Routes of Sulfate Assimilation

<p>Inorganic sulfate is adenylated by ATP sulfurylase (reaction a) to form APS. Higher plants and the majority of sulfate reducing bacteria use APS as their source of sulfite (reactions c₁ → d → e). In some organisms, APS kinase (reaction b) phosphorylates APS at the 3′-hydroxyl group to form PAPS for use as a sulfate donor for sulfotransferases or as a source of sulfite. The lower pathway of sulfate reduction (reactions c₂ → d → e) is utilized by γ-proteobacteria such as <i>E. coli</i> and some fungi. Depending on the organism, APS or PAPS is reduced to sulfite by APS reductase (reaction c₁) and PAPS reductase (reaction c₂), respectively. Sulfite is reduced to sulfide by sulfite reductase (reaction d) and incorporated into cysteine by <i>O</i>-acetylserine-(thiol) lyase (reaction e). Important metabolites such as methionine and coenzyme A are, in turn, synthesized from cysteine.