Before sulfate can act as oxidant in sulfate respiration or assimilation it needs to be „activated“ by ATP to become Adenosin 5’-Phosphosulfate (APS) or Phosphoadenosin 5’-Phosphosulfate (PAPS). APS is the electron acceptor for dissimilative sulfate reduction reactions, while the reduction of sulfate-S in assimilation processes proceeds via PAPS. An ATP-Kinase and an ATP-Sulfurylase are needed to transform sulfate into its activated forms.

Exercise 1: Which reaction is catalyzed by which enzyme ? Mark the enzymes in the rectangular boxes of the figure.
Exercise 2: Assign oxidation states to the sulfur atoms in sulfate, APS and PAPS in the circular fields of the figure.