Abstract
UDP-Glucuronosyltransferases (UGTs) are a class of enzymes (please refer to Mackenzie et al. [1] for UGT nomenclature and classification) anchored in the membrane of endoplasmic reticulum. The UGT enzyme catalyzes glucuronidation, a major metabolic pathway in humans, by transferring a glucuronic acid from the cofactor UDPglucuronic acid to the substrate. In vitro characterization has revealed that many human UGT enzymes display substrate inhibition kinetics (i.e., inhibition of enzyme activity at high substrate concentrations) [2]. It is interesting to note that plant UDP-glucosyltransferases (using a similar catalytic mechanism as human UGTs), which are water-soluble, also display substrate inhibition kinetics. For example, plant UGT73C8 and UGT88E1 from Medicago truncatula [3], and UGT78K1 from black soybean (with cyanidin as the substrate) [4].
Highlights
In vitro characterization has revealed that many human UGT enzymes display substrate inhibition kinetics [2]
Understanding substrate inhibition in glucuronidation reactions has been confused by the fact that its occurrence is UGT isoform and structure dependent
UGT1A1 and 1A3 are more prone to the substrate inhibition kinetics, which predominates in the glucuronidation reactions catalyzed by these two isoforms [2]
Summary
It is interesting to note that plant UDP-glucosyltransferases (using a similar catalytic mechanism as human UGTs), which are water-soluble, display substrate inhibition kinetics. Understanding substrate inhibition in glucuronidation reactions has been confused by the fact that its occurrence is UGT isoform and (substrate) structure dependent. UGT1A1 and 1A3 are more prone to the substrate inhibition kinetics, which predominates in the glucuronidation reactions catalyzed by these two isoforms [2].
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