Thesis-Final 03 June 2011 pdf - Jacobs University

Background Chapter 2
Biosynthesis of phenolic compounds in plants
Phenolics display a wide variety of structures, ranging from simple moieties
containing a single hydroxylated aromatic ring to highly complex polymeric
substances. 27,28 The biosynthetic pathways of phenolic compounds in plants are well
established. 29 The biosynthetic pathways of some flavonols and phenolic acids are
shown in Figure 6. The biosynthesis and accumulation of secondary compounds can
be an endogenously controlled process during developmental differentiation or it can
be regulated by exogenous factors such as light, temperature and wounding.
Phenylalanine, produced in plants via the shikimate pathway, is a common precursor
for most phenolic compounds in higher plants (Figure 6). Similarly, hydroxycinnamic
acids, and particularly their coenzyme A esters, are common structural elements of
phenolic compounds, such as cinnamate esters and amides, lignin, flavonoids and
condensed tannins 30 (Figure 6). The phenylalanine/hydroxycinnamate pathway is
defined as general phenylpropanoid metabolism. It includes reactions leading from L-
phenylalanine to the hydroxycinnamates and their activated forms.The enzymes
catalysing the individual steps in general phenylpropanoid metabolism are
phenylalanine ammonialyase (PAL), cinnamic acid 4-hydroxylase (CA4H), and
hydroxycinnamate: coenzyme A ligase (C4L). These three steps are necessary for the
biosynthesis of phenolic compounds. A growing body of evidence indicates that
phenylpropanoid and flavonoid pathways are catalysed by several membraneassociated
multienzyme complexes. 31
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