Microbial reduction of selenium oxyanions: energy-yielding and detoxification reactions

Selenium (Se), a semi-metallic chemical element in the oxygen group (group 16 [VIa]) of the periodic table, can be beneficial – even essential in some instances – for microbes and animals, including humans, when present at a suitable concentration, whereas no essential Se requirement has been shown for higher plants (Lenz & Lens, 2009; Winkel et al., 2015). Se is an essential trace element required for the biosynthesis of seleno-amino acids such as selenocysteine (Se-Cys) (Bock et al., 1991; Gromer et al., 2005) and selenomethionine (Se-Met, the major dietary form) (Schrauzer, 2000). These are potent antioxidants as well as a source of Se for the synthesis of Se-dependent antioxidant and repair proteins such as glutathione peroxidases, thioredoxin reductases, and methionine sulfoxide reductases (Flohe et al., 1973; Kim & Gladyshev, 2007; Mustacich & Powis, 2000; Zoidis et al., 2018). Multiple selenoproteins have been identified in eukaryotes, ranging from yeasts (Tastet et al., 2008) to humans (Papp et al., 2018), but Se is also found in prokaryotic proteins such as formate dehydrogenase from Methanococcus jannaschii (Jones et al., 1983) formylmethanofuran dehydrogenase from Methanopyrus kandleri (Vorholt et al., 1997), and thiol/disulfide oxidoreductase from Geobacter sulfurreducens (Kryukov & Gladyshev, 2004).