3'-Phosphoadenosine-5'-phosphosulfate (PAPS) is the universal sulfur donor for sulfotransferases. It is synthesized from ATP through two steps. In the first step, one ATP molecule is converted to adenosine-5'-phosphosulfate (APS) by ATP sulfurylase and pyrophosphate (PPi) is generated as a byproduct. In the second step, the APS is converted to PAPS by APS kinase and ADP is generated as a byproduct. To drive the reaction forward, the pyrophosphate can be further degraded to phosphate (Pi) by inorganic pyrophosphatase to eliminate feedback inhibition, and the ADP can be converted back to ATP using pyruvate kinase.