Adenylyl-sulfate reductase () is an enzyme that catalyzes the chemical reaction of the reduction of adenylyl-sulfate/adenosine-5'-phosphosulfate (APS) to sulfite through the use of an electron donor cofactor. The products of the reaction are AMP and sulfite, as well as an oxidized electron donor cofactor. Nomenclature This enzyme belongs to the family of oxidoreductases, specifically those acting on a sulfur group of donors with other acceptors. The systematic name of this enzyme class is AMP, sulfite:acceptor oxidoreductase (adenosine-5'-phosphosulfate-forming). Other names in common use include adenosine phosphosulfate reductase, adenosine 5'-phosphosulfate reductase, APS-reductase, APS reductase, AMP, sulfite:(acceptor) oxidoreductase, and (adenosine-5'-phosphosulfate-forming). This enzyme participates in selenium metabolism and sulfur metabolism. Mechanism APS reductase catalyzes the reversible transformation of APS to sulfite and AMP, which is the rate determining step of the overall reaction. The reaction catalyzed by APS reductase is as follows: Sulfate has to be activated to APS by ATP sulfurylase at the expense of one ATP, hence this reaction requires an input of energy. The reaction above occurs in a strictly anaerobic environment. The two electrons come from a reduced cofactor, in this case reduced FAD. The forward direction requires one AMP molecule; however, research suggests that the reverse reaction requires two AMP molecules (one acting on the substrate and one inhibiting the forward reaction). The reversible reaction occurs when AMP binds to the Arg317 residue, changing the confirmation of Arg317 and APS reductase as a whole, which provides the thermodynamic driving force to go in the reverse direction. APS reductases are involved in both assimilatory and dissimilatory sulfate reduction. Dissimilatory sulfate reduction takes sulfate and transforms it into sulfide, a sulfur source that can be distributed throughout the body. Assimilatory sulfate reduction takes sulfate and turns it into cysteine. Dissimilatory and assimilatory pathways both use APS reductases as a metabolic tool to produce a sulfur source and amino acids, respectively. Structure As of late 2014, 6 structures have been solved for this class of enzymes, with PDB accession codes , , , , , and . The monomer of the enzyme consists of a mix of α-helices and β-sheets (both parallel and antiparallel). The protein cofactor thioredoxin can provide the required reducing equivalents for the reaction in the form of two cysteine residues, which are ultimately oxidized to a disulfide bond. The base active form of APS reductase appears to be a heterodimer, as seen in plants. In both bacteria and