text: Sulfate reduction predominantly takes place in the leaf chloroplasts. Here, the reduction of sulfate to sulfide occurs in three steps. Sulfate needs to be activated to adenosine 5'-phosphosulfate (APS) prior to its reduction to sulfite.
  The activation of sulfate is catalyzed by ATP sulfurylase, which affinity for sulfate is rather low (Km approximately 1 mM) and the in situ sulfate concentration in the chloroplast is most likely one of the limiting/regulatory steps in sulfur reduction. Subsequently, APS is reduced to sulfite, catalyzed by APS reductase with likely glutathione as reductant.
  The latter reaction is assumed to be one of the primary regulation points in the sulfate reduction, since the activity of APS reductase is the lowest of the enzymes of the sulfate reduction pathway and it has a fast turnover rate. Sulfite is with high affinity reduced by sulfite reductase to sulfide with ferredoxin as a reductant. The remaining sulfate in plant tissue is transferred into the vacuole. The remobilization and redistribution of the vacuolar sulfate reserves appear to be rather slow and sulfur-deficient plants may still contain detectable levels of sulfate.