Search

Abstract: The thylakoid membranes of cyanobacteria are the major sites of respiratory electron transport as well as photosynthetic light reactions. The photosynthetic and respiratory electron transport chains share some components, and their presence in the same membrane opens up the possibility for a variety of “unorthodox” electron transport routes. Many of the theoretically possible electron transport pathways have indeed been detected in particular species and circumstances. Electron transport has a crucial impact on the redox balance of the cell and therefore the pathways of electron flow in the cyanobacterial thylakoid membrane must be tightly regulated. This review summarises what is known of cyanobacterial electron transport components, their interactions and their sub-cellular location. The role of thylakoid membrane organisation in controlling electron transport pathways is discussed with respect to recent evidence that the larger-scale distribution of complexes in the membrane is important for controlling electron exchange between the photosynthetic and respiratory complexes. The distribution of complexes on scales of 100nm or more is under physiological control, showing that larger-scale thylakoid membrane re-arrangement is a key factor in controlling the crosstalk between photosynthetic and respiratory electron transport. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components. [Copyright &y& Elsevier]

Abstract: Overproduction of reactive oxygen species (ROS) has been implicated in a range of pathologies. Mitochondrial flavin dehydrogenases glycerol-3-phosphate dehydrogenase (mGPDH) and succinate dehydrogenase (SDH) represent important ROS source, but the mechanism of electron leak is still poorly understood. To investigate the ROS production by the isolated dehydrogenases, we used brown adipose tissue mitochondria solubilized by digitonin as a model. Enzyme activity measurements and hydrogen peroxide production studies by Amplex Red fluorescence, and luminol luminescence in combination with oxygraphy revealed flavin as the most likely source of electron leak in SDH under in vivo conditions, while we propose coenzyme Q as the site of ROS production in the case of mGPDH. Distinct mechanism of ROS production by the two dehydrogenases is also apparent from induction of ROS generation by ferricyanide which is unique for mGPDH. Furthermore, using native electrophoretic systems, we demonstrated that mGPDH associates into homooligomers as well as high molecular weight supercomplexes, which represent native forms of mGPDH in the membrane. By this approach, we also directly demonstrated that isolated mGPDH itself as well as its supramolecular assemblies are all capable of ROS production. [Copyright &y& Elsevier]

Abstract: Parasites have developed a variety of physiological functions necessary for completing at least part of their life cycles in the specialized environments of surrounding the parasites in the host. Regarding energy metabolism, which is essential for survival, parasites adapt to the low oxygen environment in mammalian hosts by using metabolic systems that are very different from those of the hosts. In many cases, the parasite employs aerobic metabolism during the free-living stage outside the host but undergoes major changes in developmental control and environmental adaptation to switch to anaerobic energy metabolism. Parasite mitochondria play diverse roles in their energy metabolism, and in recent studies of the parasitic nematode, Ascaris suum, the mitochondrial complex II plays an important role in anaerobic energy metabolism of parasites inhabiting hosts by acting as a quinol-fumarate reductase. In Trypanosomes, parasite complex II has been found to have a novel function and structure. Complex II of Trypanosoma cruzi is an unusual supramolecular complex with a heterodimeric iron–sulfur subunit and seven additional non-catalytic subunits. The enzyme shows reduced binding affinities for both substrates and inhibitors. Interestingly, this structural organization is conserved in all trypanosomatids. Since the properties of complex II differ across a wide range of parasites, this complex is a potential target for the development of new chemotherapeutic agents. In this regard, structural information on the target enzyme is essential for the molecular design of drugs. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease. [Copyright &y& Elsevier]

Abstract: The mitochondrial inner membrane harbors the complexes of the respiratory chain and protein translocases required for the import of mitochondrial precursor proteins. These complexes are functionally interdependent, as the import of respiratory chain precursor proteins across and into the inner membrane requires the membrane potential. Vice versa the membrane potential is generated by the proton pumping complexes of the respiratory chain. Besides this basic codependency four different systems for protein import, processing and assembly show further connections to the respiratory chain. The mitochondrial intermembrane space import and assembly machinery oxidizes cysteine residues within the imported precursor proteins and is able to donate the liberated electrons to the respiratory chain. The presequence translocase of the inner membrane physically interacts with the respiratory chain. The mitochondrial processing peptidase is homologous to respiratory chain subunits and the carrier translocase of the inner membrane even shares a subunit with the respiratory chain. In this review we will summarize the import of mitochondrial precursor proteins and highlight these special links between the mitochondrial protein import machinery and the respiratory chain. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease. [Copyright &y& Elsevier]

Abstract: The production of reactive oxygen species by the mitochondrial complex II (succinate:ubiquinone oxidoreductase) recently has gained broad scientific interest. Depending on the (patho)physiological situation, ROS produced or triggered by complex II can have either beneficial or deleterious effects. This ambivalence can be explained mechanistically by the diverse role of complex II on mitochondrial ROS production: it can be a source as well as a suppressor or enhancer of ROS generation by other respiratory chain complexes. Since complex II directly links the respiratory chain to the tricarboxylic acid (TCA) cycle, the TCA-cycle intermediates – especially oxaloacetate that acts as a high affinity endogenous inhibitor – have major impact on complex II-related ROS release. The review relates the diverse effects of complex II activity on the mitochondrial ROS production that have been observed during cardioprotective ischemic or pharmacological preconditioning and the oxidative burst that occurs during ischemia/reperfusion. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease. [Copyright &y& Elsevier]

Abstract: With the arrival of the third millennium, in spite of unprecedented progress in molecular medicine, cancer remains as untamed as ever. The complexity of tumours, dictating the potential response of cancer cells to anti-cancer agents, has been recently highlighted in a landmark paper by Weinberg and Hanahan on hallmarks of cancer [1]. Together with the recently published papers on the complexity of tumours in patients and even within the same tumour (see below), the cure for this pathology seems to be an elusive goal. Indisputably, the strategy ought to be changed, searching for targets that are generally invariant across the landscape of neoplastic diseases. One such target appears to be the mitochondrial complex II (CII) of the electron transfer chain, a recent focus of research. We document and highlight this particularly intriguing target in this review paper and give examples of drugs that use CII as their molecular target. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease. [Copyright &y& Elsevier]

The bioenergetic organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from endosymbiotic bacteria. Their electron transport chains (ETCs) resemble those of free-living bacteria, but were tailored for energy transformation within the host cell. Parallel evolutionary processes in mitochondria and chloroplasts include reductive as well as expansive events: On one hand, bacterial complexes were lost in eukaryotes with a concomitant loss of metabolic flexibility. On the other hand, new subunits have been added to the remaining bacterial complexes, new complexes have been introduced, and elaborate folding patterns of the thylakoid and mitochondrial inner membranes have emerged. Some bacterial pathways were reinvented independently by eukaryotes, such as parallel routes for quinol oxidation or the use of various anaerobic electron acceptors. Multicellular organization and ontogenetic cycles in eukaryotes gave rise to further modifications of the bioenergetic organelles. Besides mitochondria and chloroplasts, eukaryotes have ETCs in other membranes, such as the plasma membrane (PM) redox system, or the cytochrome P450 (CYP) system. These systems have fewer complexes and simpler branching patterns than those in energy-transforming organelles, and they are often adapted to non-bioenergetic functions such as detoxification or cellular defense. [Copyright &y& Elsevier]

To clarify the importance of deleted protein and tRNA genes on the impairment of mitochondrial function, we performed a quantitative analysis of biochemical, genetic and morphological findings in skeletal muscles of 16 patients with single deletions and 5 patients with multiple deletions of mtDNA. Clinically, all patients showed chronic progressive external ophthalmoplegia (CPEO). The size of deletions varied between 2.5 and 9 kb, and heteroplasmy between 31% and 94%. In patients with single deletions, the citrate synthase (CS) activity was nearly doubled. Decreased ratios of pyruvate- and succinate-dependent respiration were detected in fibers of all patients in comparison to controls. Inverse and linear correlations without thresholds were established between heteroplasmy and (i) CS referenced activities of the complexes of respiratory chain, (ii) CS referenced maximal respiratory rates, (iii) and cytochrome-c-oxidase (COX) negative fibers. In patients with single and multiple deletions, all respiratory chain complexes as well as the respiratory rates were decreased to a similar extent. All changes detected in patients with single deletions were independent of deletion size. In one patient, only genes of ND5, ND4L as well as tRNALeu(CUN), tRNASer(AGY), and tRNAHis were deleted. The pronounced decrease in COX activity in this patient points to the high pathological impact of these missing tRNA genes. The activity of nuclear encoded SDH was also significantly decreased in patients, but to a lesser extent. This is an indication of secondary disturbances of mitochondria at CPEO.In conclusion, we have shown that different deletions cause mitochondrial impairments of the same phenotype correlating with heteroplasmy. The missing threshold at the level of mitochondrial function seems to be characteristic for large-scale deletions were tRNA and protein genes are deleted. [Copyright &y& Elsevier]

It is no surprise that the catalytic activity of electron-transport enzymes may be optimised at certain electrochemical potentials in ways that are analogous to observations of pH-rate optima. This property is observed clearly in experiments in which an enzyme is adsorbed on an electrode surface which can supply or receive electrons rapidly and in a highly controlled manner. In such a way, the rate of catalysis can be measured accurately as a function of the potential (driving force) that is applied. In this paper, we draw attention to a few examples in which this property has been observed in enzymes that are associated with membrane-bound respiratory chains, and we discuss its possible origins and implications for in vivo regulation. [Copyright &y& Elsevier]

The homodimeric cyanide-resistant alternative oxidase of plant mitochondria reduces oxygen to water without forming ATP. Arabidopsis thaliana alternative oxidase AOX1a is stimulated by pyruvate or other α-keto acids associating with a regulatory cysteine at position 78, by succinate in a serine-78 mutant, and by site-directed mutation of position 78 to glutamate. The mechanism of activation was explored with additional amino acid substitutions made at Cys-78 in AOX1a, which was functionally expressed in Escherichia coli. Oxidases with positively charged substitutions (Lys and Arg) were insensitive to pyruvate or succinate but were more active than the wild type without pyruvate. Uncharged substitutions (Gln, Leu) produced an inactive enzyme. These results indicate that activation may be due to conformational changes caused by charge repulsion between the dimer subunits and not through a direct role of α-keto acids in catalysis. Oxygen isotope fractionation experiments suggest that the charge of the amino acid at position 78 also affects the entry of oxygen into the active site. Therefore, the N-terminal portion of the protein containing residue 78 can indirectly affect both catalysis at the diiron active site and the path of oxygen to that site. In addition, both positively and negatively substituted alternative oxidases were stimulated by glyoxylate, suggesting the presence of a second activation site, possibly Cys-128. [Copyright &y& Elsevier]