Public Library of Science Acta Medica Okayama 1932-6203 19 3 2024 Chemical range recognized by the ligand-binding domain in a representative amino acid-sensing taste receptor, T1r2a/T1r3, from medaka fish e0300981 EN Hikaru Ishida Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University Norihisa Yasui Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University Atsuko Yamashita Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University Taste receptor type 1 (T1r) proteins are responsible for recognizing nutrient chemicals in foods. In humans, T1r2/T1r3 and T1r1/T1r3 heterodimers serve as the sweet and umami receptors that recognize sugars or amino acids and nucleotides, respectively. T1rs are conserved among vertebrates, and T1r2a/T1r3 from medaka fish is currently the only member for which the structure of the ligand-binding domain (LBD) has been solved. T1r2a/T1r3 is an amino acid receptor that recognizes various l-amino acids in its LBD as observed with other T1rs exhibiting broad substrate specificities. Nevertheless, the range of chemicals that are recognized by T1r2a/T1r3LBD has not been extensively explored. In the present study, the binding of various chemicals to medaka T1r2a/T1r3LBD was analyzed. A binding assay for amino acid derivatives verified the specificity of this protein to l-alpha-amino acids and the importance of alpha-amino and carboxy groups for receptor recognition. The results further indicated the significance of the alpha-hydrogen for recognition as replacing it with a methyl group resulted in a substantially decreased affinity. The binding ability to the protein was not limited to proteinogenic amino acids, but also to non-proteinogenic amino acids, such as metabolic intermediates. Besides l-alpha-amino acids, no other chemicals showed significant binding to the protein. These results indicate that all of the common structural groups of alpha-amino acids and their geometry in the l-configuration are recognized by the protein, whereas a wide variety of alpha-substituents can be accommodated in the ligand binding sites of the LBDs. No potential conflict of interest relevant to this article was reported.