L-Phenylalanine and L-tyrosine, as C6C3 building blocks, are precursors for a wide range of natural products. In plants, a frequent first step is the elimination of ammonia from the side-chain to generate the appropriate trans (E) cinnamic acid. In the case of phenylalanine, this would give cinnamic acid, whilst tyrosine could yield 4-coumaric acid (p-coumaric acid) (Figure 4.15). All plants appear to have the ability to deami-nate phenylalanine via the enzyme phenylalanine ammonia lyase (PAL), but the corresponding transformation of tyrosine is more restricted, being mainly limited to members of the grass family (the Graminae/Poaceae). Whether a separate enzyme tyrosine ammonia lyase (TAL) exists, or whether grasses merely have a broad specificity PAL also
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