4- Amino-3-hydroxycinnamic acid synthesis

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

Biosynthesis. The primary precursors of L. are con-iferyl, sinapyl and p-coumaiyl alcohoi, which are derived from 4-hydroxycinnamic acid. L. from conifers (i. e. from softwood) is derived chiefly from conifeiyl alcohol with variable but small proportions of sinapyl and p-coumaryl alcohol. L. from dicotyledonous an-giosperms (i.e. from hardwood), particularly deciduous trees, is formed chiefly from sinapyl (-44%) and coniferyl (-48 %) alcohol, with about 8 % p-cou-maryl alcohol. L. in grasses is formed from p-coumar-yl (-30%), coniferyl (-50%) and sinapyl (-20%) alcohol. These primary L. precursors are formed from the aromatic amino acids L-phenylalanine and L-tyro-sine by a series of reactions shown (Rg.). The first reaction is catalysed by L-Phenylalanine ammonia-lyase (EC 4.3.1.5) (see) this enzyme is induced by light in a process involving phytochrome, and it is of general importance in the synthesis of plant phenolic compounds from phenylalanine and tyrosine. 

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