Cysteine dioxygenase (CDO) is a non-heme iron enzyme that catalyzes the conversion of L-cysteine to cysteine sulfinic acid (cysteine sulfinate). CDO plays a role in cysteine catabolism, regulating intracellular levels of cysteine. As such, CDO is highly regulated and undergoes large changes in concentration and efficiency. It oxidizes cysteine to the corresponding sulfinic acid by activation of dioxygen. In addition to being found in mammals, CDO also exists in some yeast and bacteria. CDO has been implicated in various neurodegenerative diseases and cancers, which is likely related to cysteine toxicity. CDO oxidizes to cysteine sulfinic acid (which exists predominantly in the anionic sulfinate form in vivo). Overall, CDO catalyzes the addition of dioxygen (O<sub>2</sub>) to a thiol, producing a sulfinic acid. More specifically, CDO is part of the group of non-heme iron oxygenases that employ oxygen as an electron acceptor. Cysteine sulfinic acid is then metabolized further via two divergent pathways: decarboxylated to hypotaurine by sulfinoalanine decarboxylase and oxidized to taurine by hypotaurine dehydrogenase; or transaminated to a putative 3-sulfinylpyruvate intermediate, which decomposes spontaneously into pyruvate and sulfite. Sulfite can then be oxidized to sulfate by sulfite oxidase.]]
CDO is a 22.5 kDa protein in a "jelly-roll" topology. three histidine ligands are bound to iron. Heterolytic O-O bond cleavage then affords a high-valent iron (IV) oxo intermediate (C), which transfers the second oxygen to sulfur. and motor neuron diseases. In these diseases, patients display depressed sulfate levels, elevated fasting cysteine plasma concentrations, and other symptoms consistent with impaired cysteine oxidation.
The expression of CDO is altered in cancer cells Silencing of CDO1 is a critical epigenetic event in breast cancer, leading to downregulation of CDO1 activity. In particular, decreased CDO1 activity causes increased hydrogen sulfide (H<sub>2</sub>S), which has been connected to various diseases.