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. In addition, a site-directed mutant, namely, 50K573L, showed a 33% enhancement of catalytic efficiency compared to wild-type 50R573L. Our findings identify a novel cucurbitadienol synthase allele correlates with high catalytic efficiency. These results are valuable for the molecular breeding of luohanguo.
Highlights
Siraitia grosvenorii is an herbaceous perennial of the Cucurbitaceae family
The developed HPLC-MS/MS method was applied to determine the levels of the 21 mogrosides in the fruits of different S. grosvenorii varieties
When the contents of related intermediates in luohanguo were analyzed by gas chromatography mass spectrometry (GC-MS), the results showed significant variations among the different varieties
Summary
Siraitia grosvenorii (luohanguo or monk fruit) is an herbaceous perennial of the Cucurbitaceae family. Mogrosides, the major bioactive components isolated from the fruits of S. grosvenorii, are a mixture of cucurbitane-type triterpenoid glycosides that have been proven to be powerful and zero-calorie sweeteners and can be used as a sucrose substitute for patients with diabetes and patients who are obese [5]. Because of their complex structures (Table A1), the chemical synthesis of these compounds is inherently difficult [6]. With the rapid rise in market demand, the production of Molecules 2019, 24, 627; doi:10.3390/molecules24030627 www.mdpi.com/journal/molecules
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