Laboratory of Food Chemistry, Department of Food Sciences and Human Nutrition, Faculty of Agriculture, Ryukoku University, 1-5 Yokotani Seta Oe-cho, Otsu, Shiga 520-2194, Japan; Laboratory of Food Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
Laboratory of Food Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
Food Chem. 2022 Sep 30;389:132996. doi: 10.1016/j.foodchem.2022.132996. Epub 2022 Apr 18.
Thaumatin is an intensely sweet-tasting protein. Its sweetness persists when heated under acidic conditions, but disappears when heated at a pH above 7.0. To clarify how the structural features of thaumatin resist insoluble aggregation during heating under acidic conditions, we analysed its crystal structure obtained at pH 4.0, 6.0, and 8.0. Simultaneously, the melting temperature (Tm) at these pH levels was determined using differential scanning fluorimetry. At pH 4.0, the Tm of thaumatin was substantially lower and the overall B-factor value of its structure was higher than those at pH 6.0. Interestingly, the relative B-factor values for most lysine residues decreased as the pH reduced. These results suggest that the overall structure at pH 4.0 becomes flexible but the relative flexibility of some regions is lower than that at pH 6.0. Thus, the reduction in relative flexibility might play an important role in preventing thermal aggregation, thereby maintaining the sweetness.
硫氧还蛋白是一种具有强烈甜味的蛋白质。其甜味在酸性条件下加热时仍然存在，但在 pH 值高于 7.0 时加热时会消失。为了阐明硫氧还蛋白的结构特征如何在酸性条件下加热时抵抗不溶性聚集，我们分析了在 pH 值为 4.0、6.0 和 8.0 时获得的晶体结构。同时，使用差示扫描荧光法测定了这些 pH 值下的熔点（Tm）。在 pH 4.0 时，硫氧还蛋白的 Tm 显著降低，其结构的整体 B 因子值高于 pH 6.0。有趣的是，大多数赖氨酸残基的相对 B 因子值随 pH 值降低而降低。这些结果表明，pH 4.0 时的整体结构变得灵活，但某些区域的相对灵活性低于 pH 6.0。因此，相对灵活性的降低可能在防止热聚集从而保持甜味方面发挥重要作用。
