395
Some derivatives of hydroxybenzoic acid includes protocatechuic acid, gallic
acid and p-hydroxybenzoic acid. Hydroxycinnamic acids, is however more common than hydroxybenzoic acid has some of its derivatives as caffeic acid, chlorogenic acid, coumaric acid, ferulic acid and sinapic acid. These derivatives are rarely
found in free forms but they occur in fermented, frozen or sterilized foods. Berry
fruits, apple, chicory, cherry, kiwi, coffee and pear are the foods with high levels of
these phenolic acids (Manach et al. 2004).
Stilbenes, one of the sub-classes of flavonoids occur in smaller amount in human
diet. Resveratrol (Fig. 19.1) which is one of the well-studied compounds of this
group is largely found in red wine and grapes (Manach et al. 2004; Adlercreutz
2007; Pandey and Rizvi 2009). Chun et al. (2007) and Ovaskainen et al. (2008)
reported dietary intake of polyphenols is estimated to be approximately 1 g/day. The
availability of the important dietary components depend on their food source preparatory methods, their digestions in the gastrointestinal walls, their absorption rates
as well as their metabolism in the body systems (Scalbert and Williamson 2000). In
the absorption pathway of polyphenols, there must be colonic microflora or intestinal enzyme action in way of hydrolysis, which should be followed by conjugation
in the intestinal cells and later glucuronidation, methylation or sulfation by the liver
(Scalbert et al. 2002). Polyphenols, thereafter, accumulate in their target tissues to
induce biological properties and the excretion of these phenol derivatives majorly
occur in through urine and the bile (Scalbert et al. 2002).
19.4 Biosynthesis of Polyphenols
The shikimate pathway is critical to the synthesis of amino acids, which are further
acted upon by specific enzymes for the biosynthesis of phenolics, lignins and flavonoids (Tijjani et al. 2018). Other polyphenols obtained from the shikimate pathway
include phenolic compounds such as cinnamic acid and gallic acid (Tsao 2010). The
aromatic ring B of polyphenols and the chromane ring are derived from phenylalanine obtained from the shikimate pathway while ring A are obtained from the condensation of three units of malonylCoA (Fatland et al. 2004; Tsao and McCallum
2009). Phenylalanine is deaminated by the enzyme phenylalanine ammonia lyase to
trans cinnamic acid (Fig. 19.3). Cinnamate-4-hydroxylase catalyze the conversion
of trans cinnamic acid to form p-coumaric acid. CoASH is added by the enzyme
4-coumaroyl CoA ligase to form p- coumaroyl CoA (Bohm 1998; Tsao and
McCallum 2009). The p-coumaroyl-CoA formed then condenses with three molecules of malonyl-CoA to form chalcone catalyzed by the enzyme chalcone synthase. The enzyme chalcone flavanone isomerase isomerize chalcone into a
flavanone which could be further converted to dihydroflavonols or to flavan-3,
4-diols (Bohm 1998; Tsao and McCallum 2009). The glycosylation of flavonoids by
the enzyme glucosyltransferase provide addition of a sugar unit to the flavonoid
(Bohm 1998; Tsao and McCallum 2009).
19 Polyphenols: Classifications, Biosynthesis and Bioactivities
Some derivatives of hydroxybenzoic acid includes protocatechuic acid, gallic
acid and p-hydroxybenzoic acid. Hydroxycinnamic acids, is however more common than hydroxybenzoic acid has some of its derivatives as caffeic acid, chlorogenic acid, coumaric acid, ferulic acid and sinapic acid. These derivatives are rarely
found in free forms but they occur in fermented, frozen or sterilized foods. Berry
fruits, apple, chicory, cherry, kiwi, coffee and pear are the foods with high levels of
these phenolic acids (Manach et al. 2004).
Stilbenes, one of the sub-classes of flavonoids occur in smaller amount in human
diet. Resveratrol (Fig. 19.1) which is one of the well-studied compounds of this
group is largely found in red wine and grapes (Manach et al. 2004; Adlercreutz
2007; Pandey and Rizvi 2009). Chun et al. (2007) and Ovaskainen et al. (2008)
reported dietary intake of polyphenols is estimated to be approximately 1 g/day. The
availability of the important dietary components depend on their food source preparatory methods, their digestions in the gastrointestinal walls, their absorption rates
as well as their metabolism in the body systems (Scalbert and Williamson 2000). In
the absorption pathway of polyphenols, there must be colonic microflora or intestinal enzyme action in way of hydrolysis, which should be followed by conjugation
in the intestinal cells and later glucuronidation, methylation or sulfation by the liver
(Scalbert et al. 2002). Polyphenols, thereafter, accumulate in their target tissues to
induce biological properties and the excretion of these phenol derivatives majorly
occur in through urine and the bile (Scalbert et al. 2002).
19.4 Biosynthesis of Polyphenols
The shikimate pathway is critical to the synthesis of amino acids, which are further
acted upon by specific enzymes for the biosynthesis of phenolics, lignins and flavonoids (Tijjani et al. 2018). Other polyphenols obtained from the shikimate pathway
include phenolic compounds such as cinnamic acid and gallic acid (Tsao 2010). The
aromatic ring B of polyphenols and the chromane ring are derived from phenylalanine obtained from the shikimate pathway while ring A are obtained from the condensation of three units of malonylCoA (Fatland et al. 2004; Tsao and McCallum
2009). Phenylalanine is deaminated by the enzyme phenylalanine ammonia lyase to
trans cinnamic acid (Fig. 19.3). Cinnamate-4-hydroxylase catalyze the conversion
of trans cinnamic acid to form p-coumaric acid. CoASH is added by the enzyme
4-coumaroyl CoA ligase to form p- coumaroyl CoA (Bohm 1998; Tsao and
McCallum 2009). The p-coumaroyl-CoA formed then condenses with three molecules of malonyl-CoA to form chalcone catalyzed by the enzyme chalcone synthase. The enzyme chalcone flavanone isomerase isomerize chalcone into a
flavanone which could be further converted to dihydroflavonols or to flavan-3,
4-diols (Bohm 1998; Tsao and McCallum 2009). The glycosylation of flavonoids by
the enzyme glucosyltransferase provide addition of a sugar unit to the flavonoid
(Bohm 1998; Tsao and McCallum 2009).
19 Polyphenols: Classifications, Biosynthesis and Bioactivities