Direct interaction between linear electron transfer chains and solute transport systems in bacteria

Abstract In studies on alanine and lactose transport in Rhodopseudomonas sphaeroides we have demonstrated that the rate of solute uptake in this phototrophic bacterium is regulated by the rate of lightinduced cyclic electron transfer.
In the present paper the interaction between linear electron transfer chains and solute transport systems was studied in Rhodopseudomonas sphaeroides and Escherichia coli .
The results demonstrate that the activities of alanine transport in Rps. sphaeroides and lactose and proline transport in E. coli are directly controlled by the electron transfer activity in the respiratory chain, under conditions that the proton-motive force remains constant.     

The interaction between electron transfer,proton motive force and solute transport in bacteria

The properties of proton solute symport have been studied inStreptococcus cremoris, Rhodopseudomonas sphaeroides andEscherichia coli. In the homolactic fermentative organismS. cremoris the efflux of lactate is a membrane proteinmediated process, which can lead to the generation of a proton motive force. These observations support the energy-recycling model that postulates the generation of metabolic energy by end-product efflux. Studies with oxidants and reductants and specific dithiol reagents inE. coli membrane vesicles demonstrated the presence of two redox-sensitive dithiol-disulphide groups in the transport proteins of proline and lactose. The redox state of these groups is controlled by the redox potential of the environment and by the proton motive force. One redox-sensitive group is located at the inner surface, the other at the outer surface of the membrane. InRps. sphaeroides andE. coli the activity of several transport proteins depends on the activity of the electron transfer systems. On the basis of these results a redox model for proton solute transport coupled in parallel to the electron transfer system is postulated.     

细菌与古菌之间的直接电子传递

冰川占地球陆地表面的11%,储存了约10⁴ Pg有机碳。随着冰川消融有机碳被释放至下游生态系统中,刺激海洋、湖泊和径流的初级生产力进而影响其生态系统。微生物参与的固碳过程决定了冰川有机碳储量及向下游输出碳量。研究冰川固碳微生物群落构成及其生态功能,可为估算冰川碳积累量和保护下游生态系统提供数据基础。本文综述了冰川碳储量和释放量、冰川生态系统主要固碳途径、固碳微生物群落组成、固碳速率以及影响固碳速率的环境因素。最后基于研究现状展望了冰川生态系统固碳微生物的未来研究和发展方向。     

Direct electron transfer between copper-containing proteins and electrodes

The electrochemistry of some copper-containing proteins and enzymes, viz. azurin, galactose oxidase, tyrosinase (catechol oxidase), and the “blue” multicopper oxidases (ascorbate oxidase, bilirubin oxidase, ceruloplasmin, laccase) is reviewed and discussed in conjunction with their basic biochemical and structural characteristics. It is shown that long-range electron transfer between these enzymes and electrodes can be established, and the mechanistic schemes of the DET processes are proposed.     

Direct and indirect electron transfer between electrodes and redox proteins

The direct electrochemistry of redox proteins has been achieved at a variety of electrodes, including modified gold, pyrolytic graphite and metal oxides. Careful design of electrode surfaces and electrolyte conditions are required for the attainment of rapid and reversible protein-electrode interaction. The electron transfer reactions of more complex systems, such as redox enzymes, are now being examined. The 'well-behaved' electrochemistry of redox proteins can be usefully exploited by coupling the electrode reaction to enzymes for which the redox proteins act as cofactors. In systems where direct electron transfer is very slow, small electron carriers, or mediators, may be employed to enhance the rate of electron exchange with the electrode. The organometallic compound ferrocene and its derivatives have proved particularly effective in this role. A new generation of electrochemical biosensors employs ferrocene derivatives as mediators.     

The role of the proton motive force and electron flow in solute transport in Escherichia coli

Transport of lactose and methyl beta-D-thiogalactopyranoside, a melibiose analogue, was studied in intact cells of Escherichia coli. A proton motive force could drive the translocation of these solutes via these two transport systems, but the initial rates and steady-state levels of solute accumulation increased upon initiation of electron transfer. When the absolute value of the proton motive force was decreased by ionophores the steady-state levels of lactose accumulation did not decrease as expected if thermodynamic equilibrium with the proton motive force had existed. Accumulation of lactose was also observed in the absence of any measurable proton motive force as long as electron transfer took place. Since both proton/lactose and sodium/methyl beta-D-thiogalactopyranoside symport showed the same characteristics, an explanation based on local proton diffusion pathways is unlikely.     

Kinetics of electron transfer between the primary and the secondary electron acceptor in reaction centers from Rhodopseudomonas sphaeroides

Photoreduction of the two ubiquinone molecules, UQ₁ and UQ₂, bound to purified reaction center from Rhodopseudomonas sphaeroides induces different absorption band shifts of bacteriochlorophyll and bacteriopheophytin molecules depending on which ubiquinone is photoreduced. This allows us to study electron transfer between UQ₁ and UQ₂ directly by absorption spectrometry. The results support a model in which electrons are transferred one by one from UQ₁ to UQ₂ with a half-time of 200 μs, and two by two from fully reduced UQ₂ to the secondary acceptor pool.     

Novel interactions between mitochondrial superoxide dismutases and the electron transport chain

The processes that control aging remain poorly understood. We have exploited mutants in the nematode, Caenorhabditis elegans, that compromise mitochondrial function and scavenging of reactive oxygen species (ROS) to understand their relation to lifespan. We discovered unanticipated roles and interactions of the mitochondrial superoxide dismutases (mtSODs): SOD‐2 and SOD‐3. Both SODs localize to mitochondrial supercomplex I:III:IV. Loss of SOD‐2 specifically (i) decreases the activities of complexes I and II, complexes III and IV remain normal; (ii) increases the lifespan of animals with a complex I defect, but not the lifespan of animals with a complex II defect, and kills an animal with a complex III defect; (iii) induces a presumed pro‐inflammatory response. Knockdown of a molecule that may be a pro‐inflammatory mediator very markedly extends lifespan and health of certain mitochondrial mutants. The relationship between the electron transport chain, ROS, and lifespan is complex, and defects in mitochondrial function have specific interactions with ROS scavenging mechanisms. We conclude that mtSODs are embedded within the supercomplex I:III:IV and stabilize or locally protect it from reactive oxygen species (ROS) damage. The results call for a change in the usual paradigm for the interaction of electron transport chain function, ROS release, scavenging, and compensatory responses.     

A novel protein transport system involved in the biogenesis of bacterial electron transfer chains

The Tat system is a recently discovered bacterial protein transport pathway that functions primarily in the biosynthesis of proteins containing redox active cofactors. Analogous transport systems are found in plant organelles. Remarkably and uniquely the Tat system functions to transported a diverse range of folded proteins across a biological membrane, a feat that must be achieved without rendering the membrane freely permeable to protons and other ions. Here we review the operation of the bacterial Tat system and propose a model for the structural organisation of the Tat preprotein translocase.     

Direct demonstration of electron transfer between tryptophan and tyrosine in proteins

With several proteins it has been shown that electrons can be transferred intramolecularly from tyrosine to electron-deficient tryptophan units. Rates vary from ～ 10²s^?1 (in lysozyme) to ～ 2×10⁴ s^?1 (in trypsin). For β-lactoglobulin the activation energy is 45kJ mol^?1. This is incompatible with charge conduction along the polypeptide chain and rules out any mechanism involving temperature-labile hydrogen bonds as the main pathway. It seems likely that the electron transfer proceeds directly between the aromatic groups, while they are maintained at a distance from each other.     

Kinetics of electron transfer between the primary and the secondary electron acceptor in reaction centers from Rhodopseudomonas sphaeroides.

Photoreduction of the two ubiquinone molecules, UQ1 and UQ2, bound to purified reaction center from Rhodopseudomonas sphaeroides induces different absorption band shifts of bacteriochlorophyll and bacteriopheophytin molecules depending on which ubiquinone is photoreduced. This allows us to study electron transfer between UQ1 and UQ2 directly by absorption spectrometry. The results support a model in which electrons are transferred one by one from UQ1 to UQ2 with a half-time 200 micro seconds, and two by two from fully reduced UQ2 to the secondary acceptor pool.     

Electrostatic interactions during electron transfer reactions between c-type cytochromes and flavodoxin

The interaction of three different c-type cytochromes with flavodoxin has been studied by computer graphics modelling and computational methods. Flavodoxin and each cytochrome can make similar hypothetical electron transfer complexes that are characterized by nearly coplanar arrangement of the prosthetic groups, close intermolecular contacts at the protein-protein interface, and complementary intermolecular salt linkages. Computation of the electrostatic free energy of each complex showed that all were electrostatically stable. However, both the magnitude and behavior of the electrostatic stabilization as a function of solution ionic strength differed for the three cytochrome c-flavodoxin complexes. Variation in the computed electrostatic stabilization appears to reflect differences in the surface distribution of all charged groups in the complex, rather than differences localized at the site of intermolecular contact. The computed electrostatic association constants for the complexes and the measured kinetic rates of electron transfer in solution show a remarkable similarity in their ionic strength dependence. This correlation suggests electrostatic interactions influence electron transfer rates between protein molecules at the intermolecular association step. Comparative calculations for the three cytochrome c-flavodoxin complexes show that these ionic strength effects also involve all charged groups in both redox partners.     

Steady-state cyclic electron transfer through solubilized Rhodobacter sphaeroides reaction centres

The mechanism, thermodynamics and kinetics of light-induced cyclic electron transfer have been studied in a model energy-transducing system consisting of solubilized Rhodobacter sphaeroides reaction center/light harvesting-1 complexes (so-called core complexes), horse heart cytochrome c and a ubiquinone-0/ubiquinol-0 pool. An analysis of the steady-state kinetics of cytochrome c reduction by ubiquinol-0, after a light-induced steady-state electron flow had been attained, showed that the rate of this reaction is primarily controlled by the one-electron oxidation of the ubiquinol-anion. Re-reduction of the light-oxidized reaction center primary donor by cytochrome c was measured at different reduction levels of the ubiquinone-0/ubiquinol-0 pool. These experiments involved single turnover flash excitation on top of background illumination that elicited steady-state cyclic electron transfer. At low reduction levels of the ubiquinone-0/ubiquinol-0 pool, the total cytochrome c concentration had a major control over the rate of reduction of the primary donor. This control was lost at higher reduction levels of the ubiquinone/ubiquinol-pool, and possible reasons for this behaviour are discussed.     

Effects of protein-protein interactions on electron transfer: docking and electron transfer calculations for complexes between flavodoxin and c-type cytochromes

 Theoretical studies of protein-protein association and electron transfer were performed on the binary systems formed by Desulfovibrio vulgaris Hildenborough (D. v. H.) flavodoxin and D. v. H. cytochrome c ₅₅₃ and by flavodoxin and horse heart cytochrome c. Initial structures for the complexes were obtained by rigid-body docking and were refined by MD to allow for molecular flexibility. The structures thus obtained were analysed in terms of their relative stability through the calculation of excess energies. Electrostatic, van der Waals and solvation energy terms showed all to have significant contributions to the stability of complexes. In the best association solutions found for both cytochromes, these bind to different zones of flavodoxin. The binding site of flavodoxin observed for cytochrome c is in accordance with earlier works [27]. The various association modes found were characterised in terms of electron transfer using the Pathways model. For complexes between flavodoxin and horse heart cytochrome c, some correlation was observed between electron tunnelling coupling factors and conformation energy; the best conformation found for electron transfer corresponded also to the best one in terms of energy. For complexes between flavodoxin and cytochrome c ₅₅₃ this was not the case and a lower correlation was observed between electron tunnelling coupling factors and excess energies. These results are in accordance with the differences in the experimental dependence of electron transfer rates with ionic strength observed between these two cases. Received: 29 December 1998 / Accepted: 22 March 1999