Transmembrane electron channels

Electron channels, as transmembrane wires, represent the channel-type counterpart to the mobile electron carriers discussed above and will be considered in Section 8.3.2. Anion channels may also be envisaged. 

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.

Since other membranes have an integral transmembrane electron transport system, the question arises whether these electron carriers can be involved in an oxidation-reduction driven proton movement. In neutrophil as well as macrophage plasma membranes, the answer is already yes. The superoxide-producing NADPH oxidase in these membranes is associated with a channel for proton movement to accompany the electron flow when internal NADPH is oxidized by external oxygen to produce superoxide (Nanda et al., 1993). This is a relatively simple electron transport system which contains a heterodimeric cytochrome b which also binds flavin. Thus, two proteins in a transmembrane electron transport system can transfer protons across the membrane. 

Figure 1.1. Simplified scheme showing electron transport in a portion of a chloroplast thylakoid membrane. Electrons flow from water via an oxygen-evolving complex (OEC) to photosystem II (PS2), pheophytin (PHEO), plastquinone (PQ), plastocyanin (PC) to photosystem I (PSI). Aq, Chlorophyll FeS, iron-sulfur centres FD, ferredoxin. Phosphorylation is catalyzed by proton transport through a transmembrane proton channel (CFq) to the ATP-synthetase complex (CF,).

ATP synthase actually consists of two principal complexes. The spheres observed in electron micrographs make up the Fj unit, which catalyzes ATP synthesis. These Fj spheres are attached to an integral membrane protein aggregate called the Fq unit. Fj consists of five polypeptide chains named a, j3, y, 8, and e, with a subunit stoichiometry ajjSaySe (Table 21.3). Fq consists of three hydrophobic subunits denoted by a, b, and c, with an apparent stoichiometry of ajbgCg.ig- Fq forms the transmembrane pore or channel through which protons move to drive ATP synthesis. The a, j3, y, 8, and e subunits of Fj contain 510, 482, 272, 146, and 50 amino acids, respectively, with a total molecular mass... 

This picture was made by mathematically filtering electron micrographs of ordered, two-dimensional arrays of the receptor. Each of the four pentameric structures shown represents an individual acetylcholine receptor with its central transmembrane pore. The dark areas indicate the five subunits (a2fiy8). (From R. M. Stroud, M. P. McCarthy, and M. Schuster, Nicotinic acetylcholine receptor superfamily of ligand-gated ion channels,... 

Figure 11.16 Rigid-rod Jt-M-helix 28 as a supramolecular photosystem that can open up into an ion channel 29 after intercalation with the aromatic ligand 33. Transmembrane photoinduced electron transfer from EDTA donors to quinone acceptors Q is measured as formal proton pumping with light across lipid bilayers. HPTS is used to measure intravesicular deacidification with light.

At least three types of proton channel systems are recognized in animal cells. These include the Na+/H+ exchanger, the H+-ATPase, and the HCOj/Cl- exchanger. It is clear that a major part of proton release by some cells in response to transplasma membrane electron transport is by activation of the Na+/H+ exchanger. This is clear from the characteristics of the proton movement elicited and the magnitude of H+ release in relation to electron flow when electron transport is activated. Activation of electron transport can be elicited by addition of di-ferric transferrin to activate the transmembrane NADH oxidase activity or by electron flow to external ferricyanide from internal NADH. Addition of di-ferric transferrin to certain cells, especially pineal cells, elicits a remarkable proton release and internal alkaliniza-tion. The stoichiometry of H+ release to iron reduced is more than 100 to 1 (Sun et... 

Electron microscopy studies have revealed that, like acetylcholine receptors, the ion channel of the GABA receptor is formed by the pentameric assembly of hetero-oligomeric subunits (129) each subunit has four trans-membrane spanning domains and all five sub-uinits are arranged so that their second transmembrane domains comprise the ion channel wall. Cloning of the subunits from vertebrates has resulted in nearly 20 cDNAs, which have... 

An approach to the treatment of this replacement transfer problem is the analog of the simple valence bond treatment in the molecular electronic problem. The possibilities can be listed in tabular form see Table 5. In the table, L, B, and R stand for the left probe channel, the membrane gate (block), and the right transmembrane channel, as before. The labels a...f indicate wavefunction product combinations F (l) E (2)... 

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