# An Overview of Bioactive Flavonoids from _Citrus_ Fruits
## 3 _Citrus_ Flavonoids: Structure, Classification and Biosynthesis
### Biosynthesis of Flavonoids  
The flavonoidoid pathway is preceded by the general phenylpropanol pathway, in which three enzymes are involved in the conversion of the amino acid phenylalanine to 4-coumaroyl-CoA. the first enzyme, phenylalanine ammonia lyase (PAL:EC 4.3.1.24) catalyzes the conversion of the amino acid phenylalanine to _trans_-cinnamic acid, with the release of ammonia (NH3), then the two other enzymes (the enzyme cinnamate 4 \begin{table}
\begin{tabular}{c c c c} \hline \hline _Citrus Flavonoids_ & **Structure Backbone** & **Examples** & \\ \hline Flavanones & & & Hesperetin & R\({}_{1}\)=O-CH\({}_{3}\) \\ \cline{3-4}  & & & R\({}_{2}\)=OH \\ \cline{3-4}  & & & Naringenin & R\({}_{1}\)=OH \\ \cline{3-4}  & & & R\({}_{2}\)=H \\ \hline Flavanones glycosides & & & R\({}_{2}\) \\ \cline{3-4}  & & & R\({}_{1}\)=O-CH\({}_{3}\) \\ \cline{3-4}  & & & R\({}_{2}\)=H \\ \cline{3-4}  & & & R\({}_{3}\)=7-O-rutinoside \\ \cline{3-4}  & & & R\({}_{1}\)=OH \\ \cline{3-4}  & & & R\({}_{2}\)=H \\ \cline{3-4}  & & & R\({}_{3}\)=O-7-Rhamnoglucoside \\ \hline Flavones & & & R\({}_{1}\)=OH \\ \cline{3-4}  & & & R\({}_{2}\)=R\({}_{3}\)=H \\ \cline{3-4}  & & & R\({}_{1}\)=R\({}_{2}\)=OH \\ \cline{3-4}  & & & R\({}_{3}\)=H \\ \hline Polymethroxyated Flavones & & & \\ \cline{3-4}  & & & R\({}_{1}\)=H \\ \cline{3-4}  & & & R\({}_{2}\)=R\({}_{3}\)=OCH\({}_{3}\) \\ \cline{3-4}  & & & R\({}_{1}\)=R\({}_{3}\)=H \\ \cline{3-4}  & & & R\({}_{2}\)=OCH\({}_{3}\) \\ \hline Flavonols & & & \\ \cline{3-4}  & & & R\({}_{1}\)=R\({}_{2}\)=R\({}_{3}\)=OH \\ \cline{3-4}  & & & R\({}_{1}\)=R\({}_{3}\)=OH \\ \cline{3-4}  & & & R\({}_{2}\)=H \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Basic structures of Citrus flavonoid subclasses [16]. hydroxylase (C4H:EC 1.14.14.91), followed by 4-coumarate-CoA ligase (4 CL:EC 6.2.1.12)), catalyze the reaction that leads to obtain 4-coumaroyl-CoA, which is an important precursor in the flavonoid pathway [12; 13]. The biosynthesis of flavonoids is originated from the phenylpropanoids pathway and initiated by two precursors named malonyl-CoA and \(p\)-coumaroyl-CoA (Figure 1). After the condensation of three acetate units from malonyl-CoA with one molecule of \(p\)-coumaroyl-CoA, naringenin chalcones are formed. Naringenin chalcone, a major pigment of many flowers, leaves and fruits, is converted to naringenin by chalcone isomerase (CHI) or non-enzymatically in vitro [14; 15]. This reaction catalyzed by chalcone synthase (CHS:EC 2.3.1.74) is thought to be the key regulatory step in the synthesis of flavonoids. It catalyzes the stereospecific isomerization of chalcones to their corresponding (2S)-flavanones via an acid base catalysis mechanism; the unstable chalcone form is normally isomerized by the enzyme chalcone isomerase (CHI:EC 5.5.1.6) to form the structural precursors for a broad range of flavonoids, such as flavonoids, flavanones, anthocyanin glycosides and other derived compounds (Figure 1). Figure 1: Plant flavonoids biosynthetic pathway [15]. Enzymes for each step are indicated as follows: PAL, phenylalanine ammonia-lyase; C4H, cinnanate 4-hydroxylase; 4 CL, 4-coumarate-CoA ligase; CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; F3\({}^{\prime}\)H, flavonoid 3\({}^{\prime}\)-hydroxylase; DFR, dihydro-flavonol 4-reductase; FNS, flavonol synthase; FLS, flavonol synthase; LAR, leucoanthocyanidin reductase; ANS, anthocyanin synthase; UFGT, UDP-glucose: flavonoid-3-\(O\)-glycosyltransferase. (25)-Flavanones are converted stereospecifically to the respective (2,3R)-dihydroflavonols (DHFs) by flavanone 3-hydroxylase (F3H:EC 1.14.11.9) in this case to dihydrokaempfer, then converted by the enzyme flavonoid 3\({}^{\prime}\)-hydroxylase (F3\({}^{\prime}\)H:EC 1.14.14.82) to dihydroquercetin. In _Citrus_ species, the two dihydroflavonols are then converted into flavonols under the action of flavonoids synthetase (FLS:EC 1.14.20.6).