Glucosinolates (GLs) are specialized bioactive compounds characteristic of plants
in the order Brassicales, including the model plant Arabidopsis thaliana. GLs are key players in the plant's natural defense system against herbivores and
microorganisms. Additional biological functions range from flavor compounds to bio-pesticides.
Particularly, glucoraphanin (GRN), the major glucosinolate in broccoli has been associated
with broccoli's cancer-preventive properties [1]. GRN is derived from methionine that
first undergoes a series of four enzymatic reactions to form the chain-elongated dihomomethionine.
Methionine chain-elongation is partially compartmentalized to the chloroplast. Subsequently,
dihomomethionine is converted into GRN by a cytosolic seven-step pathway [2]. Recently,
we have demonstrated the feasibility to engineer the 13-step pathway of GRN biosynthesis
from A. thaliana into the non-cruciferous plant species Nicotiana benthamiana by transient expression [3]. The goal is to ultimately transfer the GRN pathway into
a microbial host organism for sustainable production. As proof-of-concept we successfully
transferred the pathway for the simple tryptophan-derived indole GLs by stable integration
of seven biosynthetic and one supporting gene into the genome of S. cerevisiae [4].
A major challenge in GRN bioengineering is the tight regulation and thus limited availability
of free methionine in plants and microorganisms. We could already attenuate this bottleneck
in our transient N. benthamiana system by co-expression of feedback-insensitive variants of cystathionine gamma-synthase.
This enzyme plays a central role in the regulation of de novo methionine biosynthesis in plants. We are currently in the process of implementing
this knowledge into our microbial expression systems yeast and E. coli.
Keywords: Glucosinolates, glucoraphanin, cancer-preventive, metabolic engineering
References:
[1] Juge N, et al. (2007), Cell Mol Life Sci. 64(9): 1105 – 1127.
[2] Sonderby IE, et al. (2010), Trends Plant Sci. 15(5): 283 – 290.
[3] Mikkelsen MD, et al. (2010), Mol Plant. 3(4): 751 – 759.
[4] Mikkelsen MD, et al. (2012), Metabolic Engineering. 14(2): 104 – 111.
