The enzyme urate oxidase (UO), uricase or factor-independent urate hydroxylase, absent in humans, catalyzes the oxidation of uric acid to 5-hydroxyisourate: Uric acid + O2 + H2O → 5-hydroxyisourate + H2O2 5-hydroxyisourate + H2O → allantoin + CO2 Structure Urate oxidase is mainly localised in the liver, where it forms a large electron-dense paracrystalline core in many peroxisomes. The enzyme exists as a tetramer of identical subunits, each containing a possible type 2 copper-binding site. Urate oxidase is a homotetrameric enzyme containing four identical active sites situated at the interfaces between its four subunits. UO from A. flavus is made up of 301 residues and has a molecular weight of 33438 daltons. It is unique among the oxidases in that it does not require a metal atom or an organic co-factor for catalysis. Sequence analysis of several organisms has determined that there are 24 amino acids which are conserved, and of these, 15 are involved with the active site. Reaction mechanism Urate oxidase is the first in a pathway of three enzymes to convert uric acid to S-(+)-allantoin. After uric acid is converted to 5-hydroxyisourate by urate oxidase, 5-hydroxyisourate (HIU) is converted to 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) by HIU hydrolase, and then to S-(+)-allantoin by 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase (OHCU decarboxylase). Without HIU hydrolase and OHCU decarboxylase, HIU will spontaneously decompose into racemic allantoin. Within the active site of urate oxidase, there is a catalytic site that holds uric acid and its analogues in the same orientation and a common catalytic site where oxygen, water, and hydrogen peroxide are sequentially driven. This mechanism is similar to that of catalase, which reverses hydrogen peroxide back to oxygen and also sequentially drives hydrogen peroxide and water through the common catalytic site. X-ray crystallography has shown that uric acid first binds to the active site as a monoanion, and is then deprotonated to a dianion. The dianion is stabilized by Arg 176 and Gln 228 of the enzyme. Oxygen will subsequently accept an electron pair from the uric acid dianion and be converted to hydrogen peroxide, which is replaced by water that performs a nucleophilic attack on the intermediate to produce 5-hydroxyisourate. Urate oxidase is known to be inhibited by both cyanide and chloride ions. This occurs because of anion-π interactions between the inhibitor and the uric acid substrate. Significance of absence in humans Urate oxidase is found in nearly all organisms, from bacteria to mammals, but