Sulfite oxidase

Sulfite oxidase is a biologically important enzyme found in all living organisms. The enzyme resides in mitochondria of the cell. Mammals posses large quantities of sulfite oxidase in their liver, kidney, and heart, and very little in their spleen, brain, skeletal muscle, and blood. It is responsible for the oxidation of sulfite to sulfate. This oxidation is the last step in metabolizing sulfur containing compounds. Sulfite Oxidase is a metallo-enzyme which utilizes a molydopterin cofactor and belongs to a super-family of enzymes including DMSO reductase, Xanthine Oxidase, and Nitrite Reductase.

Structure
As a homodimer, sulfite oxidase contains two identical subunits with an N-terminal domain and a C-terminal domain. These two domains are connected by ten amino acids forming a loop. The N-terminal domain has a heme cofactor with three adjacent antiparallel beta sheets and five alpha helices. The C-terminal domain hosts a molybdopterin cofactor that is surrounded by thirteen beta sheets and three alpha helices. The molybdopterin cofactor has a Mo(VI) center, which is bonded to a sulfur from cysteine, an ene-dithiolate from pyranopterin, and two terminal oxygens. It is at this molybdenum center that the catalytic oxidation of sulfite takes place.

Active site and mechanism of sulfite oxidase
The active site of sulfite oxidase contains the molybdopterin cofactor and supports molybdenum in its highest oxidation state, +6 (MoVI). In the enzyme's oxidized state, molybdenum is coordinated by a cysteine thiolate, the dithiolene group of molybdopterin, and two terminal oxygen atoms (oxos). Upon reacting with sulfite, one oxygen atom is transferred to sulfite to produce sulfate, and the molybdenum center is reduced by two electrons to MoIV. Water then displaces sulfate, and the removal of two protons (H+) and two electrons (e-) returns the active site to its original state. A key feature of this oxygen atom transfer enzyme is that the oxygen atom being transferred arises from water, not from dioxygen (O2).

Sulfite oxidase deficiency
Lack of functional sulfite oxidase causes a disease known as sulfite oxidase deficiency. This rare but fatal disease causes neurological disorders, mental retardation, physical deformities, the degradation of the brain, and death. Reasons for the lack of functional sulfite oxidase include a genetic defect that leads to the absence of a molybdopterin cofactor and point mutations in the enzyme. In fact studies done in the Rajagopalan Laboratory at Duke University have pointed to a G473D mutation that impairs dimerization and catalysis in human sulfite oxidase.