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Microbial anaerobes are exposed in the natural environment and in their hosts, even if transiently, to fluctuating concentrations of oxygen and its derived reactive species, which pose a considerable threat to their anoxygenic lifestyle. To counteract these stressful conditions, they contain a multifaceted array of detoxifying systems that, in conjugation with cellular repairing mechanisms and in close crosstalk with metal homeostasis, allow them to survive in the presence of O 2 and reactive oxygen species. Some of these systems are shared with aerobes, but two families of enzymes emerged more recently that, although not restricted to anaerobes, are predominant in anaerobic microbes. These are the iron-containing superoxide reductases, and the flavodiiron proteins, endowed with O 2 and/or NO reductase activities, which are the subject of this Review. A detailed account of their physicochemical, physiological and molecular mechanisms will be presented, highlighting their unique properties in allowing survival of anaerobes in oxidative stress conditions, and comparing their properties with the most well-known detoxifying systems., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Flavodiiron proteins (FDPs) are enzymes with a minimal core of two domains: a metallo-β-lactamase-like, harbouring a diiron center, and a flavodoxin, FMN containing, domains. FDPs are O 2 or NO reducing enzymes; for many pathogens, they help mitigate the NO produced by the immune system of the host, and aid survival during fluctuating concentrations concentrations of oxygen. FDPs have a mosaic structure, being predicted to contain multiple extra domains. Clostridium difficile, a threatening human pathogen, encodes two FDPs: one with the two canonical domains, and another with a larger polypeptide chain of 843 amino acids, CD1623, with two extra domains, predicted to be a short-rubredoxin-like and an NAD(P)H:rubredoxin oxidoreductase. This multi-domain protein is the most complex FDP characterized thus far. Each of the predicted domains was characterized and the presence of the predicted cofactors confirmed by biochemical and spectroscopic analysis. Results show that this protein operates as a standalone FDP, receiving electrons directly from NADH, and reducing oxygen to water, precluding the need for extra partners. CD1623 displayed negligible NO reductase activity, and is thus considered an oxygen selective FDP, that may contribute to the survival of C. difficile in the human gut and in the environment.

The increasing dependency on fossil fuels has driven researchers to seek for alternative energy sources. Renewable energy sources such as sunlight, wind, or water are the most common. However, since the 1990s, other sources for energy production have been studied. The use of microorganisms such as bacteria or archaea to produce energy is currently in great progress. These present several advantages even when compared with other renewable energy sources. Besides the energy production, they are also involved in bioremediation such as the removal of heavy metal contaminants from soils or wastewaters. Several research groups have demonstrated that these organisms are able to interact with electrodes via heme and non-heme iron proteins. Therefore, the role of iron as well as iron metabolism in these species must be of enormous relevance. Recently, the influence of cellular iron regulation by Fur in the Geobacter sulfurreducens growth and ability to produce energy was demonstrated. In this review, we aim to briefly describe the most relevant proteins involved in the iron metabolism of bacteria and archaea and relate them and their biological function with the ability of selected organisms to produce energy.

A gene encoding Bfr (bacterioferritin) was identified and isolated from the genome of Desulfovibrio vulgaris cells, and overexpressed in Escherichia coli. In vitro, H(2)O(2) oxidizes Fe(2+) ions at much higher reaction rates than O(2). The H(2)O(2) oxidation of two Fe(2+) ions was proven by Mössbauer spectroscopy of rapid freeze-quenched samples. On the basis of the Mössbauer parameters of the intermediate species we propose that D. vulgaris Bfr follows a mineralization mechanism similar to the one reported for vertebrate H-type ferritins subunits, in which a diferrous centre at the ferroxidase site is oxidized to diferric intermediate species, that are subsequently translocated into the inner nanocavity. D. vulgaris recombinant Bfr oxidizes and stores up to 600 iron atoms per protein. This Bfr is able to bind DNA and protect it against hydroxyl radical and DNase deleterious effects. The use of H(2)O(2) as an oxidant, combined with the DNA binding and protection activities, seems to indicate a DPS (DNA-binding protein from starved cells)-like role for D. vulgaris Bfr.

The isolation and characterization of a new metalloprotein containing Cu and Fe atoms is reported. The as-isolated Cu-Fe protein shows an UV-visible spectrum with absorption bands at 320 nm, 409 nm and 615 nm. Molecular mass of the native protein along with denaturating electrophoresis and mass spectrometry data show that this protein is a multimer consisting of 14+/-1 subunits of 15254.3+/-7.6 Da. Mössbauer spectroscopy data of the as-isolated Cu-Fe protein is consistent with the presence of [2Fe-2S](2+) centers. Data interpretation of the dithionite reduced protein suggest that the metallic cluster could be constituted by two ferromagnetically coupled [2Fe-2S](+) spin delocalized pairs. The biochemical properties of the Cu-Fe protein are similar to the recently reported molybdenum resistance associated protein from Desulfovibrio, D. alaskensis. Furthermore, a BLAST search from the DNA deduced amino acid sequence shows that the Cu-Fe protein has homology with proteins annotated as zinc resistance associated proteins from Desulfovibrio, D. alaskensis, D. vulgaris Hildenborough, D. piger ATCC 29098. These facts suggest a possible role of the Cu-Fe protein in metal tolerance.