Bacterial 3-mercaptopropionate dioxygenase (MDO) is a mononuclear nonheme iron dioxygenase that catalyzes the O₂-dependent oxidation of 3-mercaptopropioniate (3MPA) to produce 3-sulfinopropionate (3SPA). MDO is a member of the cupin superfamily of proteins and therefore shares a similar active site architecture as mammalian and bacterial cysteine dioxygenases (CDOs). However, unlike CDOs, which exhibit near exclusive specificity for l-cysteine (CYS), MDOs are more flexible with respect to accommodating other thiol-bearing substrates of comparable size to 3MPA. The active site of MDO and CDO share two major structural features. First, they both contain a mononuclear nonheme iron site coordinated by three histidine residues. Since all protein-derived ligands occupy one face of an octahedron, binding of dioxygen and organic substrate occurs at the opposite face of the 3-His facial triad. Second, both enzymes share a conserved sequence of outer Fe-coordination sphere amino acids (Ser, His, and Tyr) positioned adjacent to the iron site (∼ 4 Å). These residues participate in an extended proton relay network which ultimately donates an H-bond to the enzymatic Fe-site. To date, MDOs have only been identified among Gram-negative soil proteobacteria. It is believed that the high concentration of 3MPA present in coastal sediments can be attributed to bacterial catabolism of organosulfur compounds present in saline environments. Indirect evidence also suggests that 3MPA may play a more central role in bacterial sulfur metabolism than currently understood.