**2.1 Enzymes**

264 Biochemistry

In nature, flavonoid acylation is catalyzed by various acyltransferases which are responsible for the transfer of aromatic or aliphatic acyl groups from a CoA-donor molecule to hydroxyl residues of flavonoid sugar moieties (Davies & Schwinn, 2006). Acylation is widespread especially among anthocyanins; more than 65% are reported to be acylated (Andersen & Jordheim, 2006). While the exact role of plant acylation is not yet fully understood, it is known that these modifications modulate the physiological activity of the resulting flavonoid ester by altering solubility, stability, reactivity and interaction with cellular targets (Ferrer et al., 2008). Acylation might be a prerequisite molecular tag for efficient vacuolar uptake of flavonoids (Kitamura, 2006; Nakayama et al., 2003). Some acylated flavonoids have been found to be involved in plant-insect interactions; they act as phytoalexins, oviposition stimulants, pollinator attractants (Iwashina, 2003), and insect antifeedants (Harborne & Williams, 1998). With respect to novel biological activities, acylation of flavonoids can result in changes in pigmentation (Bloor, 2001), insect antifeedant activity

(Harborne & Williams, 1998) and antioxidant properties (Alluis & Dangles, 1999).

aromatic acids (Tab.1) (Nakajima et al., 2000; Fujiwara et al., 1998).

hydroxycinnamoyl-CoA:anthocyanin

hydroxycinnamoyl-CoA:anthocyanin

hydroxycinnamoyl-CoA:anthocyanidin

malonyl-CoA:anthocyanidin

1999; Nagasawa & Yamada, 1995).

malonyl-CoA:anthocyanin 3-*O*-glucosid-

Over the past 15 years, there has been a substantial effort to take advantage of this naturally occurring phenomenon and to implement acylation methods into laboratories. However, the use of acyltransferases as modifying agents is rather inconvenient, as they require corresponding acylcoenzyme A, which must be either in stoichiometric amounts or regenerated *in situ*. Natural acyltransferases and cell extracts from *Ipomoea batatas* and *Perilla frutescens* containing acyltransferases were applied for selective flavonoid modification with

**Acyltransferase Plant source References** 

et al., 2000

3-*O*-glucosid-6''-*O*-acyltransferase *Perilla frutescens* Yonekura-Sakakibara

<sup>6</sup>′′-*O*-malonyltransferase *Dahlia variabilis* Wimmer et al., 1998

5-*O*-glucosid-6''-*O*-acyltransferase *Gentiana triflora* Tanaka et al., 1996

3-rutinosid acyltransferase *Petunia hybrida* Brugliera & Koes, 2003

To solve this problem, the chemical approach was first investigated. It possessed a low degree of regioselectivity of esterification and drastic reaction conditions had to be applied (Patti et al., 2000). Later on, hydrolytic enzymes (lipases, esterases and proteases) have been recognized as useful agents due to their large availability, low cost, chemo-, regio- and enantioselectivity, mild condition processing and no need of cofactors (Collins & Kennedy,

Since the enzymatic preparation of flavonoid derivatives is a matter of several years, commercial applications have just been emerging. There are several patented inventions available to date, oriented on the flavonoid ester production and their use for the manufacture of pharmaceutical, dermopharmaceutical, cosmetic, nutritional or agri-foodstuff compositions

5-*O*-glucosid-6''-*O*-malonyltransferase *Salvia splendens* Suzuki et al., 2001

Table 1. Acyltransferase catalysis of flavonoid acylation and their nature sources.

To date, the use of proteases, esterases, acyltransferases and lipases has been investigated in order to find the most potent biocatalyst for selective flavonoid acylation. These enzymes are often in the immobilized form which improves enzyme stability, facilitates product isolation, and enables enzyme reuse (Adamczak & Krishna, 2004).
