Lignin precursors ammonia-lyase

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

The biosynthetic pathway for isoflavonoids in soybean and the relationship of the isoflavonoids to several other classes of phenylpropanoids is presented in Fig. 8.2. Production of /i-coumaryl-CoA from phenylalanine requires phenylalanine ammonia lyase to convert phenylalanine to cinnamate, cinnamic acid hydroxylase to convert cinnamate to /7-coumarate, and coumaraterCoA ligase to convert jt -coumarate to -coumaroyl-CoA. Lignins may be produced from j3-coumaroyl-CoA or from />-coumarate. Chalcone synthase catalyzes the condensation of three molecules of malonyl CoA with p-coumaroyl-CoA to form 4, 2 , 4 , 6 -tetrahydroxychalcone, which is subsequently isomerized in a reaction catalyzed by chalcone isomerase to naringenin, the precursor to genistein, flavones, flavonols, condensed tannins, anthocyanins, and others. 

Under the action of phenylalanine ammonia lyase (EC 4.3.1.5) Phe is transformed to rrans- cinnamic acid. This plays an important role in plants as a precursor of numerous phenolic secondary products such as flav-onoids, lignin, etc. D-Phe is a component of grami-cidin S and tyrocidines. L-Phe tastes weakly bitter, D-Phe tastes sweet. 

Lignin precursors p-coumaryl (p-CA), coniferyl (CA) and sinapyl (SA) alcohols, are synthesized through the shikimate and cinamic acid pathways, starting from phenylalanine which under the action of phenylalanine ammonia-lyase (PAL) is deaminated followed by hydroxylations of the aromatic ring, methyla-tions and reductions of therminal acidic group to an alcohol leading to the formation of the monolignols (Fig. 8.2), [37, 48]. 

Higher plants accumulate a wide variety of phenolic substances, including lignins and flavonoids, which are biogenetically derived from the common precursor cinnamic acid. The enzyme, which catalyzes the formation of cinnamic acid from phenylalanine, phenylalanine ammonia-lyase (PAL,sc 4.3,1,5), is responsible for channeling a considerable amount of carbon from primary metabolism (shikimate pathway) into these secondary products. Be-... 

Further work on the mode of formation of the phenylpropanoid precursors of lignin in higher plants has also been published. Confirmatory observations on the stereochemical mode of action of tyrosine ammonia lyase have been made and a review of the biochemical properties of phenylalanine ammonia lyase has been compiled by Towers and Camm . Zenk and his group . using a cell free system from cambial tissue of Salix alba and a cell free preparation from phytotron grown Forsythia species, have demonstrated for the first time the reduction of ferulate (7) to coniferyl alcohol (8) in a higher plant. The conversion is dependent on ATP, co-enzyme A and reduced pyridine nucleotides and Zenk has formulated the reaction sequence as shown below (7 - 8). 

Coniferyl alochol is a plant metabolite that is a precursor to both lignin and a number of flavor constituents. It arises from action of specific lyases, which cleave out ammonia from phenylalanine and tyrosine (Figure 21.12). 

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