the ultimate acceptor, A2. In this case S, S+, A2 and AJ should... 

As already mentioned in Sect. 2, there are two alternative pathways of transmembrane electron transfer (i) direct transfer via electron exchange reactions between the molecules located in the inner and outer monolayers of the membrane, such as reactions (8) and (14), and (ii) carrier-mediated transfer, provided by the diffusion of an electron carrier or a hole carrier from one side of the membrane to the other, such as reaction (26). 

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