Comparative modelling of the human UDP-glucuronosyltransferases: insights into structure and mechanism.

UDP-glucuronosyltranferases (UGTs) affect the disposition of drugs and other xenobiotics by catalysing the conjugation of glucuronic acid to available oxygen, nitrogen, and sulfur atoms. Several related mammalian isoforms of UGT are expressed that have different binding affinities and turnover rates for the substrates they encounter in the liver and other tissues. Because no high-resolution structural information is available to dissect the enzyme-substrate interactions that give rise to different specificities, a search was conducted to find the best available templates to use for comparative protein modelling. Sequence identity analysis was used to identify some recently crystallized plant UGTs as homologues of microsomal UGTs. Because UGTs contain a Rossman fold motif predicted to bind the UDP-containing sugar donor substrate, this consensus sequence was used to aid sequence alignment, as were other conserved residues thought to be involved in catalysis or substrate binding, and the predicted secondary structure. Docking of UDP-glucuronic acid to a model of UGT1A1 resulted in a root mean square deviation of only 0.37 angstroms vs. UDP co-crystallized with the plant UGT71G1 template. The significance of a comparative model generated for UGT1A1 with respect to both the sugar donor and aglycone binding sites, and mechanism, is discussed.