Glucuronidation (Homo sapiens)

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Metabolism of xenobiotic compounds consists of phase I and a phase II biotransformation reactions, being compound modification and conjugation reactions respectively. In phase I biotransformation, the compound is modificated via oxidation, reduction, hydrolysis, or other minor reactions, to reveal a reactive group to which a conjugation molecule can react to. In phase II, a small conjugation molecule reacts with the phase I modified molecule, producing a much more water-soluble molecule that can be excreted more easily. Glucuronidation is a phase II biotransformation reaction in which glucuronide acts as a conjugation molecule and binds to a substrate via the catalysis of glucuronosyltransferases. First, in a series of reactions the cosubstrate uridine diphosphate glucuronic acid (UDPGA) is formed. The glucuronosyltransferases (UGTs) then catalyze the transfer of glucuronic acid from UDPGA to a substrate resulting in a glucuronidated substrate and leaving uridine 5'-diphosphate. UGTs are a very broad and divers group of enzymes and count as the most significant group of conjugation enzymes in xenobiotic metabolism, qualitatively because glucuronic acid can be coupled to a large diversity of functional groups and quantitatively because of the large and divers number of substrates that are formed.