# A Synthetic Approach for Biosynthesis of Miquelianin and Scutellarin A in _Escherichia coli_
## 1 Introduction  
Flavonoids have always been recognized as a promising factor in human health, as they refer to various biological responses, such as an inherent ability to modify the body's reaction to allergens, viruses, and carcinogens. Experimental pieces of evidences have always been positive in terms of their activities as anti-allergens, anti-inflammatory, anti-microbial, anti-cancer, etc. [1; 2; 3]. Flavonoids cover a large group of plant secondary metabolites with a profound research interest in various biological and therapeutic applications, since its genetic and biosynthetic investigations have been well-documented over the decades [4]. These aromatic molecules are derived from \(p\)-coumaroyl-CoA and malonyl-CoA. They cover a diverse class of natural products, including; the chalcones, flavones, isoflav, flavonoids, flavonols, flavandiols, anthocyanins, and condensed tannins [5; 6]. Flavonols (quercetin, kaempfer, myricetin, isorhamnetin) represent their diversity, and stem from the 3-OH phenolic group. They are present in a wide variety of fruits and vegetables (apples, berries, onions, broccoli, grapes, flowers and bark), including botanicals [6]. They are well-known for their various health-promoting effects, which include antioxidant, anti-inflammatory, anti-angiogenic, anti-proliferative, and neuro-pharmacologicalproperties [1; 2; 3]. However, flavones (apigenin, luteolin, chrysin, baicalein) have recently been generating enormous public and scientific interest due to their beneficial effects against atherosclerosis, osteoporosis, diabetes mellitus, and certain cancers [7; 8; 9]. Interestingly, both classes of flavonoids act as inhibitors of cytochrome (CYP) P450 enzymes, such as CYP2C9 and CYP3A4, enzymes that help in the metabolism of xenobiotics in the human body [10; 11]. Thus, polyphenols are worth intaking naturally as they have substantial health benefits; however, there are limitations in direct consumption. The limitations could be rooted in their water solubility, limited bioavailability or xenobiotic neutralizations, which challenges the necessity of their modifications by glycosylation/hydroxylation (to enhance solubility), and methylation (to increase lipophilicity, cell recognition) [12; 13; 14]. Nevertheless, conjugation of carbohydrate appendages in such promising metabolites has always improved pharmaceutical values [15]. The glucuronic acid itself is often linked to the xenobiotic metabolism in the human body, and the substances resulting from glucuronidation are typically much more water-soluble [16]. In plants and animals, glucuronic acid is considered to be a precursor of vitamin C [17]. Thus, producing plant polyphenols conjugated with sugars such as glucuronic acid could help to develop such molecules as prospective food additives. Biological synthesis of regio- and stereo-selective phytochemicals using an industrially important _Escherichia coli_ system has been widely studied in current research trends [14]. Most of the bacteria, including _E. coli_, can produce nucleotide sugars, which are building blocks of bacterial polysaccharides and have been well-studied for various reasons [14; 18]. These activated nucleotide diphosphate sugars are utilized as sugar donors by glycosyltransferase (UGTs), which are central players in decorating these valuable natural plant products with sugar moieties [19]. Although sugars like UDP-glucose, UDP-galactose, and UDP-glucuronic acid are common nucleotide diphosphate sugars present in _E. coli_, engineering or overexpression of the pathway-specific genes increase their availability for the biotransformation. The current research employs _Vitis vinifera_ glucuronosyltransferase; _VvGT5_ catalyzes the 3-O-specific glucuronic acid conjugation, and flavonoid 7-O-specific glucuronosyltransferase, _UGT88D8_ from the blue flowers of _Veronica persica_ to produce uniquelianin and scutellarin A in _E. coli_. Both plant UGTs were codon optimized, synthesized, and functionally expressed in _E. coli_.