Chapter 9 - Whole-Cell Bioconversion of Citrus Flavonoids to Enhance Their Biological Properties

Abstract

Abstract Flavonoids are known as natural soft drugs that have biological activities and applications in the pharmaceutical industry. These include antioxidant, antiinflammatory, anticarcinogenic, antimicrobial, and antiviral activities, among others (Ribeiro et al., 2008; Karabin et al., 2015). The flavonoids are a group of phenolic compounds with structures comprising three rings, having antioxidant activities. Two of the rings are benzenes connected by a pyrone ring that has an oxygen atom. It is the structure of the last ring which permits their classification into six subclasses: flavons, flavonols, flavonons, flavanols (catechins), anthocyanins, and isoflavonoids (Zhang, 2007). Hydroxylation or methoxylation of flavones increases their antioxidant properties and reduces their low-density lipoproteins (Morin et al., 2008). Natural synthesis of flavonoids is only carried out in plants. Due to its importance in terms of functionality, it has been discovered that their biological activities can be enhanced by hydroxylation or methylation; these enhanced properties can be obtained through bioconversion processes using enzymes or whole cells. Ullrich and Hofrichter (2007) reviewed the enhancement of biological activities of flavonoids by the selective hydroxylation of aromatic compounds, one of the most challenging chemical reactions in synthetic chemistry, using isolated enzymes or whole microbial cells. They gave an overview of the different enzymes and mechanisms used to introduce oxygen atoms into aromatic molecules. Several enzymes have been used to enrich flavonoids. One example is the use of monooxygenases. These are enzymes that incorporate one hydroxyl group into a substrate. One such monooxygenase, cytochrome P450 105D7 from Streptomyces avermitilis, can catalyze the hydroxylation of two flavanones—naringenin and pinocembrin (Liu et al., 2016). The unique catalytic properties of oxygenases have undoubted biosynthetic value (Duetz et al., 2001). Prasetyo et al. (2011) carried out the enrichment of naringenin with simple phenolic compounds, rich in hydroxyl or methoxy groups, using laccases to enhance its antioxidant activity. In oxidative enzymatic reactions where cofactor regeneration is needed, with high regioselectivity and stereoselectivity, whole-cell bioconversion is an attractive and economic alternative to the cell-free enzyme system (Carvalho, 2016). Holland and Weber (2000) examined the manipulation of microbial hydroxylating enzymes, in both whole-cell and cell-free environments, in the context of controlling the regioselectivity and stereoselectivity of the hydroxylation reaction. Kitamura et al. (2013) reported the bioconversion of flavonoids (flavonons, isoflavons, and chalcons) to their corresponding hydroxylated products using recombinant cells of Escherichia coli, with expression of the monooxygenase enzyme cytochrome P450 BM3. An increase in antioxidant activity in some of the products was observed. However, a deep review of the kinetics and engineering might be helpful to analyze and stimulate discussion about the potential of the whole-cell bioconversion of flavonoids. Therefore the aim of this chapter is to analyze and discuss the potential of the whole-cell bioconversion of flavonoids as an attractive alternative for enhancing their biological properties.