Identification of key amino acid residues in the hTGR5–nomilin interaction and construction of its binding model

TGR5, a member of the G protein-coupled receptor (GPCR) family, is activated by bile acids. Because TGR5 promotes energy expenditure and improves glucose homeostasis, it is recognized as a key target in treating metabolic diseases. We previously showed that nomilin, a citrus limonoid, activates TGR5 and confers anti-obesity and anti-hyperglycemic effects in mice. Information on the TGR5–nomilin interaction regarding molecular structure, however, has not been reported. In the present study, we found that human TGR5 (hTGR5) shows higher nomilin responsiveness than does mouse TGR5 (mTGR5). Using mouse–human chimeric TGR5, we also found that three amino acid residues (Q77(ECL1), R80(ECL1), and Y89(3.29)) are important in the hTGR5–nomilin interaction. Based on these results, an hTGR5–nomilin binding model was constructed using in silico docking simulation, demonstrating that four hydrophilic hydrogen-bonding interactions occur between nomilin and hTGR5. The binding mode of hTGR5–nomilin is vastly different from those of other TGR5 agonists previously reported, suggesting that TGR5 forms various binding patterns depending on the type of agonist. Our study promotes a better understanding of the structure of TGR5, and it may be useful in developing and screening new TGR5 agonists.