Modeling Sweet Protein-Receptor Interactions The goal of this research proposal is to understand the nature of the interactions between sweet proteins and sweet taste receptor (STR). STR is a heterodimer of T1R2 and T1R3, which is a G protein-coupled receptor (GPCRs) from Family C. Growing evidence shows that STR has multiple ligand binding sites for different sweeteners. Although no crystal structure of STR is available at this time, homology models of STR can be developed based on the appropriate templates. The Venus Flytrap Module (VFTM) and Cysteine Rich Domain (CRD) can be modeled based on the crystal structures of metabotropic glutamate receptors (mGluRs). Transmembrane Domain (TMD) of T1R2 and T1R3 can be modeled based on the crystal structure of bovine rhodopsin. We have successfully used homology models of the sweet taste receptors, molecular docking of sweet ligands to the receptors, and site-directed mutagenesis of the receptors to identify potential ligand binding sites of the sweet taste receptor. Our studies show that the VFTM of hT1R2 is the binding site for sweeteners aspartame and neotame, while the TMD of hT1R3 is the binding site for the sweetener cyclamate and the sweet inhibitor lactisole. Recent chimera and mutagenesis studies show that both the VFTM and CRD of STR are involved in sweet protein-receptor interactions. To investigate the interactions of sweet proteins with STR we propose to construct models of the VFTM-CRD in complex with sweet proteins. The crystal structures of the VFTM-CRD of mGluR-I/II, which were resolved recently, provide us an ideal template to develop homology models for the VFTM-CRD of STR. Here we propose to develop several VFTM-CRD of STR homology models, which represent different conformational states, based on the crystal structures of mGluR templates. The refined VFTM-CRD models of STR will be used for docking the sweet proteins by Brownian Dynamics (BD) simulations to understand sweet protein-receptor interactions. In combination with site-directed point mutational experiments and BD docking simulations, we will identify potential complexes of the VFTM-CRD of STR with sweet proteins. The predicted complexes models will be tested via mutagenesis studies to assess the success of the predictions. These studies should lead to a better understanding the function mechanism of STR.
Obesity and diabetes have reached epidemic proportions in developedsocieties. Replacing sugar with low-or non-caloric sweeteners may be of benefit. To design moreeffective sweeteners it is important to understand at the molecularlevel how the sweet taste receptor functions.
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