Adenylyl-sulfate reductase (EC 1.8.99.2) 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.

[4] Dissimilatory and assimilatory pathways both use APS reductases as a metabolic tool to produce a sulfur source and amino acids, respectively.

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.

[5] Sulfur is a vital component in biological life and a key element in amino acids cysteine and methionine.

[6] APS reductase controls the rate limiting step of endogenous sulfur assimilation, which is the process of producing hydrogen sulfide from sulfite.

[6] APS reductase controls the flow of inorganic sulfur to cysteine, which is involved in many biological processes in plants such as growth, development, and responses to biotic and abiotic stresses.

[6] In fact, studies have shown that when cells are starved of sulfur, APS reductase gene expression fluctuates, indicating that when the plants are exposed to metabolic and regulatory stress, APS reductase is likely a crucial enzyme in producing hydrogen sulfide and restoring homeostasis.

[6] Bacteria use APS reductases to engage in assimilatory and dissimilatory sulfate reduction, which make them prime candidates to appear in wastewater treatment environments.

[8] APS reductase does not exist within the proteome of human cells; consequently, these enzymes have become the targets of research for various environmental and medical reasons.

[9] Such inhibitors have also been studied in the context of obtaining oil and gas from reservoirs in order to better control the souring of such products.

APR2, the dominant APS reductase isozyme in the model plant Arabidopsis thaliana, has been implicated in the involvement of selenate tolerance and selenite metabolism.