The enzyme cyanase (, also known as cyanate hydratase or cyanate lyase), catalyses the bicarbonate dependent metabolism of cyanate to produce ammonia and carbon dioxide. The systematic name of this enzyme is carbamate hydrolyase. In E. coli, cyanase is an inducible enzyme and is encoded for by the cynS gene. Cyanate is a toxic anion, and cyanase catalyzes the metabolism into the benign products of carbon dioxide and ammonia. Enzyme Reaction Cyanase catalyzes the metabolic conversion of toxic cyanate to carbamate (H2NCOO−). The enzyme does this by first catalyzing the conversion of cyanate to carbamate, which then spontaneously decomposes to carbon dioxide and ammonia: cyanate (OCN−) + HCO3− + H+ carbamate (H2NCOO−) + CO2 carbamate (H2NCOO−) + H+ NH3 + CO2 (spontaneous) The resulting net reaction is: cyanate (OCN−) + HCO3− + 2 H+ NH3 + 2 CO2 The kinetic mechanism is a rapid equilibrium where bicarbonate and cyanate both act as substrates. Bicarbonate acts a recycling substrate to cyanase in the production carbamate which then decomposes to complete the metabolism, rather than metabolism via a hydrolysis reaction as formerly believed. A positively charged active site forms a pocket with two anion binding sites, bonded via ionic interaction. Positively charged amino acids that make up the protein help form this site between subunits. This adjacent anion binding site structure accommodates a complex of cyanate and bicarbonate, allowing the carbamate producing reaction to proceed. Enzyme Structure Some bacteria can overcome the toxicity of environmental cyanate by degrading it via this enzyme. Cyanate hydratase is found in bacteria and plants. Cyanase is functionally active as a homodecamer, composed of 5 complexes of dimers. Each of these monomer subunits is 17 kDa and supports half-site binding of substrates. The cyanase monomer is composed of two domains. The N-terminal domain shows structural similarity to the DNA-binding alpha-helix bundle motif. The C-terminal domain has an 'open fold' with no structural homology to other proteins. The dimer structure reveals the C-terminal domains to be intertwined, and a decamer is formed by a pentamer of these dimers. The active site of the enzyme is located between dimers and is composed of residues from four adjacent subunits of the homodecamer. Connecting these two domains in the monomer is the single cysteine residue. The disulfide bond links 156 amino acid units. Forming the larger complexes of the pentamer and decamer requires the presence of cyanate, or a substrate analog