Requiring thioredoxin as an electron donor, phosphoadenosine phosphosulphate reductase (CysH) catalyses the reduction of phosphoadenosine phosphosulphate (PAPS) to sulphite and phosphoadenosine phosphate (PAP). It is part of the pathway that synthesises sulfite from sulfate. This pathway for introducing sulfur into biological molecules which utilises phosphoadenylyl reductase is only present in prototrophic organsisms.