Adenosine-5'-phosphosulfate reductase (APSR) is an iron sulfur protein that catalyzes the reduction of adenosine-5'-phosphosulfate (APS) to sulfite. APSR coordinates to a [4Fe-4S] cluster via a conserved CC-X-similar to 80-CXXC motif; and the cluster is essential for catalysis. Despite extensive functional, structural, and spectroscopic studies, the exact role of the iron sulfur cluster in APS reduction remains unknown. To gain an understanding into the role of the cluster, density functional theory (DFT) analysis and extended X-ray fine structure spectroscopy (EXAFS) have been performed to reveal insights into the coordination, geometry, and electrostatics of the [4Fe-4S] cluster. X-ray absorption near-edge structure (XANES) data confirms that the cluster is in the [4Fe-4S](2+) state in both native and substrate-bound APSR while EXAFS data recorded at similar to 0.1 angstrom resolution indicates that there is no significant change in the structure of the [4Fe-4S] cluster between the native and substrate-bound forms of the protein. On the other hand, DFT calculations provide an insight into the subtle differences between the geometry of the cluster in 1 native and APS-bound forms of APSR A comparison between models with and without the tandem cysteine pair coordination of the cluster suggests a role for the unique coordination in facilitating a compact geometric structure and "fine-tuning" the electronic structure to prevent reduction of the cluster. Further, calculations using models in which residue Lys144 is mutated to Ala confirm the finding that Lys144 serves as a crucial link in the interactions involving the [4Fe-4S] cluster and APS.