Phenylalanine ammonia lyase (PAL) represents the first committed step to phenylpropanoid metabolism, and thus to a wide range of phenolic products (lignins, lignans, hydroxycin — namic acids, flavonoids, suberins, etc.). Discovered by Koukol and Conn (84) in 1961, this enzymatic conversion required no added cofactor. Moreover, the ammonium ion released during the deamination step had also been subsequently demonstrated to be recycled via GS/GOGAT, with the glutamate so formed then serving as the amino donor for arogenate (35) formation (Figure 7.8) (85-88). In this way, the means to both recycle the nitrogen and sustain phenylpropanoid metabolism can occur without any apparent further need for an additional nitrogen source. A gene encoding a PAL was reported by Edwards et al. in 1985 (89); an earlier description by others (90, 91) of PAL cloning was later shown not to be PAL (89, 92).

In Arabidopsis, there are four bona fide PAL gene family members present [as determined by both analysis of the genome sequence (93), and also by recombinant protein characteri­zation (94)]. In terms of its biochemical mechanism, the X-ray crystal structure of PAL was also recently described (95); additionally, it was deduced that a highly conserved tripep­tide sequence (Ala-Ser-Gly) in PAL undergoes spontaneous dehydration/cyclization to give MIO (3,5 dihydro-5-methylidene-4H-imidazol-4-one) (95) based on earlier studies with histidine ammonia lyase (HAL) (96, 97). MIO is envisaged to serve as an internal cofactor for catalysis. Interestingly, other studies using both tyrosine ammonia lyase (TAL) (from Rhodobacter sphaeroides) (98) and PAL (from Arabidopsis) (99) have demonstrated that the interconversion of PAL to TAL activity can be effectuated by a single amino acid substitu­tion (i. e., H89F converts TAL to PAL in R. sphaeroides, whereas in Arabidopsis, the PAL/TAL switch is F144H).