THE BIOSYNTHETIC PATHWAY OF THE NATURAL NON-SUGAR SWEETENER MOGROSIDE V FROM SIRAITA GROSVENORII

Sweetness is a fundamental hedonic pleasure, but the consumption of sweeteners, natural sugars as well as synthetic, is implicated in an array of modern day health problems. Therefore, non-sugar natural sweeteners are of increasing interest. We report here the biosynthetic pathway of the sweet triterpenoid glycoside mogroside V, with a sweetening strength of approximately 250-times that of sucrose, derived from mature fruit of luo-han-guo (Siraitia grosvenorii). A whole genome sequencing of Siraitia, leading to a preliminary draft of the genome was combined with an extensive transcriptome of fruit development. A functional expression survey of nearly 200 candidate genes identified the members of the five enzyme families responsible for the synthesis of mogroside V: squalene epoxidases, triterpenoid synthases, epoxide hydrolases, cytochrome P450s and glucosyltransferases. Protein modelling and docking studies corroborated the experimentally proven functional enzyme activities. A comparison of the genomic organization and expression patterns of the Siraitia genes with those of other Cucurbitaceae species does not support a role for gene clustering in this novel secondary metabolic pathway but does implicate coordinated expression of the pathway comprising tandem gene duplications and gene functionalization in the evolution of this unique and valuable pathway.