The haem proteins TDO (tryptophan 2,3-dioxygenase) and IDO (indoleamine 2,3-dioxygenase) are specific and powerful oxidation catalysts that insert one molecule of dioxygen into L -tryptophan in the first and rate-limiting step in the kynurenine pathway. Recent crystallographic and biochemical analyses of TDO and IDO have greatly aided our understanding of the mechanisms employed by these enzymes in the binding and activation of dioxygen and tryptophan. In the present paper, we briefly discuss the function, structure and possible catalytic mechanism of these enzymes.

Hydrogen metabolism is closely related to other important metabolic and energetic processes of bacterial cells, such as photosynthesis, anaerobic respiration and sulphur metabolism. Even small environmental changes influence these networks through different regulatory systems. The presence or absence of oxygen is one of the most important signals; how the cascades evolved to transmit this signal in different bacteria is summarized. In many instances, hydrogen is released only under anoxic conditions, because of bioenergetic considerations. Most [NiFe] hydrogenases are inactivated by oxygen, but many of them can be re-activated under reducing conditions. In addition to direct inactivation of the hydrogenases, oxygen can also regulate their expression. The global regulatory systems [FNR ( f umarate and n itrate r eduction regulator), ArcAB ( a erobic r espiratory c ontrol) and RegAB], which respond to alterations in oxygen content and redox conditions of the environment, have an important role in hydrogenase regulation of several bacteria. FNR-like proteins were shown to be important for the regulation of hydrogenases in Escherichia coli , Thiocapsa roseopersicina and Rhizobium leguminosarum , whereas RegA protein modulates the expression of hupSL genes in Rhodobacter capsulatus .

Agitated bacterial tank bioleaching reactors are currently sparged with air to satisfy both oxygen and CO 2 requirements of microbial cells. Under high-sulphide loading conditions, as is the case with high-grade metal sulphide concentrates, the microbial and chemical demand for oxygen is significantly increased during the bioleaching process. Sparging with enriched oxygen gas may offer an alternative process option to increased agitation and sparged aeration, to overcome the mass transfer difficulties at elevated temperatures where thermophilic Archaea, rather than Bacteria, are used. In the case of air sparging, the DO (dissolved oxygen) concentration in tank reactors could not be increased to a point where it would become inhibitory due to the limited oxygen content of air (20.9% O 2 ). The use of enriched oxygen in such reactors at large scale does, however, pose its own set of process risks. The first aim of this investigation was, therefore, to determine the effects of various DO concentrations, in both the limiting and inhibitory ranges, on the microbial activity of Sulfolobus sp. U40813, a typical thermophilic mineral-leaching archaeon. Secondly, the effect of CO 2 concentration on the rate of ferrous iron oxidation was investigated. Both the oxygen and CO 2 kinetics were examined in controlled batch cultures at 78°C, using ferrous sulphate and potassium tetrathionate as energy sources. The optimal DO concentration for iron oxidation was found to be between 1.5 and 4.1 mg·l −1 . The use of elevated DO concentrations (above 4.1 mg·l −1 ) inhibited the ferrous oxidation rates. The optimal gas CO 2 concentration for ferrous iron oxidation was found to be in the range 7–17% (v/v). The iron oxidation rates were, however, severely limited at CO 2 concentrations less than 7%, indicating that the CO 2 supply was limiting in this range and inhibited the microbial growth rate.

This article reviews the role of free radicals in causing oxidative stress during exercise. High intensity exercise induces oxidative stress and although there is no evidence that this affects sporting performance in the short term, it may have longer term health consequences. The mechanisms of exercise-induced oxidative stress are not well understood. Mitochondria are sometimes considered to be the main source of free radicals, but in vitro studies suggest they may play a more minor role than was first thought. There is a growing acceptance of the importance of haem proteins in inducing oxidative stress. The release of metmyoglobin from damaged muscle is known to cause renal failure in exercise rhabdomyolysis. Furthermore, levels of methaemoglobin increase during high intensity exercise, while levels of antioxidants, such as reduced glutathione, decrease. We suggest that the free-radical-mediated damage caused by the interaction of metmyoglobin and methaemoglobin with peroxides may be an important source of oxidative stress during exercise.