Membrane pump theory

Many pieces of experimental evidence exist in support of the AI hypothesis and against the membrane-pump theory. The reader must consult the aforementioned monograph for a full discussion. Here I shall limit our discussion to two issues the adsorbed state of and the bulk phase water in living cells. 

The establishment of localized K adsorption in frog muscle by itself represents a disproof of one basic tenet of the membrane-pump theory. There are other important secondary implications ... 

The proposed model for the so-called sodium-potassium pump should be regarded as a first tentative attempt to stimulate the well-informed specialists in that field to investigate the details, i.e., the exact form of the sodium and potassium current-voltage curves at the inner and outer membrane surfaces to demonstrate the excitability (e.g. N, S or Z shaped) connected with changes in the conductance and ion fluxes with this model. To date, the latter is explained by the theory of Hodgkin and Huxley U1) which does not take into account the possibility of solid-state conduction and the fact that a fraction of Na+ in nerves is complexed as indicated by NMR-studies 124). As shown by Iljuschenko and Mirkin 106), the stationary-state approach also considers electron transfer reactions at semiconductors like those of ionselective membranes. It is hoped that this article may facilitate the translation of concepts from the domain of electrodes in corrosion research to membrane research. 

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. 

A prediction of the chemiosmotic theory is that, because the role of electron transfer in mitochondrial ATP synthesis is simply to pump protons to create the electrochemical potential of the proton-motive force, an artificially created proton gradient should be able to replace electron transfer in driving ATP synthesis. This has been experimentally confirmed (Fig. 19-20). Mitochondria manipulated so as to impose a difference of proton concentration and a separation of charge across the inner membrane synthesize ATP in the absence of an oxidizable substrate the proton-motive force alone suffices to drive ATP synthesis. 

According to the chemiosmotic theory, flow of electrons through the electron-transport complexes pumps protons across the inner membrane from the matrix to the intermembrane space. This raises the pH in the matrix and leaves the matrix negatively charged with respect to the intermembrane space and the cytosol. Protons flow passively back into the matrix through a channel in the ATP-synthase, and this flow drives the formation of ATP. 

This energy transduction step [64], although functionally equivalent to those involving the redox-linked pumping of protons across the membrane [65], can be achieved in aqueous solution in the absence of a membrane confinement, and is in full agreement with the Williams localized theory for energy transduction [66]. 

In these equations, Pj and P2 are the two conformational states of the transport protein, and equilibrium constants (K) and rate constants (k) in an electric field are shown to be these constants in zero field multiplied by a nonlinear term that is the product of A Me and the electric field across the membrane, Em. The r in these equations is the apportionation constant and has a value between 0 and 1 (14). This property of a membrane protein has been explored, and a model called electroconformational coupling has been proposed to interpret data on the electric activation of membrane enzymes (13-17). A four-state membrane-facilitated transport model has been analyzed and shown to absorb energy from oscillating electric fields to actively pump a substrate up its concentration gradient (see the section entitled Theory of Electroconformational Coupling). 

One of the tenets of the chemiosmotic theory is that energy from the oxidation-reduction reactions of the electron transport chain is used to transport protons from the matrix to the intermembrane space. This proton pumping is generally facilitated by the vectorial arrangement of the membrane spanning complexes. Their stracture allows them to pick up electrons and protons on one side of the membrane and release protons on the other side of the membrane as they transfer an electron to the next component of the chain. The direct physical link between proton movement and electron transfer can be illustrated by an examination of the Q cycle for the b-Ci complex (Fig. 21.9). The Q cycle involves a double cycle of CoQ reduction and oxidation. CoQ accepts two protons at the matrix side together with two electrons it then releases protons into the intermembrane space while donating one electron back to another component of the cytochrome b-Ci complex and one to cytochrome c. 

The next important postulate of Mitchell s theory concerns the consumers of the energy produced by primary pumps and presupposes the presence in the organella membranes of secondary proton pumps which use the transmembrane proton flow for ATP synthesis and a number of other processes. Essential to this theory is the... 

Our recent work has partially resolved the difficulties of interpretation of potentiometric experiments and provided a quantitative method for deciphering potentiometric data. Using this theory, we have performed quantitative analysis of the data of Belevich et al, in which the experimental amplitudes and rates are related to specific residues that exchange electrons and protons, and generate the observed membrane potential. Using this theory, we have tested proposed candidates for the proton pump site of the enzyme against experimental potentiometric data [37]. 

LIQUID MEMBRANES THEORY AND APPLICATIONS To Vacuum Pump... 

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