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Sulfur Assimilation01:20

Sulfur Assimilation

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Sulfur is an essential element in biological systems, contributing to synthesizing key biomolecules, including amino acids such as cysteine and methionine, and cofactors such as coenzyme A and biotin. Microorganisms primarily assimilate sulfur as sulfate (SO₄²⁻) from the environment, which must undergo a series of biochemical transformations before it can be incorporated into cellular components. As sulfate is highly oxidized, it must undergo assimilatory sulfate reduction to...
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Preparation of Amines: Reduction of Oximes and Nitro Compounds01:29

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Oximes can be reduced to primary amines using catalytic hydrogenation, hydride reduction, or sodium metal reduction. The reduction of aliphatic and aromatic nitro compounds to primary amines takes place by either catalytic hydrogenation or by using active metals like Fe, Zn, and Sn in the presence of an acid.
Though catalytic hydrogenation can reduce nitrobenzenes, the reduction is nonselective in the presence of other functional groups. For instance, if nitrobenzene contains an aldehyde group,...
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Preparation of Amines: Reduction of Amides and Nitriles01:13

Preparation of Amines: Reduction of Amides and Nitriles

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Nitriles can be reduced to primary amines using reducing agents like lithium aluminum hydride or catalytic hydrogenation. The reduction introduces an amino group with an extra carbon in the skeleton. Nitriles are formed from the reaction between alkyl halides and sodium cyanide through the SN2 mechanism. Primary alkyl halides are the preferred substrates to prepare nitriles.
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Nitriles to Amines: LiAlH4 Reduction00:55

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Nitriles are reduced to amines in the presence of strong reducing agents like lithium aluminum hydride through a typical nucleophilic acyl substitution. The reaction requires two equivalents of the reducing agent. The reducing agent acts as a source of hydride ions.
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Inorganic Nitrogen Assimilation01:22

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Nitrogen is an essential element in biological systems, forming a crucial component of proteins, nucleic acids, and other cellular constituents. Many bacteria and archaea acquire nitrogen in the form of nitrate (NO₃⁻) or ammonia (NH₃), which are then assimilated into biomolecules through specific enzymatic pathways.Assimilatory Nitrate ReductionWhen nitrate enters the cell, it undergoes a two-step reduction process known as assimilatory nitrate reduction. Initially, the enzyme...
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Microbes and the Sulfur Cycle01:29

Microbes and the Sulfur Cycle

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Sulfur is a vital element in Earth's biogeochemical systems. It transitions through various inorganic states, including sulfate (SO₄²⁻), elemental sulfur (S⁰), and sulfide (S²⁻). Abiotic and biological mechanisms across oxic and anoxic environments intricately mediate these transformations. Sulfate, the most oxidized form of sulfur, is predominantly stored in rocks, marine sediments, and oceanic waters, acting as a long-term reservoir in the global sulfur...
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Measurement of the Potential Rates of Dissimilatory Nitrate Reduction to Ammonium Based on 14NH4+/15NH4+ Analyses via Sequential Conversion to N2O
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Sulfite Is Not Required for N2 Reduction Catalyzed by Mo-Nitrogenase.

Zhi-Yong Yang1, Dmitriy A Lukoyanov2, Ana Pérez-González3

  • 1Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States.

Journal of the American Chemical Society
|April 17, 2026
PubMed
Summary
This summary is machine-generated.

Sulfite is not required for Mo-nitrogenase to convert nitrogen (N2) into ammonia (NH3). Studies show the enzyme functions effectively without sulfite, both in vitro and in vivo, challenging previous hypotheses.

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Area of Science:

  • Biochemistry
  • Enzymology
  • Nitrogen Fixation

Background:

  • Mo-nitrogenase catalyzes N2 to NH3 conversion via the FeMo-cofactor.
  • A recent hypothesis proposed sulfite is essential for NH3 release and cofactor regeneration.

Purpose of the Study:

  • To investigate the necessity of sulfite in Mo-nitrogenase activity.
  • To test the hypothesis that sulfite is required for N2 reduction and cofactor regeneration.

Main Methods:

  • Turnover studies of Mo-nitrogenase under sulfite-free conditions.
  • Using reduced viologen as a reductant and preparing proteins without dithionite or sulfite.
  • Electron paramagnetic resonance (EPR) spectroscopy analysis.
  • Assessing diazotrophic growth in *Azotobacter vinelandii*.

Main Results:

  • Mo-nitrogenase efficiently catalyzed N2 and proton reduction without sulfite.
  • Steady-state turnover and H2-formed/N2-reduced ratios were comparable to dithionite-supported reactions.
  • EPR confirmed FeMo-cofactor regeneration in the absence of sulfite.
  • *Azotobacter vinelandii* growth was unaffected by bypassing sulfite formation.

Conclusions:

  • Sulfite is not required for Mo-nitrogenase-catalyzed N2 reduction.
  • The enzyme functions effectively both *in vitro* and *in vivo* without sulfite.
  • This finding refutes the proposed essential role of sulfite in the catalytic cycle.