Phospholipids electron transfer systems

The majority of studies on the distance dependence in electron transfer systems have involved modified enzymatic substrates/" " potential models of chlorophyll photosystems/ and donor-acceptor pairs separated by phospholipid bilayers/ There has been considerable progress in these areas despite the formidable synthetic problems, the difficulty in systematically varying only one parameter in a series of related studies, and the difficulty in evaluating k u in such complicated systems. Some reviews of this work have appeared. Electrochemical studies, using organic bridging groups attached to metal surfaces, should also be noted. ... 

The main barrier for solutes is the cytoplasmic membrane which consists of a liquid-crystalline bilayer of phospholipids in which proteins are embedded. The selective movement of solutes through this membrane is catalyzed by specific carrier proteins. In addition several other cellular functions are located in the cytoplasmic membrane such as the energy. transducing cyclic and linear electron transfer systems and the Ca + Mg -dependent ATPase complex. 

The digitonin preparation of PSI-RC has also been demonstrated to be able to catalyze a light-induced proton uptake when incorporated in phospholipid liposomes and illuminated in the presence of ascorbate and phenazine methosulphate [47] incorporation of chloroplast ATPase in the same system yielded the reconstitution of photophosphorylation in a model system. The PSI-RC preparation therefore seems to possess all the functional features of PSI for the vectorial transmembrane electron transfer [48] (see Fig. 4.7). 

An appropriate electron carrier such as PMS (5-N-methyl-phenazonium methylsulfate) can mediate a cyclic electron transfer around photosystem 1. PMS mediates the cyclic electron flow by serving as an electron acceptor on the reducing side of PS 1 with the reduced PMS serving as a donor of electrons directly to photooxidized P700, as illustrated in Fig. 9 (A). Thus a simple system containing just photosystem I embedded in a closed membrane system that also contains the ATP synthase plus the electron carrier PMS should in principle carry out photophosphorylation. Indeed, Hauska, Samoray, Orlich and Nelson prepared such a simple, minimum system using purified individual membrane complexes of chloroplasts, namely, photosystem I and ATP synthase, plus a soybean phospholipid (asolectin) as the membrane matrix to demonstrate its expected effectiveness. 

Coon, M.J., A.P. Autor, and H.W. Strobel (1971). Role of phospholipid in electron transfer in a reconstituted liver microsomal enzyme system containing cytochrome P-450. Chem. Biol. Interact. 3, 248-250. 

Electron transport systems perform important functions concerning respiration and energy metabolism in eucaryotes [22, 23], The electron transport reactions occur at the mitochondria inner membrane formed by electron transport proteins [24] and the lipid bilayer built up by the self-assembly of phospholipids as vital smfactants [25, 26]. The electron transport proteins include redox catalysts such as nicotinamide, iron [27, 28], and quinones [29]. The electrons produced by these redox reactions transfer through the lipid bilayer. While the relationship between the electron transport mechanisms and the molecular self-assembly in vivo has been clarified, control of the self-assembly by electron transport has been applied for an artificial polymeric surfactant. 

It is, however, of particular significance that the liver system has been solubilized and found to be made of four essential fractions cytochrome P450, NADPH cytochrome P450 reductase, and a heat-labile phospholipid fraction essential for electron transfer with phosphatidylcholine as the active component [194]. 

Idilayer formed by the lipid. In addition to vesicles and liposomes formed by biologically derived phospholipids, synthetic vesicles formed by surfactant derivatives such as dioctadecyl dimethyl ammonium chloride (DODAC) have been reported recently. Vesicles, unlike micelles are static entities and can accomodate a substantial number of guest molecules per aggregate. As in micelles, these systems can organise donors and acceptors, lower ionisation potentials and more importantly through their interfaces or electrical double layer allow for some kinetic control of electron transfers. 

Phosphatidylcholine was demonstrated to play an essential role in the biological activity of the microsomal hydroxylation system (Lu et al., 1969), functioning in some poorly understood manner in electron transfer from NADPH to cytochrome P-450. Reconstitution studies of the microsomal hydroxylation system from its isolated components have demonstrated the ability of a number of nonionic detergents in low concentration to substitute for phospholipid in the process of benzphetamine N-demethylation (Lu et al., 1974). While restoration of catalytic activity was not complete, it is likely that the natural lipid and the detergents act in a similar manner, perhaps enhancing the interaction of the two proteins of this system. 

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