Trans-cinnamic acid hydroxylation

The coumarins are synthesized after an o-hydroxyl group is added to a cinnamic acid derivative. The o-hydroxyl group subsequently undergoes glucosylation, which facilitates the isomerization of trans-cinnamic acid derivatives to the cis form. The hydrolysis of the sugar then leads to spontaneous cyclization. 

The possible pathway for the biosynthesis of benzoylacetic acid as building block of phenolic side chain is the following (Fig. 11.18). Phenylalanine is converted into trans-Cinnamic acid by the enzyme phenylalanine ammonia-lyase (PAL). Hydroxylation of Cinnamic acid gave 3-hydroxy-3-phenylpropanoic acid which is transformed to benzoylacetic acid by (3-oxidation (chain degradation). 

Benzoic and salicylic acids. Two of the simplest plant acids arising from trans-cinnamate are benzoic acid, accumulated in plums and cranberries, and salicylic acid, present in all green plants and accumulated as methyl esters or glycosides in some plants, e.g., those of the willow family. Salicylic acid is made by hydroxylation of benzoic acid, which can be formed from trans-cinnamate by P oxidation as depicted in Fig. 25-8, but it may also arise from isochoris-mate as shown in Fig. 25-2. Salicylic acid plays a central role in resistance of plants to a variety of... 

Cinnamate 4-hydroxylases catalyze the hydroxylation of frans-cinnamic acid into trans-p-coumaric acid. The ability to monitor this enzyme activity in Jerusalem artichoke allowed isolation of the P450 enzyme CYP73A1 using conventional chromatography and generation of specific antibodies . ... 

Cinnamic acid 4-hydroxyla a (EC 1.14.13.11) a mixed function monooxygenase, present in plants, which catalyses an early reaction in Flavonoid (see) biosynthesis, i.e. the insertion of an atom of oxygen into cinnamic acid to form 4-hydroxydnnamic acid (4-coumaric acid) with concomitant oxidation of one molecule of NADPH. The enzyme is a cytochrome P4S0 system associated with the microsom fraction, and is specific for the trans isomer of cinnamic acid. During the hydroxylation, hydrogen at position 4 (experimentally tritium in position 4) is retained, i.e. there is an NIH shift (see). In vitro, a thiol, e.g. 2-mercaptoethanol, is required for activity. (P.R.Rich C.J.Lamb Ear. J.Biochem. 72 (1977) 353-360)... 

The coumarins in question are derivatives of o-coumaric acid. Cinnamic acids which may carry hydroxy and methoxy groups, are hydroxylated in the o-posi-tion to give the coumarins shown in Table 49. An important step in this biosynthetic pathway is a light-dependent, nonenzymatic cis-trans-isomerization of the cinnamic acid side chain. Glucosylated derivatives are important intermediates. Glucosides of coumarinic acid derivatives predominate in fresh plants. Cleavage of glucose and formation of coumarins usually is the result of the break down of... 

The further biosynthetic pathway to anthocyanins involves the formation of L-phenylalanine from chorismic acid. L-phenylalanine is then converted to trans-cmrL mic acid through fra 5-elimination of ammonia. In a second step fra 5-cinnamic acid is hydroxylated and activated to yield 4-coumaroyl-CoA. In the next step, 4-coumaroyl-CoA is condensed with 3 molecules of malonyl-CoA to yield naringenin chalcone. naringenin chalcone is rapidly and stereospecifically isomerized to naringenin. 

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