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Subatomic structure of hyper-sweet thaumatin D21N mutant reveals the importance of flexible conformations for enhanced sweetness
- Masuda, Tetsuya, Okubo, Kyohei, Murata, Kazuki, Mikami, Bunzo, Sugahara, Michihiro, Suzuki, Mamoru, Temussi, Piero Andrea, Tani, Fumito
- Biochimie 2019 v.157 pp. 57-63
- electrostatic interactions, hydrogen bonding, mutants, sweetness, taste receptors, tropical and subtropical fruits
- One of the sweetest proteins found in tropical fruits (with a threshold of 50 nM), thaumatin, is also used commercially as a sweetener. Our previous study successfully produced the sweetest thaumatin mutant (D21N), designated hyper-sweet thaumatin, which decreases the sweetness threshold to 31 nM. To investigate why the D21N mutant is sweeter than wild-type thaumatin, we compared the structure of the D21N mutant solved at a subatomic resolution of 0.93 Å with that of wild-type thaumatin determined at 0.90 Å. Although the overall structure of the D21N mutant resembles that of wild-type thaumatin, our subatomic resolution analysis successfully assigned and discriminated the detailed atomic positions of side-chains at position 21. The relative B-factor value of the side-chain at position 21 in the D21N mutant was higher than that of wild-type thaumatin, hinting at a greater flexibility of side-chain at 21 in the hyper-sweet D21N mutant. Furthermore, alternative conformations of Lys19, which is hydrogen-bonded to Asp21 in wild-type, were found only in the D21N mutant. Subatomic resolution analysis revealed that flexible conformations at the sites adjacent to positions 19 and 21 play a crucial role in enhancing sweet potency and may serve to enhance the complementarity of electrostatic potentials for interaction with the sweet taste receptor.