Phenylalanine ammonia lyase catalyzes the initial reaction of phenylpropanoid metabolism
Phenylalanine ammonia lyase, abbreviated PAL, catalyzes a deamination of phenylalanine (Fig. 18.2): a carbon-carbon double bond is formed during the release of NH3, yielding trans-cinnamic acid. In some grasses, tyrosine is converted to 4-hydroxycinnamic acid in an analogous way by tyrosine ammonia lyase. The released NH3 is probably refixed by the glutamine syn-thetase reaction.
PAL is one of the most intensively studied enzymes of plant second-ary metabolism. The enzyme consists of a tetramer with subunits of 77 to 83 kDa. The formation of phenylpropanoid phytolalexins after fungal infection involves a very rapid induction of PAL. PAL is inhibited by its product trans-cinnamic acid. The phenylalanine analogue aminoxyphenyl-propionic acid (Fig. 18.3) is also a very potent inhibitor of PAL.