SO42- is activated by the attachment of a phosphate of ATP by ATP sulfurylase, forming adenosine phosphosulfate (APS). There are two pathways: Dissimilative sulfate reduction: APS is reduced directly to SO32- by the enzyme APS reductase with the release of AMP. Assimilate sulfate reduction: A second phosphate is added to APS, forming phosphoadenosine phosphosulfate (PAPS). PAPS is reduced to SO32-, oxidizing NADPH to NADP+. SO32- is reduced to H2S by sulfite reductase