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Obligate anaerobes are periodically exposed to oxygen, and it has been conjectured that on such occasions their low-potential biochemistry will predispose them to rapid ROS formation. We sought to identify scavenging enzymes that might protect the anaerobe Bacteroides thetaiotaomicron from the H2 O2 that would be formed. Genetic analysis of eight candidate enzymes revealed that four of these scavenge H2 O2  in vivo: rubrerythrins 1 and 2, AhpCF, and catalase E. The rubrerythrins served as key peroxidases under anoxic conditions. However, they quickly lost activity upon aeration, and AhpCF and catalase were induced to compensate. The AhpCF is an NADH peroxidase that effectively degraded low micromolar levels of H2 O2 , while the catalytic cycle of catalase enabled it to quickly degrade higher concentrations that might arise from exogenous sources. Using a non-scavenging mutant we verified that endogenous H2 O2 formation was much higher in aerated B. thetaiotaomicron than in Escherichia coli. Indeed, the OxyR stress response to H2 O2 was induced when B. thetaiotaomicron was aerated, and in that circumstance this response was necessary to forestall cell death. Thus aeration is a serious threat for this obligate anaerobe, and to cope it employs a set of defences that includes a repertoire of complementary scavenging enzymes., (© 2013 John Wiley & Sons Ltd.)

Hemerythrins and hemocyanins are respiratory proteins present in some of the most ecologically diverse animal lineages; however, the precise evolutionary history of their enzymatic domains (hemerythrin, hemocyanin M, and tyrosinase) is still not well understood. We survey a wide dataset of prokaryote and eukaryote genomes and RNAseq data to reconstruct the phylogenetic origins of these proteins. We identify new species with hemerythrin, hemocyanin M, and tyrosinase domains in their genomes, particularly within animals, and demonstrate that the current distribution of respiratory proteins is due to several events of lateral gene transfer and/or massive gene loss. We conclude that the last common metazoan ancestor had at least two hemerythrin domains, one hemocyanin M domain, and six tyrosinase domains. The patchy distribution of these proteins among animal lineages can be partially explained by physiological adaptations, making these genes good targets for investigations into the interplay between genomic evolution and physiological constraints.

The methyl-accepting chemotaxis protein, DcrH, from the anaerobic sulfate-reducing bacterium, Desulfovibrio vulgaris (Hildenborough), has a hemerythrin-like domain, DcrH-Hr, at its C terminus. DcrH-Hr was previously shown to contain a diiron site that binds O2, suggesting an O2-sensing function. X-ray crystal structures of diferric (met-), azido-diferric (azidomet-), and diferrous (deoxy-) DcrH-Hr reveal a "substrate tunnel" distinct from that in invertebrate hemerythrins. This tunnel is proposed to facilitate the rapid autoxidation of oxy-DcrH-Hr and suggests that sensing is triggered by O2 binding and subsequent oxidation of the diferrous active site. The N-terminal loop of DcrH-Hr is highly ordered in both met- and azidomet-DcrH-Hr but is disordered in deoxy-DcrH-Hr. These redox-dependent conformational differences presumably transduce the sensory signal of DcrH-Hr to the neighboring methylation domain in the full-length receptor. Given the putative cytoplasmic localization of its Hr-like O2-sensing domain, DcrH is proposed to serve a role in negative aerotaxis (anaerotaxis).

Oxygen-transport proteins are multisubunit, circulating molecules that provide an efficient supply of oxygen to metabolically active metazoans. Hemoglobins, hemerythrins and hemocyanins have evolved in both structural and functional diversity and exhibit functional repertoires beyond that of simple, monomeric tissue myoglobins. Their phylogenetic distribution is intriguing, especially with respect to those organisms that express more than one type of oxygen-transport protein. An animal can modify the delivery of oxygen to its tissues by varying the rate of synthesis of these proteins or by selective expression of individual subunits and/or molecules. Changes in levels of allosteric modifiers that affect the protein's oxygenation properties will also modify oxygen delivery; some organisms have more ability than others to control concentrations of modulators. Hemoglobins have assumed functions in addition to oxygen transport, while hemocyanins have diversified through multiple gene duplications and functional specializations. Understanding the mechanisms of regulation of expression, synthesis and modulator levels is a key focus of current investigations.

The single sulfhydryl group (Cys-50) of the methemerythrin from Phascolosoma gouldii reacts with 5,5'dithiobis(2-nitrobenzoic acid) to form a mixed disulfide. The pulse radiolytically generated formate radical reduces this mixed disulfide to its radical anion. In turn, the disulfide radical anion reduces the protein two-iron center over a nominal distance of 13 A. The rate constant for this intramolecular electron transfer is approximately 15 s-1 at room temperature, pH 7. Between the two redox centers there is an equilibrium driving force of 0.78 V, measured by differential pulsed polarography.

The distribution of the O2 carrying proteins suggests that the original transport system was a hemoglobin similar to the alpha-chain of hemoglobin A and packaged in a nucleated red blood cell. These molecules, which occur in large open fluid compartments, function as O2 stores for regular periods of hypoxia as well as carriers between sites of gas exchange. When the closed circulatory system first arose, the red blood cell was abandoned in favor of extracellular heme proteins, and the O2 storage function became less important. Alternative O2 carriers, hemerythrins, appear in the blood at about the same phylogenetic level as the intracellular hemoglobins, and their respiratory functions appear to be similar. The presence of hemoglobins instead of hemerythrins in the vertebrates may be an evolutionary accident. Still other O2 carriers, hemocyanins, arose separately in two specialized groups that left no descendants. Their O2 binding has all the adaptive features of vertebrate hemoglobin O2 binding, with unique features also. The respiratory function of the hemocyanins is largely limited to O2 transport, which makes a far greater contribution to aerobic metabolism than the O2 carriers found in simpler systems.

Oxygen consumption, anaerobic metabolism, and oxygen supply of inner tissues were analysed in Sipunculus nudus at different oxygen tensions. Oxygen consumption, energy expenditure, and the PO2 in the coelomic fluid decreased linearly with declining ambient PO2. Below a certain range of PO2, which was a function of the size of the animals, the rate of oxygen consumption deviated progressively from the linear PO2/MO2 function. In the same range of ambient PO2 the coelomic PO2 levelled off. Anaerobic glycolysis, phosphagen degradation, and the succinate-propionate pathway became apparent with concentration changes of anaerobic metabolites first occurring in inner tissues. In extension of the conventional definition (Prosser, 1973; Dejours, 1981) the term critical PO2 (Pc) is applied to the oxyconforming Sipunculus nudus. The Pc is redefined as the steady-state PO2 below which environmental oxygen availability becomes insufficient for complete aerobic metabolism (as indicated by the onset of anaerobic energy production). It is discussed to be closely linked to the oxygen supply of inner tissues. This redefined critical PO2 is shifted to higher partial pressures with increasing size of the animals because of the diffusion distance related decrease in coelomic PO2. Accordingly, with decline of ambient PO2, oxygen starts to be released from haemerythrin at higher ambient PO2 values in larger animals. The pigment, which is likely to function as an oxygen store, defers anaerobiosis and, thereby, supports compensation of a higher Pc in large individuals by means of an increased haematocrit. The Pc is discussed as crucial factor for survival of individual animals in intertidal oxygen-depleted environments.