Phenylacetaldehyde synthase

Kaminaga, Y, Schnepp, ]., Peel, G., Kish, C.M., Ben-Nissan, G., Weiss, D., Orlova, L, Lavie, O., Rhodes, D., Wood, K., Porterfield, D.M., Cooper, A.J.L., Schloss, J.V., Pichersky, E., Vainstein, A. and Dudareva, N. (2006) Plant phenylacetaldehyde synthase is a bifunctional homotetrameric enzyme that catalyzes phenylalanine decarboxylation and oxidation.. Biol. Chem., 281,23357-66. 

The biosynthesis of other volatile phenyl-propanoid-related compoimds such as phenyla-cetaldehyde and 2-phenylethanol, does not occur via trans-cinnamic acid and competes with PAL for Phe utilization [90, 96, 97]. Phenylacetaldehyde biosynthesis from Phe requires the removal of both the carboxyl and amino groups. A classical sequential two-step removal is believed to occur in tomato where Phe was shown to be first converted to phe-nylethylamine by aromatic amino acid decarboxylase (AADC) and further required the action of a hypothesized amine oxidase, dehydrogenase, or transaminase for phenylacetaldehyde formation [97]. On the other hand, in petunia, one bifunctional enzyme, phenylacetaldehyde synthase (PAAS) catalyzes the unprecedented efficient coupling of Phe decarboxylation to oxidation resulting in... 

An aldehyde, 4-hydroxy-phenylacetaldehyde, operating under the influence of the enzyme norcoclaurine synthase, condenses with dopamine to afford (5)-nor-coclaurine, the progenitor of all benzylisoquinoline... 

Norcoclaurine synthase catalyzes the coupling of dopamine with p-hydroxy-phenylacetaldehyde, a step in morphine biosynthesis. 

STEP 3 OF FIGURE 25.6 COUPLING The coupling of dopamine andp-hydroxy-phenylacetaldehyde is catalyzed by (S)-norcoclaurine synthase and is relatively straightforward. The reaction proceeds through initial formation of an intermediate iminium ion, followed by intramolecular electrophilic aromatic substitution at a position para to one of the hydroxyl groups (Figure 25.9). 

---