Hydroxycinnamic metabolism

The colonic microflora are a central site of hydroxycinnamate metabolism in addition to the small intestine and liver. The majority of ingested hydroxy-cinnamates from dietary sources reach the colon, where they are exposed to the... 

The metabolism of ferulic acid (3-methoxy-4-hydroxycinnamic acid) by Ent. cloacae to a nnmber of products including phenylpropionate and benzoate (Fignre 2.8) (Grbic-Galic 1986). 

Gasson Ml, Y Kitamura, WR McLaughlan, A Narbad, AJ Parr, ELH Parsons, J Payne, MJC Rhodes, NK Walton (1998) Metabolism of ferulic acid to vanillin. A bacterial gene of the enoyl-SCoA hydratase/ isomerase superfamily encodes an enzyme for the hydration and cleavage of a hydroxycinnamic acid SCoA thioester. J Biol Chem 273 4163-4170. 

The key reaction that links primary and secondary metabolism is provided by the enzyme phenylalanine ammonia lyase (PAL) which catalyzes the deamination of l-phenylalanine to form iran.v-cinnamic acid with the release of NH3 (see Fig. 3.3). Tyrosine is similarly deaminated by tyrosine ammonia lyase (TAL) to produce 4-hydroxycinnamic acid and NH3. The released NH3 is probably fixed by the glutamine synthetase reaction. These deaminations initiate the main phenylpropanoid pathway. 

Some of the pathways of animal and bacterial metabolism of aromatic amino acids also are used in plants. However, quantitatively more important are the reactions of the phenylpropanoid pathway,173-1743 which is initiated by phenylalanine ammonia-lyase (Eq. 14-45).175 As is shown at the top of Fig. 25-8, the initial product from phenylalanine is trails-cinnam-ate. After hydroxylation to 4-hydroxycinnamate (p-coumarate) and conversion to a coenzyme A ester,1753 the resulting p-coumaryl-CoA is converted into mono-, di-, and trihydroxy derivatives including anthocyanins (Box 21-E) and other flavonoid compounds.176 The dihydroxy and trihydroxy methylated products are the starting materials for formation of lignins and for a large series of other plant products, many of which impart characteristic fragrances. Some of these are illustrated in Fig. 25-8. 

The shikimate/arogenate pathway leads to the formation of three aromatic amino acids L-phenylalanine, L-tyrosine, and L-tryptophane. This amino acids are precursors of certain homones (auxins) and of several secondary compounds, including phenolics [6,7]. One shikimate/arogenate is thought to be located in chloroplasts in which the aromatic amino acids are produced mainly for protein biosynthesis, whereas the second is probably membrane associated in the cytosol, in which L-phenylalanine is also produced for the formation of the phenylpropanoids [7]. Once L-phenylalanine has been synthesized, the pathway called phenylalanine/hydroxycinnamate begins, this being defined as "general phenylpropanoid metabolism" [7]. 

Studies have shown that phenylpropanoid metabolism can be stimulated by ozone. The activity of PAL increased in soybean [91], Scots pine (Pinus sylvestris L.) [92], and parsley (Petroselinum crispum L.) [93] soon after treatment with 150-200 nmol O3 mol 1. Rapid increases in transcript levels for PAL in response to ozone have been observed in parsley [93], Arabidopsis thaliana L. Heynhold [94] and tobacco (Nicoticma tabacum L.) [95]. Transcript levels for 4-coumarate CoA ligase (4CL), the last enzyme in the general phenylpropanoid pathway, increased commensurately with PAL transcripts in ozone-treated parsley seedlings [93]. Phenolic compunds reported to accumulate in leaf tissue in response to ozone include hydroxycinnamic acids, salicylic acid, stilbenes, flavonoids, furanocoumarins, acetophenones, and proanthocyanidins [85, 92, 93, 96, 97]. 

Heresztyn, T. (1986). Metabolism of phenolic compounds from hydroxycinnamic acids by Bret-tanomyces yeasts. Arch. Microbiol., 146, 96-98. 

As early as 1964 it was recognized that 4-ethyl phenol and 4-ethyl guaiacol were produced by yeast and bacteria during fermentation by the decarboxylation of the hydroxyciimamic acids p-coumaric and fendic acid (88). Later it was reported that among yeast only Brettanomyces species possess the metabolic ability to enzymatically decarboxylate hydroxycinnamic acids to produce ethyl derivatives (29, 89). Heresztyn was the first to identify 4-ethyl phenol and 4t-ethyl guaiacol as the major volatile phenolic compounds formed by Brettanomyces yeast (84). ... 

Lactic acid bacteria, including the typical "wine lactic acid bacteria" Leuconostoc oenos (85, 90), can produce ethyl and vinyl derivatives by hydroxycinnamic acid metabolism (91) although, the minimal concentration produced in red wines by Leuconostoc oenos is insignificant compared to the odor threshold (85, 87). 

Keywords phenylpropanoid metabolism hydroxycinnamic acid conjugates lignans coumarins furanocoumarins gallotannins eUagitarmins metabolic channelling... 

Davin, F.B. and Fewis, N.G. (2003) An historical perspective on lignan biosynthesis monolignol, allylphenol and hydroxycinnamic acid coupling and downstream metabolism. Phytochemistry Rev., 2, 257-88. 

Strack, D. and Mock, H.P. (1993) Hydroxycinnamic acids and lignins, in Methods in Plant Biochemistry, Vol. 9, Enzymes of Secondary Metabolism (eds P.M. Dey and J.B. Harborne). Academic Press, London, pp. 45-97. 

It would be instructive to compare the metabolic cost of synthesizing and allocating specific flavonoids, salicylic and hydroxycinnamic acids and their efficacy in filtering biologically active UV radiation. This information could explain their ecological roles in the adaptation or acclimation process. 

Polyphenols include flavonoids, proanthocyanidins, stilbenes, microbial metabolites of lignan, and hydroxycinnamates (Fig. 2). Flavonoid metabolism, while still far from being fully understood, has been the most widely studied and will therefore form the basis of this chapter. Six main subclasses of flavonoids are widely consumed by humans flavonols, flavones, flavanones, isoflavonoids, flavanols (catechins), and anthocyanins these posses the generic structure shown in Fig. 3. These classes differ in the degree of saturation and the nature and position of reactive groups on their three rings examples of substitution patterns for selected flavonoids are given in Table 1. 

---