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(1) Tungsten: Tungstoenzymes, replacement for Mo
There are a small set of tungstoenzymes that obligately require W for function. Similar to Mo, W-containing enzyme rely on W bound to the pterin cofactor (tungstopterin) (Milojevic, 2022). The true tungstoenzymes include formate dehydrogenase and aldehyde:ferredoxin-oxidoreductases from Bacteria and Archaea, including methanogens, acetogens, hyperthermophiles, and thermophilic archaea, (L'Vov N et al., 2002). Tungstoenzymes are proposed to be evolutionarily ancient, and perhaps evolved in the anaerobic and sulfidic environment of the Archean, when W was more available than Mo (Hille, 2002). Tungstoenzymes may also play a role in allowing specific metabolic processes to continue when Mo is limiting, an example of elemental substitution.
(2) Substituting Moco and Wco Enzymes
There are several examples of enzyme substitutions of Mo and W as part of a metal-sparing response.
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The sulfate-reducing bacterium Desulfovibrio alaskensis encodes two formate dehydrogenases. One isozyme (W-FDH) requires W, whereas the other can function with either W or Mo (Mo/W-FDH). Growth in the presence of Mo leads to the upregulation of the Mo/W isozyme whereas addition of 10mM W to the medium led to the exclusive synthesis of the W-FDH isozyme. These results suggest that this organism modulates synthesis of these two enzymes in response to metal availability for cofactor synthesis (Mota et al., 2011).
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Other organisms may also have functionally redundant pathways that can utilize either Mo- or W-cofactored enzymes. This has been suggested, for example, in the Archaean Methanosarcina acetivorans, where genome analysis suggests the presence of paralogous gene clusters encoding Mo- and W-dependent forms of formylmethanofuran dehydrogenases (Rohlin and Gunsalus, 2010).