34
Se
Selenium
78.96
Essential:
many Eukarya, Bacteria, Archaea
Beneficial extracellularly: methanogens
Selenium
Major functions in cells: (1)
-Selenoproteins are essential in humans and many other organisms
-Some anaerobes can respire Se(VI)
Environmental and health impacts:
-Se deficiency is endemic in some areas
-Deficiency is associated with various human diseases
-High concentrations are toxic; kill waterfowl
Reduce:
-Substitute Sec-containing enzymes with Cys-containing isozymes (methanogens) (2)
Learn More!
(1) Selenium: An Essential Trace Element
Selenium is an essential trace element for many organisms, including humans, who have a recommended daily dose of 55 ug per day (Goldhaber, 2003). Unlike many trace elements that are incorporated into proteins after translation as an inorganic cofactor, selenium is essential for many biological functions because it is cotranslationally inserted into proteins as the 21st amino acid, selenocysteine (Sec). Insertion of Sec occurs at UGA stop codons within the specific context of mRNAs with a selenocysteine insertion sequence. Most selenoproteins have a single Sec residue, which typically serves a catalytic role as in, for example, Se-containing glutathione peroxidases.
Selenoproteins are found throughout all three domains of life (Labunskyy et al., 2014). There are Sec-containing proteins in 20% of Bacterial and 14% of Archaeal genomes (Kryukov and Gladyshev, 2004). Selenoproteins are essential for human life. For those organisms that lack Sec, the corresponding selenoproteins may still be present but typically have a Cys residue in place of Sec. Substitution of Sec for Cys has occurred many times in evolution, as has the converse.
In addition to the assimilatory role of Se in selenoproteins, some prokaryotes are also capable of dissimilatory respiration of Se. The bacterium Thauera selenatis reduces Se(VI) to Se(IV) (Macy et al., 1993). Later studies using random transposon mutagenesis found a second respiratory Se(VI) reductase in Bacillus selenatarsenatis (Kuroda et al., 2011). A single organism, Bacillus selenitireducens, has also been found to be capable of reducing selenite (Se(IV)) (Wells et al., 2019).
There are many questions that remain unresolved in the field of selenium metabolism. Although Sec utilization emerged early on in earth’s history, it is unknown why it has persisted in certain lineages for 3.7 Gya and has been lost in many other lineages (Wells et al., 2021). A recent hypothesis potentially links selenium assimilation to the evolution of antioxidant defense (Reich and Hondal, 2016); however, many questions in the field still remain.
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(2) Substituting Sec with Cys
Some organisms can substitute Cys when Sec is not available. The methanogen Methanococcus maripaludis encodes 10 Sec proteins and 8 of these are involved in methanogenesis. Despite this abundance of Sec-containing proteins, M. maripaludis is able to grow in the absence of Sec by the expression of alternate enzymes containing Cys in place of Sec. The only exception is the formate dehydrogenase, which requires Se for both isozymes. As a result, inactivation of Sec incorporation prevented growth on formate, but not growth by methanogenesis (Rother et al., 2003). The substitute Cys-containing methanogenesis proteins are transcriptionally regulated by a Se-sensing regulatory protein, HrsM (Sun and Klein, 2004). This LysR family regulator represses expression in the presence of Se. Although the mechanisms are not yet defined, it has also been proposed that M. maripaludis may have a hierarchy for Sec insertion into proteins with formate hydrogenase (which lacks a Cys-containing alternate) given priority under conditions of limited Sec synthetic capacity (Stock et al., 2011).