Figure 1. Routes of Sulfate Assimilation.

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 E. coli 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 O-acetylserine-(thiol) lyase (reaction e). Important metabolites such as methionine and coenzyme A are, in turn, synthesized from cysteine.