Polyphenols are naturally occurring compounds that contain multiple phenolic groups and are often found in plants. Polyphenols with antioxidant ability 1, 2 and protein–ligand interactions have been used as drugs to treat diseases such as AIDS, cancer, and heart ailments [3]. In addition, polyphenols are often used as the major ingredients in the nutraceuticals. Given the high applicability of polyphenolic compounds, purification and analysis of new polyphenolics, subsequent polyphenol modifications, and their overproduction have been extensively studied in recent years [4]. As polyphenols have repeated structures and multiple hydroxyl groups, regioselective modification via chemical methods is very difficult. Therefore, regiospecific enzymatic reactions are an alternative for polyphenol modification [5]. To understand the properties of such enzymes in detail, heterologous expression of recombinant enzymes using Escherichia coli is the most common strategy for acquiring sufficient products for further analysis. Furthermore, protein structural analysis and mutation studies help elucidate the enzymatic mechanisms in detail. These results are useful for improving the proper functional activity of the enzyme, thus leading to higher product yield and efficiency. In this review, trends and recent findings related to two major polyphenol modifications — that is, hydroxylation and glycosylation, are discussed, and the associated enzymes are introduced.

Regioselective polyphenol hydroxylation leads to the formation of very interesting products. Depending on selectivity, three types of hydroxylated products are obtained: ortho-, meta-, and para-hydroxylated polyphenols. Previously, o-hydroxylation was extensively studied with cytochrome P450. However, polyphenol oxidase (PPO) has attracted increasing attention due to its high specificity and much greater productivity than P450 as well as does not require cofactor reduced form of nicotinamide adenine dinucleotide phosphate (NAD(P)H) [6]. To date, various enzymes responsible for m- and p-hydroxylation reactions have been reported. Among them, cytochrome P450 and monooxygenase have emerged as the most promising and interesting enzymes owing to their regio- and stereospecific features.

Glycosylated polyphenols can be obtained using three main strategies. First, polyphenols and nucleotide diphosphate (NDP)-sugars are individually synthesized and combined in a modified cell using a metabolic engineering approach. Second, to supply sufficient polyphenols as glycan acceptors, polyphenols are fed into the cell, with endogenous NDP-sugars used as glycan donors. Finally, aglycones of polyphenol and sucrose are supplied as either exogenous or endogenous substrates. Thus, by using the amylosucrase (AS) reaction, glycosylated polyphenols are generated via transglycosylation. The AS method has attracted substantial attention for its utility in mass industrial production as it requires sucrose, which is inexpensive, rather than expensive NDP-sugars [7]. Herein, we focus on recent findings regarding enzymatic hydroxylation and glycosylation of polyphenols using PPOs, P450, and AS.