Abstract
All UDP-glucuronosyltransferase enzymes (UGTs) share a common cofactor, UDP-glucuronic acid (UDP-GlcUA). The binding site for UDP-GlcUA is localized to the C-terminal domain of UGTs on the basis of amino acid sequence homology analysis and crystal structures of glycosyltransferases, including the C-terminal domain of human UGT2B7. We hypothesized that the 393DQMDNAK399 region of human UGT1A10 interacts with the glucuronic acid moiety of UDP-GlcUA. Using site-directed mutagenesis and enzymatic analysis, we demonstrated that the D393A mutation abolished the glucuronidation activity of UGT1A10 toward all substrates. The effects of the alanine mutation at Q394,D396, and K399 on glucuronidation activities were substrate-dependent. Previously, we examined the importance of these residues in UGT2B7. Although D393 (D398 in UGT2B7) is similarly critical for UDP-GlcUA binding in both enzymes, the effects of Q394 (Q399 in UGT2B7) to Ala mutation on activity were significant but different between UGT1A10 and UGT2B7. A model of the UDP-GlcUA binding site suggests that the contribution of other residues to cosubstrate binding may explain these differences between UGT1A10 and UGT2B7. We thus postulate that D393 is critical for the binding of glucuronic acid and that proximal residues, e.g., Q394 (Q399 in UGT2B7), play a subtle role in cosubstrate binding in UGT1A10 and UGT2B7. Hence, this study provides important new information needed for the identification and understanding of the binding sites of UGTs, a major step forward in elucidating their molecular mechanism.
Footnotes
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This work was supported in part by National Institutes of Health (NIH) Grants DK56226, DK60109, and GM075893 (A.R.-P.). A.R.-P. is also supported by tobacco settlement funds from the University of Arkansas for Medical Sciences. The work in Helsinki was supported by grants from the Academy of Finland [Projects 207535 (M.F.) and 1105157 and 1114752 (A.G.)] and the Sigrid Jusélius Foundation (M.F. and A.G.) and by Biocentrum Helsinki (A.G.). The work at University of North Caroline was supported in part by NIH Grant CA98468 (M.R.R.)
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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doi:10.1124/dmd.107.016469.
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ABBREVIATIONS: UGT, UDP-glucuronosyltransferase; UDP-GlcUA, UDP-glucuronic acid; pNP, p-nitrophenol; 2-OH-E1, 2-hydroxyestrone; TCC, tetrachlorocatechol; TLC, thin-layer chromatography; VvGT1, plant UDP-glucose:flavonoid 3-O-glycosyltransferase.
- Received May 1, 2007.
- Accepted November 26, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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