The Interaction Between Small Molecule Sweetener And Sweet/Bitter Taste Receptors And The Prediction Of Structure-Sweetness Relationship

Sweeteners are essential food additives and play the indispensable role in food,medicine and other fields.In order to overcome the shortcomings including high calorie of traditional sweeteners such as sucrose,novel non-nutritive sweeteners with high sweetness but low calorie have gradually attracting attention,such as steviol glycosides and dipeptide sweeteners.However,due to the lack of understanding of the structure-activity relationship between the structure and sweetness and bitterness of these sweeteners,researchers wasted a lot of time and money on the process of synthesis,separation,and sensory evaluation of new sweeteners.Therefore,in order to deepen the understanding of the structure-sweetness/bitterness of sweeteners,and to indicate the direction for designing novel sweetener,this paper built the homology models of human taste receptors that play the decisive role in the perception of sweetness and bitterness in vivo,then studied the interaction between steviol glycosides or dipeptide compounds and taste receptors by molecular docking.The main contents are as follows:Firstly,this paper constructed the human sweet taste receptor hT1R2/hT1R3 and bitter taste receptor hT2R4 models with high-confidence because the protein structures of taste receptors have not been resolved.Homology models of hT1R2 and hT1R3 were built using different templates and sequence comparison methods by modeling software MODELLER,and the quality of models were evaluated by Ramachandran plot and Verify 3D scoring.The final hT1R2 and hT1R3 models are established by the 5K5 T and 5X2 M templates with the appropriate two-template modeling method.For the human bitter taste receptor hT2R4,the homology model was established using eight templates by online modeling server I-TASSER and was evaluated by TM-Score and Ramachandran plot.Compared to the taste receptor models constructed in previous studies,the new models have better evaluation results,which prove that the hT1R2,hT1R3 and hT2R4 models have higher reliability and are more qualified for subsequent molecules docking calculation.Secondly,with the constructed taste receptor models,molecular docking calculations were carried out with steviol glycosides to study the structure-activity relationship between the sweetness/bitterness and the structures of steviol glycosides.The natural steviol glycosides with known sweetness are firstly docked by two semi-flexible molecular docking methods CDOCKER and Libdock.The CDOCKER method was found to be more suitable for predicting the sweetness of steviol glycosides.The interaction energies between steviol glycosides and receptors have better positive correlation with the sweetness and bitterness of steviol glycosides.CDOCKER can fit a regression equation with higher correlation.The sweetness and bitterness of modified glucosylated steviol glycosides were predicted by this method and the probable sweeteners with better edulcorant qualities were screened.The glycosyl groups at C-13 and C-19 positions could interact with numerous amino acid residues of taste receptors in different combination and produce specific flavors.By analyzing the calculation results,it was found that the changes in the glucose residues at C-13 and C-19 positions had different effects on sweetness and bitterness of steviol glycosides: the more glucose residues at C-13 leads to the stronger sweetness and weaker bitterness,while the more glucose residues at C-19 leads to both weaker sweetness and bitterness.Considering the ligand-receptor interaction patterns and the docking poses,it was found that this difference may be due to the different spatial structures of sweet/bitter taste receptors.Adding glucose residue at C-13 may be a viable method for designing novel steviol glycosides sweetener.Finally,with the taste receptor models,molecular docking was carried out with dipeptide compounds to study the structure-activity relationship between the sweetness/bitterness and the structures of dipeptide.The dipeptide sweeteners with known sweetness are firstly docked by CDOCKER and Libdock methods;CDOCKER was still found to could fit a regression equation with higher correlation and be more suitable for predicting the sweetness of dipeptide compounds.Then the amino acids with strong sweetness were screened out by calculation method and were combined into different dipeptide compounds.Possible dipeptide sweeteners with high quality were selected according to the sweetness prediction results of dipeptide compounds,wherein D-Trp-D-Trp and D-Trp-D-Phe were predicted to possess the highest sweetness about 1400 times than sucrose.The results showed that the interaction energies between ligands and the sweet/bitter taste receptors were usually positively correlated.The interaction patterns and the docking poses indicated that phenomenon may be due to the easy entry of amino acids or dipeptides into the binding sites of taste receptors.Relatively bigger ligands are more likely to generate stronger interactions with the amino acid residues of receptor.Amino acids or dipeptide with benzene ring structure generally have higher sweetness because the benzene ring is more susceptible to hydrophobic interactions with amino acid residues of receptors.This conclusion provides a theoretical basis for the design of novel dipeptide sweeteners.