ATP sulfurylase molecule. Computer model showing the structure of an ATP sulfurylase (ATPS) enzyme. ATPS catalyses the primary step of sulphate activation within cells. This involves the reaction of inorganic sulphate with ATP (adenosine triphosphate) to form adenosine-5'-phosphosulfate (APS) and pyrophosphate (PPi). It plays a role in the following 3 metabolic pathways: purine metabolism, selenoamino acid metabolism, and sulphur metabolism.