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Storage of the vital metal tungsten in a dominant SCFA-producing human gut microbe Eubacterium limosum and implications for other gut microbes

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The authors identify Tub, a small protein that stores tungstate in Eubacterium limosum, a human gut bacterium that makes short-chain fatty acids from lactate using tungsten enzymes. Tub binds tungstate very tightly and assembles into a hexamer that holds about eight tungstate ions. Gene analysis suggests similar tungstate-binding proteins are widespread across human gut microbes.

Key numbers

  • Purified recombinant Tub held 7.8 +/- 0.1 g-atoms of tungsten per hexamer, equivalent to 1.3 tungsten per monomer (Results, Fig. 1B).
  • Isothermal titration gave an estimated association constant of 1.00 pM with a saturating tungsten to Tub ratio of 1.44 +/- 0.04 to 1 (Results; Fig. S4).
  • Cultures were grown with 100 nM tungstate and 100 nM molybdate (Results; Methods).
  • Background statements cited by the authors from earlier work: tungstate in the human gut is typically well below 100 nM, and lactate-grown cells took up about 60-fold more tungsten (6.0 +/- 0.1 micromol per g protein) than molybdenum (Introduction).

Methods (brief)

Cell extracts were fractionated by anion exchange and size exclusion chromatography with metal tracking by ICP-MS. Recombinant Tub was produced in Escherichia coli and characterized by native gel shift assays, isothermal titration calorimetry, thermal stability tests and X-ray crystallography. Genomic co-occurrence analysis surveyed related binding proteins in gut microbiome data.

Evidence fitness

Supports mechanistic statements on how a gut bacterium stores tungsten for enzymes involved in short-chain fatty acid production. It does not measure tungsten in foods or people and does not show that dietary tungsten changes gut function.

Limitations

Single organism in culture at experimenter-set tungsten levels. Gut tungstate levels and the uptake ratio are cited from earlier work rather than measured here. The authors note that the binding parameters of the two sites could not be resolved individually.

Update history

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