Kinetics of electron transfer through the respiratory chain

We show that the rate at which electrons pass through the respiratory chain in mitochondria and respiring prokaryotic cells is described by the product of three terms, one describing electron donation, one acceptance, and a third, the thermodynamic drive. We apply the theory of nonequilibrium thermodynamics in the context of the chemiosmotic model of proton translocation and energy conservation. This approach leads to a closed-form expression that predicts steady-state electron flux as a function of chemical conditions and the proton motive force across the mitochondrial inner membrane or prokaryotic cytoplasmic membrane. The rate expression, derived considering reverse and forward electron flow, is the first to account for both thermodynamic and kinetic controls on the respiration rate. The expression can be simplified under specific conditions to give rate laws of various forms familiar in cellular physiology and microbial ecology. The expression explains the nonlinear dependence of flux on electrical potential gradient, its hyperbolic dependence on substrate concentration, and the inhibiting effects of reaction products. It provides a theoretical basis for investigating life under unusual conditions, such as microbial respiration in alkaline waters.     

Polarons and conformons

Orgel's “error catastrophe” theory of cellular senescence may be adapted as a theory of oncogenesis which is capable of explaining tumour progression as well as tumour induction. The theory gives rise to a number of predictions and suggestions for experiment.     

Cadmium inhibits the electron transfer chain and induces reactive oxygen species

Recent research indicates that cadmium (Cd) induces oxidative damage in cells; however, the mechanism of the oxidative stress induced by this metal is unclear. We investigated the effects of Cd on the individual complexes of the electron transfer chain (ETC) and on the stimulation of reactive oxygen species (ROS) production in mitochondria. The activity of complexes II (succinate:ubiquinone oxidoreductase) and III (ubiquinol:cytochrome c oxidoreductase) of mitochondrial ETC from liver, brain, and heart showed greater inhibition by Cd than the other complexes. Cd stimulated ROS production in the mitochondria of all three tissues mentioned above. The effect of various electron donors (NADH, succinate, and 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinol) on ROS production was tested separately in the presence and in the absence of Cd. ESR showed that complex III might be the only site of ROS production induced by Cd. The results of kinetic studies and electron turnover experiments suggest that Cd may bind between semiubiquinone and cytochrome b566 of the Q0 site of cytochrome b of complex III, resulting in accumulation of semiubiquinones at the Q0 site. The semiubiquinones, being unstable, are prone to transfer one electron to molecular oxygen to form superoxide, providing a possible mechanism for Cd-induced generation of ROS in mitochondria.     

Comparative and evolutionary aspects of the photosynthetic electron transfer system of purple bacteria

The cyclic electron transfer system in purple bacteria is composed of the photosynthetic reaction center, the cytochromebc ₁ complex, cytochromec ₂, and ubiquinone. These components share many characteristics with those of photosynthesis and respiration in other organisms. Studies of the cyclic electron transfer system have provided useful insights about the evolution and general mechanisms of membranous energy-conserving systems. The photosynthetic system in purple bacteria may represent a prototype of highly efficient, energy-conserving electron transfer systems in the organisms. Recipient of the Botanical Society Award of Young Scientists, 1992     

Role of the photosynthetic electron transfer chain in electrogenic activity of cyanobacteria

Certain anaerobic bacteria, termed electrogens, produce an electric current when electrons from oxidized organic molecules are deposited to extracellular metal oxide acceptors. In these heterotrophic “metal breathers”, the respiratory electron transport chain (R-ETC) works in concert with membrane-bound cytochrome oxidases to transfer electrons to the extracellular acceptors. The diversity of bacteria able to generate an electric current appears more widespread than previously thought, and aerobic phototrophs, including cyanobacteria, possess electrogenic activity. However, unlike heterotrophs, cyanobacteria electrogenic activity is light dependent, which suggests that a novel pathway could exist. To elucidate the electrogenic mechanism of cyanobacteria, the current studies used site-specific inhibitors to target components of the photosynthetic electron transport chain (P-ETC) and cytochrome oxidases. Here, we show that (1) P-ETC and, particularly, water photolysed by photosystem II (PSII) is the source of electrons discharged to the environment by illuminated cyanobacteria, and (2) water-derived electrons are transmitted from PSII to extracellular electron acceptors via plastoquinone and cytochrome bd quinol oxidase. Two cyanobacterial genera (Lyngbya and Nostoc) displayed very similar electrogenic responses when treated with P-ETC site-specific inhibitors, suggesting a conserved electrogenic pathway. We propose that in cyanobacteria, electrogenic activity may represent a form of overflow metabolism to protect cells under high-intensity light. This study offers insight into electron transfer between phototrophic microorganisms and the environment and expands our knowledge into biologically based mechanisms for harnessing solar energy.     

Structural aspects of vectorial electron transfer in photosynthetic reaction centers

Structural aspects of photosynthetic reaction centers in bacteria and plants are discussed in relation with the ability of these structures to perform a photoinduced electron transfer from one side of the membrane to the other. A comparison is made with recently synthesized artificial models. Functional similarities between the acceptor sides of bacterial and of Photosystem-II centers are utilized to hypothesize on their structure.This review corresponds to a lecture delivered at the 3rd European Bioenergetics Conference, Hannover, September 1984.     

Extensive domain motion and electron transfer in the human electron transferring flavoprotein.medium chain Acyl-CoA dehydrogenase complex

The crystal structure of the human electron transferring flavoprotein (ETF).medium chain acyl-CoA dehydrogenase (MCAD) complex reveals a dual mode of protein-protein interaction, imparting both specificity and promiscuity in the interaction of ETF with a range of structurally distinct primary dehydrogenases. ETF partitions the functions of partner binding and electron transfer between (i) the recognition loop, which acts as a static anchor at the ETF.MCAD interface, and (ii) the highly mobile redox active FAD domain. Together, these enable the FAD domain of ETF to sample a range of conformations, some compatible with fast interprotein electron transfer. Disorders in amino acid or fatty acid catabolism can be attributed to mutations at the protein-protein interface. Crucially, complex formation triggers mobility of the FAD domain, an induced disorder that contrasts with general models of protein-protein interaction by induced fit mechanisms. The subsequent interfacial motion in the MCAD.ETF complex is the basis for the interaction of ETF with structurally diverse protein partners. Solution studies using ETF and MCAD with mutations at the protein-protein interface support this dynamic model and indicate ionic interactions between MCAD Glu(212) and ETF Arg alpha(249) are likely to transiently stabilize productive conformations of the FAD domain leading to enhanced electron transfer rates between both partners.     

Effect of cross-linking agents on electron transport and proton transfer in the mitochondrial respiratory chain

The effect of protein cross-linkage on proton translocation and electron transport in mitochondria respiratory chain was studied. Dimethylsuberimidate (1 mM) or dicyclohexyl carbodiimide (50 g/ml) inhibit proton translocation with concomitant stimulation of respiration. It is concluded that the definite level of dynamic mobility of proteins is needed for proton translocation.     

Cardiolipin requirement for electron transfer in complex I and III of the mitochondrial respiratory chain

Almost complete phospholipid depletion has been achieved for Complex I and III of the mitochondrial respiratory chain using a technique that involves elution on Sephadex LH-20 in the presence of Triton X-100. Enzymic activity may be regenerated by replenishment with phospholipid. However, restoration of enzymic activity in phospholipid-depleted Complex I and III has been shown to require the presence of cardiolipin. These results are, therefore, similar to findings on the absolute catalytic requirement of cardiolipin for cytochrome oxidase activity (Fry, M., and Green, D. E. (1980) Biochem. Biophys. Res. Commun. 93, 1238-1246). At least two roles for phospholipid involvement in electron transfer processes are proposed, a catalytic role provided specifically by cardiolipin and a dispersive role that may be provided by various phospholipids or detergents. The absolute requirement of enzymic activity for cardiolipin suggests that this phospholipid plays a crucial role in the coupled electron transfer process.     

Dehydroascorbate reduction in plant mitochondria is coupled to the respiratory electron transfer chain

The reduction of dehydroascorbate (DHA) was investigated in plant mitochondria. Mitochondria isolated from Bright Yellow-2 tobacco cells were incubated with 1 m M of DHA, and the ascorbate generation was followed by high-performance liquid chromatography. Mitochondria showed clear ability to reduce DHA and to maintain a significant level of ascorbate. Ascorbate generation could be stimulated by the respiratory substrate succinate. The complex I substrate malate and the complex I inhibitor rotenone had no effect on the ascorbate generation from DHA. Similarly, the complex III inhibitor antimycin A, the alternative oxidase inhibitor salicylhydroxamic acid, and the uncoupling agent 2,4-dinitrophenol were ineffective on mitochondrial ascorbate generation both in the absence and in the presence of succinate. However, the competitive succinate dehydrogenase inhibitor malonate almost completely abolished the succinate-dependent increase in ascorbate production. The complex IV inhibitor KCN strongly stimulated ascorbate accumulation. These results together suggest that the mitochondrial respiratory chain of plant cells – presumably complex II – plays important role in the regeneration of ascorbate from its oxidized form, DHA.     

FAD oxidizes the ERO1-PDI electron transfer chain: the role of membrane integrity

The molecular steps of the electron transfer in the endoplasmic reticulum from the secreted proteins during their oxidation are relatively unknown. We present here that flavine adenine dinucleotide (FAD) is a powerful oxidizer of the oxidoreductase system, Ero1 and PDI, besides the proteins of rat liver microsomes and HepG2 hepatoma cells. Inhibition of FAD transport hindered the action of FAD. Microsomal membrane integrity was mandatory for all FAD-related oxidation steps downstream of Ero1. The PDI inhibitor bacitracin could inhibit FAD-mediated oxidation of microsomal proteins and PDI, but did not hinder the FAD-driven oxidation of Ero1. Our data demonstrated that Ero1 can utilize FAD as an electron acceptor and that FAD-driven protein oxidation goes through the Ero1-PDI pathway and requires the integrity of the endoplasmic reticulum membrane. Our findings prompt further studies to elucidate the membrane-dependent steps of PDI oxidation and the role of FAD in redox folding.     

Isolation and reconstitution of two electron transfer components of tryptophan side chain oxidase.

The isolation and reconstitution of two electron transfer components of tryptophan side chain oxidase from Pseudomonas (ATCC 29574) are described. The dehydrogenase component abstracts electrons from the substrate and transfers them to oxidation-reduction dyes such as potassium ferricyanide and 2,6-dichlorophenolindophenol but not to molecular oxygen. It is composed of a single polypeptide chain with a molecular weight of 72,000 and exhibits the absorption spectrum of a reduced b-type cytochrome with maxima at 563, 532, 433, 323, and 278 nm. The oxidase component transfers electrons, derived from the former component, to oxygen, and has a molecular weight of 48,000. The absorption spectrum exhibits broad peaks at 680, 438, and 358 nm, and a peak at 280 nm. On sucrose gradient centrifugation and polyacrylamide gel electrophoresis, these two components are shown to form a molecular complex, which has the reconstituted oxidase activity. The turnover number of the reconstituted enzyme is comparable to that of the native enzyme.