172
11
What You Will Learn in This Chapter
Aromatic amino acids are produced via the shikimate pathway in the plastids, which are the
precursors of phenylpropanoids. Phenylpropanoids contribute to the taste of many plantderived food, but their interaction with the human body is in many cases not well documented. Many of these compounds act as precursors for alkaloids.
11.1 Shikimate Pathway (Phenylalanine, Tyrosine and Tryptophan)
More than 8000 aromatic metabolites of the phenylpropanoid pathway have been identified
in plants. They have various functions in plant growth, development, plant- environment
interactions and protection from biotic and abiotic stresses (Fraser and Chapple 2011;
Quideau et al. 2011; Cheynier et al. 2013; Brunetti et al. 2015). The phenylpropanoid pathway assimilates about 30–40% of the organic carbon on earth, most of it as the lignin
polymer in the cell walls.
Phenylalanine is the most important entry point into this pathway, itself being synthesized by the so-called the shikimate pathway (. Fig. 11.1). The shikimate pathway is
localized in plastids and therefore absent in animals (Tzin and Galili 2010; Tohge et al.
2013). The starting points are phosphoenolpyruvate (from the glycolysis) and erythrose
4-phosphate (from the pentose phosphate pathway). The first intermediate is 3-deoxy-Darabino-heptulosonate 7-phosphate (DAHP). The DAHP synthase is the first committing
enzyme of the shikimate pathway; as a key enzyme, it regulates the metabolic flux into
this pathway (Tzin et al. 2012). The next major intermediates are shikimate and shikimate
3-phosphate. The 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase and the chorismate synthase catalyse the synthesis of chorismate. EPSP is known to be the target of
the herbicide glyphosate (Roundup) (Schonbrunn et al. 2001). Dehydroshikimate leads to
gallic acid and further to gallotannins. Chorismate serves as a common precursor for the
synthesis of ubiquinone via hydroxybenzoate, phylloquinone (Vitamin K1) via isochorismate, and tetrahydrofolate (Vitamin B9) via aminobenzoate.
The three aromatic amino acids phenylalanine (Phe), tyrosine (Tyr) and tryptophan
(Trp) are products of this pathway. As they are produced in plastids and humans and
animals cannot produce them themselves, they have to be taken up with food (“essential
amino acids”). Anthranilate is the main precursor for tryptophan. Prephenate leads to the
biosynthesis of tyrosine and phenylalanine mainly via arogenate but also via phenylpyruvate to form phenylalanine or hydroxyphenylpyruvate to form tyrosine. At this stage, the
primary metabolism is ending, and the secondary metabolism is beginning.
Tryptophan is a precursor for alkaloids via tryptamine (indole alkaloids, see 7 Sect. 12.2
in 7 Chap. 12), for auxin (IAA), for camalexin and for glucosinolates. Tyrosine can lead
to the formation of cyanogenic glucosides (such as dhurrin), dopa, betalains, tocopherols (vitamin E) and plastoquinone (via 4-hydroxyphenylpyruvate). Tyrosine can also be
deaminated to tyramine, which is a precursor for suberin, hydroxycinnamate amide and
the isoquinoline alkaloids (see 7 Sect. 12.3 in 7 Chap. 12). Although tyrosine can interchange with phenylalanine, phenylalanine is thought to be the main pathway to most secondary metabolites. Extensive regulation and feed-back control regulates the abundancy
of the three amino acids and their flow into the downstream metabolic pathways (Tzin
and Galili 2010; Tohge et al. 2013).
Phenylalanine is transferred from the plastid to the cytosol where it undergoes further
reactions (. Fig. 11.2). The committing enzyme for phenylpropanoid biosynthesis is the
Chapter 11 · Phenylpropanoids
11
What You Will Learn in This Chapter
Aromatic amino acids are produced via the shikimate pathway in the plastids, which are the
precursors of phenylpropanoids. Phenylpropanoids contribute to the taste of many plantderived food, but their interaction with the human body is in many cases not well documented. Many of these compounds act as precursors for alkaloids.
11.1 Shikimate Pathway (Phenylalanine, Tyrosine and Tryptophan)
More than 8000 aromatic metabolites of the phenylpropanoid pathway have been identified
in plants. They have various functions in plant growth, development, plant- environment
interactions and protection from biotic and abiotic stresses (Fraser and Chapple 2011;
Quideau et al. 2011; Cheynier et al. 2013; Brunetti et al. 2015). The phenylpropanoid pathway assimilates about 30–40% of the organic carbon on earth, most of it as the lignin
polymer in the cell walls.
Phenylalanine is the most important entry point into this pathway, itself being synthesized by the so-called the shikimate pathway (. Fig. 11.1). The shikimate pathway is
localized in plastids and therefore absent in animals (Tzin and Galili 2010; Tohge et al.
2013). The starting points are phosphoenolpyruvate (from the glycolysis) and erythrose
4-phosphate (from the pentose phosphate pathway). The first intermediate is 3-deoxy-Darabino-heptulosonate 7-phosphate (DAHP). The DAHP synthase is the first committing
enzyme of the shikimate pathway; as a key enzyme, it regulates the metabolic flux into
this pathway (Tzin et al. 2012). The next major intermediates are shikimate and shikimate
3-phosphate. The 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase and the chorismate synthase catalyse the synthesis of chorismate. EPSP is known to be the target of
the herbicide glyphosate (Roundup) (Schonbrunn et al. 2001). Dehydroshikimate leads to
gallic acid and further to gallotannins. Chorismate serves as a common precursor for the
synthesis of ubiquinone via hydroxybenzoate, phylloquinone (Vitamin K1) via isochorismate, and tetrahydrofolate (Vitamin B9) via aminobenzoate.
The three aromatic amino acids phenylalanine (Phe), tyrosine (Tyr) and tryptophan
(Trp) are products of this pathway. As they are produced in plastids and humans and
animals cannot produce them themselves, they have to be taken up with food (“essential
amino acids”). Anthranilate is the main precursor for tryptophan. Prephenate leads to the
biosynthesis of tyrosine and phenylalanine mainly via arogenate but also via phenylpyruvate to form phenylalanine or hydroxyphenylpyruvate to form tyrosine. At this stage, the
primary metabolism is ending, and the secondary metabolism is beginning.
Tryptophan is a precursor for alkaloids via tryptamine (indole alkaloids, see 7 Sect. 12.2
in 7 Chap. 12), for auxin (IAA), for camalexin and for glucosinolates. Tyrosine can lead
to the formation of cyanogenic glucosides (such as dhurrin), dopa, betalains, tocopherols (vitamin E) and plastoquinone (via 4-hydroxyphenylpyruvate). Tyrosine can also be
deaminated to tyramine, which is a precursor for suberin, hydroxycinnamate amide and
the isoquinoline alkaloids (see 7 Sect. 12.3 in 7 Chap. 12). Although tyrosine can interchange with phenylalanine, phenylalanine is thought to be the main pathway to most secondary metabolites. Extensive regulation and feed-back control regulates the abundancy
of the three amino acids and their flow into the downstream metabolic pathways (Tzin
and Galili 2010; Tohge et al. 2013).
Phenylalanine is transferred from the plastid to the cytosol where it undergoes further
reactions (. Fig. 11.2). The committing enzyme for phenylpropanoid biosynthesis is the
Chapter 11 · Phenylpropanoids