plants, two heterodimers tend to form together and produce a heterotetramer. The active site cleft in bacterial APS reductase has a few key elements. Residue sequences that appear to be necessary for catalysis are the P-loop (residues 60-66), the Arg-loop (residues 162-173), and the LDTG motif (residues 85-88). The P-loop, or phosphate-binding loop, is an especially important consecutive sequence of resides which aids in the recognition of the phosphate group in APS and, as a result, influences the substrate specificity for APS reductase. The C-terminal Cys256 is also catalytically essential, and seems to have a role in changing the conformation of the enzyme during catalysis. One notable chemical motif that distinguishes APS reductase from the related 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase is the presence of a conserved cysteine motif, CC-X~80-CXXC, which occurs in addition to the universally conserved catalytic cysteine residue. This motif is correlated with the presence of a [4Fe-4S] cluster; therefore, these iron-sulfur clusters are not present within PAPS reductase. When the iron-sulfur cluster is present, it is required for catalytic activity and coordinated to the four cysteine residues in the conserved motif on the other side of the active site cleft. Function Sulfur is a vital component in biological life and a key element in amino acids cysteine and methionine. APS reductase controls the rate limiting step of endogenous sulfur assimilation, which is the process of producing hydrogen sulfide from sulfite. Hydrogen sulfite is one of the major sources of sulfur in plants. 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. 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. Bacteria use APS reductases to engage in assimilatory and dissimilatory sulfate reduction, which make them prime candidates to appear in wastewater treatment environments. Biofoulants can contain a number of sulfate reducing bacteria, and studies have shown that if wastewater plants are left untreated sulfate levels will decrease. These studies have further solidified APS reductase’s crucial role in the global sulfur cycle by giving organisms another unique way to obtain sulfur when it's unavailable. Clinical significance APS reductase does not exist within the proteome