Mapping the binding site of the nucleoside transporter protein: a 3D-OSAR study

The nucleoside transporter is an intrinsic membrane protein that mediates salvage of nucleosides from the extracellular medium. In this report, its binding sites have been characterized by a 3D-QSAR (three-dimensional structure-directed quantitative structure-activity relationships) receptor mapping technique. REMOTEDISC. The algorithm is applied to a set of 19 nucleoside analogues, each of which binds to the transporter. The methodology includes: (i) conformational analysis of each ligand; (ii) estimation of physicochemical properties of each ligand at the atomic level; (iii) structural comparison of the low energy conformation of each ligand in the series with a reference structure on the basis of physicochemical property matching; (iv) construction of a predicted binding site cavity from the alignments of step (iii); and (v) multiple regression analysis of the binding data with respect to the 3-dimensional physicochemical descriptors in different 'site-pockets' of the binding cavity. The pharmacophore model that emerges consists of the geometry of the binding site cavity and the relative weights of various properties in different pockets for each of the ligands considered. The study suggests that binding free energy is sensitive to the composition, size and hydrophobicity of the heterocyclic base in the ligand. Though both syn and anti conformations are tried as active forms, the anti conformation gives a better solution and is chosen for modeling the binding site cavity. The best model obtained divides the binding site into six pockets and uses nine independent variables, fitting the observed data with a correlation coefficient of 0.94, a standard deviation of 0.22 and an explained variance of 0.80. Results of our model are consistent with a hypothesis that the 5'-OH group hydrogen bonds with the receptor. This model provides tentative design criteria for development of new nucleoside drugs and transport inhibitors. The model will undoubtedly continue to evolve (i) as the 3D-QSAR algorithm is further refined, and (ii) as data on additional nucleoside analogues become available.