Solution structure of the third extracellular loop of human thromboxane A2 receptor

The extracellular domains of the thromboxane A2 receptor (TP receptor) were found to be involved in the specific ligand recognition. Determination of the three-dimensional (3D) structure of the extracellular loops would help to explain the mechanism of the ligand binding to its receptor with regard to the tertiary structure. Based on our previous studies on the extracellular loop of the human TP receptor, the synthetic loop peptides, whose termini are constrained to 10 to 14-A separations, are more likely to mimic the native structure of the extracellular loops. In this study, a peptide with the sequence of the third extracellular loop (eLP3, residues 271-289) of the TP receptor was synthesized, and its termini were constrained by the formation of a disulfide bond between the additional homocysteines located at both ends. Fluorescence spectroscopic studies showed that the fluorescence intensity of this constrained loop peptide could be increased by the addition of SQ29,548, a TP receptor antagonist, which indicated the interaction between the peptide and the ligand. The structure of this peptide was then studied by two-dimensional 1H nuclear magnetic resonance (NMR) spectroscopy. 1H NMR assignments of the peptide were obtained and structure constraints were derived from nuclear Overhauser effects and J-coupling constants. The solution structure of the peptide was then calculated based on these constraints. The overall structure shows a beta turn from residues 278 to 281. It also shows a distance of 9.45A between the ends of the N and C termini of the peptide, which agrees with the distance between the two residues at the ends of the transmembrane helices connecting the eLP3 on the TP receptor working model generated using molecular modeling, based on the crystal structure of bovine rhodopsin. These results provide valuable information for the characterization of the complete 3D structure of the extracellular domains of the human TP receptor.