Electrogenic transfer

A perfect selective proton transfer mechanism is easily conceivable and its role appears particularly suggestive in relation to the paradigmatic frame of reference of the chemiosmotic theories, 13, However, we may fairly ask the question whether experimental proofs and theoretical argumentation are nowadays sufficiently conclusive in order to retain the predominant proton pump as the most plausible model for steady state electrogenic transfer in these cells. 

Neurotransmitter transport can be electrogenic if it results in the net translocation of electrical charge (e.g. if more cations than anions are transferred into the cell interior). Moreover, some transporters may direction-ally conduct ions in a manner akin to ligand-gated ion channels this ion flux is not coupled to substrate transport and requires a separate permeation pathway associated with the transporter molecule. In the case of the monoamine transporters (DAT, NET, SERT) the sodium current triggered by amphetamine, a monoamine and psychostimulant (see Fig. 4) is considered responsible for a high internal sodium concentration... 

From the point of view of the stoichiometry of the transported ions during active transport, the electroneutral pump, where there is no net charge transfer or change in the membrane potential, must be distinguished from the electrogenic pump connected with charge transfer. 

The difference in the hydrogen ion electrochemical potential, formed in bacteria similarly as in mitochondria, can be used not only for synthesis of ATP but also for the electrogenic (connected with net charge transfer) symport of sugars and amino acids, for the electroneutral symport of some anions and for the sodium ion/hydrogen ion antiport, which, for example, maintains a low Na+ activity in the cells of the bacterium Escherichia coli. 

The malate-aspartate shuttle is the most important pathway for transferring reducing equivalents from the cytosol to the mitochondria in brain. This shuttle involves both the cytosolic and mitochondrial forms of aspartate aminotransferase and malate dehydrogenase, the mitochondrial aspartate-glutamate carrier and the dicarboxylic acid carrier in brain (Fig. 31-5) [69]. The electrogenic exchange of aspartate for glutamate and a... 

A third, clearer explanation of the electron transfer, proton translocation cycle is given by Saratse. Each ubiquinol (QH2) molecule can donate two electrons. A hrst QH2 electron is transferred along a high-potential chain to the [2Fe-2S] center of the ISP and then to cytochrome Ci. From the cytochrome Cl site, the electron is delivered to the attached, soluble cytochrome c in the intermembrane space. A second QH2 electron is transferred to the Qi site via the cytochrome b hemes, bL and bn. This is an electrogenic step driven by the potential difference between the two b hemes. This step creates part of the proton-motive force. After two QH2 molecules are oxidized at the Qo site, two electrons have been transferred to the Qi site (where one ubiquinone (Qio) can now be reduced, requiring two protons to be translocated from the matrix space). The net effect is a translocation of two protons for each electron transferred to cytochrome c. Each explanation of the cytochrome bci Q cycle has its merits and its proponents. The reader should consult the literature for updates in this ongoing research area. 

The transport is, therefore, electronentral. If the transport does, in fact, resnlt in the transfer of a net charge, it is termed electrogenic (e.g. the proton pnmps in the inner membrane). 

Figure 9.19 Adenine nucleotide translocase and phosphate transfer into the matrix. Phosphate is transported into the mitochondria with protons in a symport transport system. The adenine nucleotide translocase transports ADP into and ATP out of the mitochondria, i.e. it is electrogenic. The charge is neutralised by H movement into the matrix from the proton motive force which utilises about 25% of the energy in the proton motive force.

The transport process is electrogenic if the export of one molecule coupled with the import of another molecule yields a net charge difference across the membrane. In general terms, transfer of A3- from the matrix and A3- into the matrix yields a net negative charge on the cytoplasmic side of the membrane. Electrogenic processes are driven by the membrane potential (A P). 

Konstantinov, A. A., Siletsky, S., Mitchel, D., Kaulen, A., and Gennis, R., 1997, The roles of the two proton input channels in cytochrome c oxidase from Rhodobactor spheroides probed by the effects of site-directed mutations on time-resolved electrogenic intraprotein proton transfer, Proc. Nad. Acad. Sci. USA 94 9085n9090. 

Bacteriorhodopsin translocates one H ion per photon which causes the aW-trans — 13-c 5 photoisomerization of chromophore, retinal. At least two H -acceptor groups are shown to be directly involved in the H transfer by bacteriorhodopsin, namely, (a) the Schiff base forming a link between the retinal and the e-amino group of Lys-216, and (b) the Asp-96 carboxylic group. The involvement of the Schiff base is confirmed by many independent pieces of evidence (e.g., the electrogenic H transfer disappears at a pH below 3.5, i.e., below the pK value of the Schiff base in the M-intermediate of bacteriorhodopsin photocycle reviewed in ref. [7]). As to Asp-96, its participation in the H transfer relay was recently demonstrated by site-directed mutagenesis studies [13-19]. 

The mechanism by which the exergonic methyl-transfer reaction is coupled with vectorial electrogenic Na translocation across the membrane is not known. An electron transport chain appears not to be involved in Na transport. Sodium ion transport... 

In bacterial chromatophores the RC and the b/c, complex are arranged to form a cyclic electron transfer system possibly mediated by the diffusion of ubiquinone and cyt. Cj these carriers are, however, also coupled to other multienzyme complexes forming the respiratory chain and perform the aerobic metabolism of these facultative photosynthetic organisms [254]. The electrogenic steps of the photosynthetic cycle take place both within the RC and the 6/cj complexes and can be monitored by the electrochromic spectral shift of endogenous carotenoids and on the basis of their response to specific inhibitors and kinetics. When induced by a short laser flash the carotenoid signal displays three distinct kinetic phases (r,/2 10 h/i 5 jas... 

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