Fig. 2  A simplified sulfate assimilation pathway. Inorganic sulfate is converted to adenosine 5'-phosphosulfate (APS) by ATP sulfurylase (EC 2.7.7.4 and/or EC 2.7.7.5, Class I). APS is phosphorylated by APS kinase (EC 2.7.1.25, Class II) to 3'-phosphoadenosine-5'-phosphosulfate (PAPS). PAPS is either transferred to 3'-phosphoadenosine-5'-phosphate (PAP) or reconverted to APS by 3'-phosphoadenosine-5'-phosphatase (EC 3.1.3.7, Class III).