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Urate oxidase
The enzyme urate oxidase (UO), also known as uricase or factor-independent urate hydroxylase, is found in nearly all species from bacteria to mammals, but is not expressed in humans, great apes, and certain New World monkeys, in which it exists as a pseudogene. It catalyzes the oxidation of uric acid to 5-hydroxyisourate:
Urate oxidase is found in nearly all organisms, from bacteria to mammals, but is a pseudogene in humans, great apes, and certain New World monkeys, having been lost during primate evolution. This means that instead of producing allantoin as the end product of purine oxidation, the pathway ends with uric acid. This leads to humans and many primates having much higher and more highly variable levels of urate in the blood than most other mammals.
Urate oxidase is a notable example of the existence of non-homologous isofunctional enzymes, proteins with independent evolutionary origin catalyzing the same chemical reaction.
Besides the cofactorless urate oxidase (UOX), which is found in all three domains of life, other bacterial proteins are known that catalyze the same reaction without being evolutionarily related to UOX. These are two different oxidases (named HpxO and HpyO) that use FAD and NAD+ as cofactors, and one integral membrane protein (named PuuD) that additionally contains a cytochrome c protein domain.
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.
Urate oxidase is the first enzyme 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.
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Urate oxidase
The enzyme urate oxidase (UO), also known as uricase or factor-independent urate hydroxylase, is found in nearly all species from bacteria to mammals, but is not expressed in humans, great apes, and certain New World monkeys, in which it exists as a pseudogene. It catalyzes the oxidation of uric acid to 5-hydroxyisourate:
Urate oxidase is found in nearly all organisms, from bacteria to mammals, but is a pseudogene in humans, great apes, and certain New World monkeys, having been lost during primate evolution. This means that instead of producing allantoin as the end product of purine oxidation, the pathway ends with uric acid. This leads to humans and many primates having much higher and more highly variable levels of urate in the blood than most other mammals.
Urate oxidase is a notable example of the existence of non-homologous isofunctional enzymes, proteins with independent evolutionary origin catalyzing the same chemical reaction.
Besides the cofactorless urate oxidase (UOX), which is found in all three domains of life, other bacterial proteins are known that catalyze the same reaction without being evolutionarily related to UOX. These are two different oxidases (named HpxO and HpyO) that use FAD and NAD+ as cofactors, and one integral membrane protein (named PuuD) that additionally contains a cytochrome c protein domain.
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.
Urate oxidase is the first enzyme 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.