 In eukaryotic cells the activated sulfate donor is 3'-phosphoadenosine-5'-phosphosulfate (PAPS) (Figure 1).PAPS is synthesized in the cytosol from ATP and sulfate through the sequential action of ATP sulfurylase and APS kinase.ATP sulfurylase first generates adenosine-5'-phosphosulfate (APS) and then APS kinase transfers a phosphate from ATP to APS to create PAPS.