Maxim Itkin, Rachel Davidovich-Rikanati, Shahar Cohen, Vitaly Portnoy, Adi Doron-Faigenboim, Elad Oren, Shiri Freilich, Galil Tzuri, Nadine Baranes, Shmuel Shen, Marina Petreikov, Rotem Sertchook, Shifra Ben-Dor, Hugo Gottlieb, Alvaro Hernandez, David R. Nelson, Harry S. Paris, Yaakov Tadmor, Yosef Burger, Efraim Lewinsohn, Nurit Katzir, and Arthur Schaffer

Significance

We identified the biosynthetic pathway for the nonsugar sweetener mogroside V, a noncaloric with a sweetening strength 250-fold that of sucrose. This compound is produced by the fruit of the endemic Chinese cucurbit Siraitia grosvenoriii, also known as monk fruit and luo-han-guo. The metabolic pathway was identified using a combination of genomic and transcriptomic databases of the Siraitia plant, together with a large-scale functional expression of candidate genes. The novelty of the pathway could be attributed to a highly coordinated gene expression pattern responsible for the unique epoxidations, hydroxylations, and glucosylations leading to the sweet mogrosides. These discoveries will facilitate the development of alternative natural sweeteners.

Abstract

The consumption of sweeteners, natural as well as synthetic sugars, is implicated in an array of modern-day health problems. Therefore, natural nonsugar sweeteners are of increasing interest. We identify here the biosynthetic pathway of the sweet triterpenoid glycoside mogroside V, which has a sweetening strength of 250 times that of sucrose and is derived from mature fruit of luo-han-guo (Siraitia grosvenorii, monk fruit). A whole-genome sequencing of Siraitia, leading to a preliminary draft of the genome, was combined with an extensive transcriptomic analysis of developing fruit. 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 UDP-glucosyltransferases. Protein modeling and docking studies corroborated the experimentally proven functional enzyme activities and indicated the order of the metabolic steps in the pathway. A comparison of the genomic organization and expression patterns of these Siraitia genes with the orthologs of other Cucurbitaceae implicates a strikingly coordinated expression of the pathway in the evolution of this species-specific and valuable metabolic pathway. The genomic organization of the pathway genes, syntenously preserved among the Cucurbitaceae, indicates, on the other hand, that gene clustering cannot account for this novel secondary metabolic pathway.

See http://www.pnas.org/content/113/47/E7619.abstract.html?etoc

PNAS November 22 2016; vol.113; no.47: E7619–E7628

Fig. 1.

Schematic diagram of the proposed pathway for mogroside biosynthesis in fruit of S. grosvenorii. The left portion of the schematic represents the steps leading to the nonglycosylated tetra-hydroxycucurbitane, mogrol. The right side indicates the successive glucosylations. Enzyme names and numbers are described in the text.