How Many and Which Amino Acids Are Responsible for the Large Activity Differences between the Highly Homologous UDP-Glucuronosyltransferases (UGT) 1A9 and UGT1A10?
作者:Katriina Itäaho、Liisa Laakkonen、Moshe Finel
DOI:10.1124/dmd.109.031229
日期:2010.4
The amino acid sequences of the human UDP-glucuronosyltransferases (UGTs) 1A9 and 1A10 are 93% identical, yet there are large differences in their activity and substrate selectivity. For example, the regioselectivity in propranolol glucuronidation, the regioselectivity in dobutamine glucuronidation, and the glucuronidation rate of α- and β-estradiol differ greatly between UGT1A9 and UGT1A10. To identify the residue responsible for the activity differences, we divided the N-terminal half of the two UGTs into five comparable segments by inserting four unique restriction sites at identical positions in both genes and constructing chimeras in which segments of UGT1A9 were individually replaced by the corresponding segments from UGT1A10. Activity analyses of the resulting mutants, 910A [1A10(1–83)/1A9(84–285)], 910B [1A9(1–83)/1A10(84–147)/1A9(148–285)], 910C [1A9(1–147)/1A10(148–181)/1A9(182–285)], 910D [1A9(1–181)/1A10(182–235)/1A9(236–285)], and 910E [1A9(1–235)/1A10(236–285)] indicated that more than one residue is responsible for the differences between UGT1A9 and UGT1A10. We next prepared four double chimeras, in which two of the above UGT1A9 segments were replaced simultaneously by the corresponding UGT1A10 segments. However, none of the double chimeras glucuronidated either estradiol, propranolol, or dobutamine at rates that resembled those of UGT1A10. On the other hand, studying the kinetics of 1-naphthol glucuronidation yielded more focused results, indicating that residues within segment B (84–147) contribute directly to the K m value for this substrate. Further mutagenesis and activity assays suggested that Phe117 of UGT1A9 participates in 1-naphthol binding. In addition, it appears that residues within segment C of the N-terminal domain, mainly at positions 152 and 169, contribute to the higher glucuronidation rates of UGT1A10.
人UDP-葡萄糖醛酸基转移酶(UGT) 1A9和1A10的氨基酸序列有93%相同,但它们的活性和底物选择性存在很大差异。例如,普萘洛尔葡萄糖醛酸化的区域选择性、多巴酚丁胺葡萄糖醛酸化的区域选择性以及α-和β-雌二醇的葡萄糖醛酸化率在UGT1A9和UGT1A10之间存在很大差异。为了鉴定造成活性差异的残基,我们通过在两个基因的相同位置插入四个独特的限制性位点并构建嵌合体,将两个 UGT 的 N 端一半分成五个可比较的片段,其中 UGT1A9 的片段分别被替换UGT1A10 的相应片段。所得突变体的活性分析,910A [1A10(1–83)/1A9(84–285)]、910B [1A9(1–83)/1A10(84–147)/1A9(148–285)]、910C [ 1A9(1–147)/1A10(148–181)/1A9(182–285)]、910D [1A9(1–181)/1A10(182–235)/1A9(236–285)]和 910E [1A9 (1–235)/1A10(236–285)]表明不止一个残基造成UGT1A9和UGT1A10之间的差异。接下来,我们制备了四个双嵌合体,其中上述 UGT1A9 片段中的两个同时被相应的 UGT1A10 片段替换。然而,没有一个双嵌合体以与 UGT1A10 类似的速率对雌二醇、普萘洛尔或多巴酚丁胺进行葡萄糖醛酸化。另一方面,研究 1-萘酚葡萄糖醛酸化的动力学产生了更集中的结果,表明片段 B (84-147) 内的残基直接影响该底物的 K m 值。进一步的诱变和活性测定表明 UGT1A9 的 Phe117 参与 1-萘酚结合。此外,N 端结构域 C 段内的残基(主要位于位置 152 和 169)似乎有助于提高 UGT1A10 的葡萄糖醛酸化率。