Miraculin (1-20)

Miraculin (1-20) is active component of R. dulcifica that modifies or converts sourness to sweetness. Miraculin is a taste-modifying protein that exhibits extremely unusual properties and is famous for its unique taste characteristics ^[2]. Taste-modifying effect of Miraculin is specific to humans but not to rodents,he role of Miraculin varies among different types of acids^[4,7].

The TAS1Rs belong to the class C G-protein-coupled receptor (GPCR) family and consist of three principal domains: an amino-terminal domain (ATD) and a cysteine-rich domain (CRD) located in the extracellular region and a transmembrane domain (TMD) ^[6].

Miraculin interacts with the human sweet receptor subunit hTAS1R2. Miraculin-applied cells displayed a pH dependence with citric acid (weak acid) being right shifted to that with hydrochloric acid (strong acid). When histidine residues in both the intracellular and extracellular region of hTAS1R2 were exchanged for alanine, taste-modifying effect of Miraculin was reduced or abolished. Stronger intracellular acidification of HEK293 cells was induced by citric acid than by HCl and taste-modifying effect of Miraculin was proportional to intracellular pH regardless of types of acids. Intracellular acidity is required for full activation of the sweet taste receptor by Miraculin ^[1，3].

Recombinant Miraculin resembled native Miraculin in the secondary structure and the taste-modifying activity to generate sweetness at acidic pH. Since the observed pH-sweetness relation seemed to reflect the imidazole titration curve, suggesting that histidine residues might be involved in the taste-modifying activity.Both H30A and H30,60A mutants have lost the taste-modifying activity. Histidine-30 residue is important for the taste-modifying activity of Miraculin ^[5].

References:
[1]: Misaka T. Molecular mechanisms of the action of miraculin, a taste-modifying protein. Semin Cell Dev Biol. 2013 Mar;24(3):222-5. doi: 10.1016/j.semcdb.2013.02.008. Epub 2013 Mar 4. PMID: 23466289.
[2]: Kurihara K, Beidler LM. Taste-modifying protein from miracle fruit. Science. 1968 Sep 20;161(3847):1241-3. doi: 10.1126/science.161.3847.1241. PMID: 5673432.
[3]: Sanematsu K, Kitagawa M, et,al. Intracellular acidification is required for full activation of the sweet taste receptor by miraculin. Sci Rep. 2016 Mar 10;6:22807. doi: 10.1038/srep22807. PMID: 26960429; PMCID: PMC4785348.
[4]: Brouwer JN, Glaser D, et,al. The sweetness-inducing effect of miraculin; behavioural and neurophysiological experiments in the rhesus monkey Macaca mulatta. J Physiol. 1983 Apr;337:221-40. doi: 10.1113/jphysiol.1983.sp014621. PMID: 6875928; PMCID: PMC1199104.
[5]: Ito K, Asakura T, et,al. Microbial production of sensory-active miraculin. Biochem Biophys Res Commun. 2007 Aug 24;360(2):407-11. doi: 10.1016/j.bbrc.2007.06.064. Epub 2007 Jun 19. PMID: 17592723.
[6]: Kunishima N, Shimada Y, et,al. Structural basis of glutamate recognition by a dimeric metabotropic glutamate receptor. Nature. 2000 Oct 26;407(6807):971-7. doi: 10.1038/35039564. PMID: 11069170.
[7]: KURIHARA, K., BEIDLER, L. Mechanism of the Action of Taste-modifying Protein. Nature 222, 1176-1179 (1969). https://doi.org/10.1038/2221176a0

Miraculin (1-20) 是 R. dulcifica 的活性成分，可改变酸味或将酸味转化为甜味。 Miraculin 是一种味觉修饰蛋白，具有极其不寻常的特性，并以其独特的味觉特征而闻名^[2]。 Miraculin 的味觉调节作用对人类有特异性，但对啮齿动物没有，Miraculin 的作用因酸的不同而异^[4,7]。

TAS1R 属于 C 类 G 蛋白偶联受体 (GPCR) 家族，由三个主要结构域组成:氨基末端结构域 (ATD) 和位于细胞外区域的富含半胱氨酸结构域 (CRD) 和跨膜结构域 (TMD) ^[6]。

奇迹蛋白与人类甜味受体亚基 hTAS1R2 相互作用。应用 Miraculin 的细胞表现出 pH 依赖性，柠檬酸（弱酸）右移到盐酸（强酸）。当 hTAS1R2 细胞内和细胞外区域的组氨酸残基都被丙氨酸交换时，Miraculin 的味觉调节作用减弱或消失。柠檬酸比 HCl 诱导的 HEK293 细胞内酸化更强，而无论酸的类型如何，Miraculin 的味觉调节作用都与细胞内 pH 值成正比。 Miraculin ^[1，3] 完全激活甜味受体需要细胞内酸度。

重组的 Miraculin 在二级结构和在酸性 pH 下产生甜味的味道调节活性方面与天然 Miraculin 相似。由于观察到的pH-甜度关系似乎反映了咪唑滴定曲线，表明组氨酸残基可能参与了味觉修饰活性。H30A和H30,60A突变体都失去了味觉修饰活性。组氨酸 30 残基对 Miraculin 的味觉修饰活性很重要^[5]。