molecules Article Identification of a Novel Specific Cucurbitadienol Synthase Allele in Siraitia grosvenorii Correlates with High Catalytic Efficiency Jing Qiao 1, Zuliang Luo 1, Zhe Gu 1, Yanling Zhang 2, Xindan Zhang 2 and Xiaojun Ma 1,* 1 Institute of Medicinal Plant Development, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing 100193, China;
[email protected] (J.Q.);
[email protected] (Z.L.);
[email protected] (Z.G.) 2 Guilin GFS Monk Fruit Corp., Guilin 541006, China;
[email protected] (Y.Z.);
[email protected] (X.Z.) * Correspondence:
[email protected]; Tel.: +86-10-57833155 Received: 7 December 2018; Accepted: 24 January 2019; Published: 11 February 2019 Abstract: Mogrosides, the main bioactive compounds isolated from the fruits of Siraitia grosvenorii, are a group of cucurbitane-type triterpenoid glycosides that exhibit a wide range of notable biological activities and are commercially available worldwide as natural sweeteners. However, the extraction cost is high due to their relatively low contents in plants. Therefore, molecular breeding needs to be achieved when conventional plant breeding can hardly improve the quality so far. In this study, the levels of 21 active mogrosides and two precursors in 15 S. grosvenorii varieties were determined by HPLC-MS/MS and GC-MS, respectively. The results showed that the variations in mogroside V content may be caused by the accumulation of cucurbitadienol. Furthermore, a total of four wild-type cucurbitadienol synthase protein variants (50R573L, 50C573L, 50R573Q, and 50C573Q) based on two missense mutation single nucleotide polymorphism (SNP) sites were discovered. An in vitro enzyme reaction analysis indicated that 50R573L had the highest activity, with a specific activity of 10.24 nmol min−1 mg−1.