Pydi Sai P, Jaggupilli Appalaraju, Nelson Ken M, Abrams Suzanne R, Bhullar Rajinder P, Loewen Michele C, Chelikani Prashen
†Department of Oral Biology, University of Manitoba, Winnipeg, MB R3E 0W4, Canada.
∥Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK S7N 5C9, Canada.
Biochemistry. 2015 Apr 28;54(16):2622-31. doi: 10.1021/acs.biochem.5b00265. Epub 2015 Apr 13.
Bitter taste receptors (T2Rs) belong to the G protein-coupled receptor superfamily. In humans, 25 T2Rs mediate bitter taste sensation. In addition to the oral cavity, T2Rs are expressed in many extraoral tissues, including the central nervous system, respiratory system, and reproductive system. To understand the mechanistic roles of the T2Rs in oral and extraoral tissues, novel blockers or antagonists are urgently needed. Recently, we elucidated the binding pocket of T2R4 for its agonist quinine, and an antagonist and inhibitory neurotransmitter, γ-aminobutyric acid. This structure-function information about T2R4 led us to screen the plant hormone abscisic acid (ABA), its precursor (xanthoxin), and catabolite phaseic acid for their ability to bind and activate or inhibit T2R4. Molecular docking studies followed by functional assays involving calcium imaging confirmed that ABA is an antagonist with an IC50 value of 34.4 ± 1.1 μM. However, ABA precursor xanthoxin acts as an agonist on T2R4. Interestingly, molecular model-guided site-directed mutagenesis suggests that the T2R4 residues involved in quinine binding are also predominantly involved in binding to the novel antagonist, ABA. The antagonist ability of ABA was tested using another T2R4 agonist, yohimbine. Our results suggest that ABA does not inhibit yohimbine-induced T2R4 activity. The discovery of natural bitter blockers has immense nutraceutical and physiological significance and will help in dissecting the T2R molecular pathways in various tissues.
苦味受体（T2Rs）属于G蛋白偶联受体超家族。在人类中，25种T2Rs介导苦味感觉。除口腔外，T2Rs在许多口腔外组织中表达，包括中枢神经系统、呼吸系统和生殖系统。为了了解T2Rs在口腔和口腔外组织中的作用机制，迫切需要新型阻滞剂或拮抗剂。最近，我们阐明了T2R4与其激动剂奎宁以及拮抗剂和抑制性神经递质γ-氨基丁酸的结合口袋。关于T2R4的这种结构-功能信息促使我们筛选植物激素脱落酸（ABA）、其前体（黄质醛）及其分解代谢物相酸与T2R4结合并激活或抑制T2R4的能力。随后进行涉及钙成像的功能测定的分子对接研究证实，ABA是一种拮抗剂，IC50值为34.4±1.1μM。然而，ABA前体黄质醛对T2R4起激动剂作用。有趣的是，分子模型指导的定点诱变表明，参与奎宁结合的T2R4残基也主要参与与新型拮抗剂ABA的结合。使用另一种T2R4激动剂育亨宾测试了ABA的拮抗能力。我们的结果表明，ABA不会抑制育亨宾诱导的T2R4活性。天然苦味阻滞剂的发现具有巨大的营养保健和生理意义，并将有助于剖析各种组织中的T2R分子途径。
