Sulfite Oxidase

Sulfite oxidase (EC 1.8.3.1) is a homodimeric metallo-enzyme localized in the intermembrane space of mitochondria of all eukaryotes. It is a member of the cytochromes b5 and also belongs to the enzyme super-family of molybdenum oxotransferases. Each subunit of sulfite oxidase consists of a heme group and a molybdopterin-binding domain. The molybdopertin cofactor is critical to the enzyme since the molybdenum center serves as the active site and guarantees electron transport. Sulfite oxidase catalyzes the two-electron oxidization of harmful sulfite to sulfate with oxygen and/or heme-coordinated iron ions as the final electron acceptor, which is the last step in the oxidative metabolism of sulfur-containing compounds like sulfur amino acids cysteine and methionine. The generated sulfate is finally excreted. The produced electrons during this procedure are transferred to the electron transport chain via cytochrome c, allowing the generation of ATP through oxidative phosphorylation. In mammals, sulfite oxidases are actively expressed in kidney, liver, and heart, and very little can be detected in brain, spleen, blood, and skeletal muscle. Point mutations in sulfite oxidase and defects in the biosynthesis of molybdenum cofactor can ultimately lead to a lethal genetic disease, sulfite oxidase deficiency.

Structure

Sulfite oxidase isolated from different sources has molecular weight in the range 101-110 kDa. As a homodimer, sulfite oxidase is composed of two identical subunits with an N-terminal domain and a C-terminal domain, which are connected by a loop consisting of ten amino acids. The smaller N-terminal domain contains a b5 cytochrome cofactor with five alpha helices and three adjacent antiparallel beta sheets. The larger C-terminal domain harbors a molybdopterin cofactor that is encompassed by three alpha helices and thirteen beta sheets. The molybdopterin cofactor provides a molybdenum center that is bonded to a sulfur atom from cysteine, an ene-dithiolate from pyranopterin, and two terminal oxygens. The catalytic oxidation of sulfite takes place at the molybdenum center.

Figure 1. Three Dimensional Structure of sulfite oxidase.

Action Mechanism

The active site of sulfite oxidase supports molybdenum to exhibit its highest oxidation state, Mo⁶⁺. When the enzyme stays in an oxidized state, molybdenum is coordinated with a cysteine thiolate, the dithiolene group of molybdopterin, and two terminal oxygen atoms. Upon reacting with sulfite, one oxygen atom is combined with sulfite to obtain sulfate, and the molybdenum center is consequently reduced to Mo⁴⁺ by two electrons. Intramolcular electron transfer from Mo⁴⁺ to Fe³⁺ leads to Fe²⁺ and Mo⁵⁺. Then the single electron photochemical reduction moves Fe²⁺ to the exterior cytochrome and generates Fe³⁺. The displacement of sulfate by water, and the removal of two protons (H⁺) and two electrons (e⁻) allow the active site to recover to its original state. A distinct feature of this oxygen atom transferring enzyme is that the oxygen atom being transferred comes from water, not from molecular O₂. Tryptophan is a crucial amino acid in this enzyme in that it plays a major role in the electron transfer. It is suggested that multi-step electron tunneling is helpful to enhance charge transfer rates, where redox active amino acid side chains function as intermediate donors or acceptors.

Figure 2. Catalytic mechanism of sulfite oxidase. (D'Errico G; et al. 2006)

Biological Importance

In all organisms, sulfite oxidases are of great significance in cell detoxification by oxidizing sulfite, a strong nucleophilic, reducing and harmful compound, to a more innocuous form sulfate. All living cells are exposed to exogenous environmental sulfite derived from the pollutant sulfur dioxide, which is produced by some geological and industrial processes as well as some other processes. Sulfite can also be accumulated intracellularly as a metabolic product of involving organic sulfur compounds, such as sulfur-containing amino acids. Therefore, sulfite oxidases give us robust protection against detriments from both exogenous and endogenous sulfite. In organisms that demonstrate mixotrophic, heterotrophic, chemolithoautotrophic, and phototrophic growth pattern, elemental sulfur oxidation via sulfite oxidase is one of the most energy-yielding reactions. Indeed, elemental sulfur functions as an electron donor in aerobic and facultative aerobic archaea, where it is oxidized via sulfite and thiosulfate in a pathway implicating sulfite:acceptor oxidases and has recently been proven to be associated with the aerobic respiratory chain, signifying a potential connection between oxygen reduction and sulfur oxidation.

Sulfite Oxidase Deficiency

Sulfite oxidase is essential to degrade the sulfur-containing amino acids cysteine and methionine in foods. The lack of functional sulfite oxidase induces a disease termed sulfite oxidase deficiency, which is a rare autosomal inherited disorder of the normal metabolism of sulfated amino acids. Sulfite oxidase deficiency is resulted from a genetic defect causing the absence of a molybdopterin cofactor and point mutations in the enzyme. Individuals attacked with this disease most commonly exist in the neonatal period with characteristic dysmorphic features, profound mental retardation, neurological disorders, physical deformities, brain degradation, and eventually death.

Reference

1. D' Errico G, Di Salle A, La Cara F, Rossi M, Cannio R. Identification and characterization of a novel bacterial sulfite oxidase with no heme binding domain from Deinococcus radiodurans. J Bacteriol, 2006, 188(2):694–701.