(Inorganic sulfate occurs abundantly on Earth; terrestrial organisms must use sulfate assimilation to convert it to sulfide.)
5-alpha reductase | HMG-CoA reductase | Rubredoxin—NAD(P)+ reductase | Ribonucleoside-triphosphate reductase | GMP reductase | Flavanone 4-reductase | D-xylose reductase |