The human phosphoadenosine-phosphosulfate synthase (PAPS) system is a bifunctional enzyme: ATP sulfurylase, which catalyzes the formation of adenosine 5'-phosphosulfate (APS) from ATP and inorganic sulfate and the second step is catalyzed by the APS kinase portion of 3'-phosphoadenosine 5'-phosphosulfate (PAPS) synthase, which involves the formation of PAPS from enzyme bound APS and ATP.