Plant aromatic L-amino acid decarboxylases: evolution, biochemistry, regulation, and metabolic engineering applications

A comprehensive survey of the extensive literature relevant to the evolutio n, physiology, biochemistry, regulation, and genetic engineering applicatio ns of plant aromatic L-amino acid decarboxylases (AADCs) is presented. AADC s catalyze the pyridoxal-5'-phosphate (PLP)-dependent decarboxylation of se lect aromatic L-amino acids in plants, mammals, and insects. Two plant AADC s, L-tryptophan decarboxylase (TDC) and L-tyrosine decarboxylase (TYDC), ha ve attracted considerable attention because of their role in the biosynthes is of pharmaceutically important monoterpenoid indole alkaloids and benzyli soquinoline alkaloids, respectively. Although plant and animal AADCs share extensive amino acid homology, the enzymes display striking differences in their substrate specificities. AADCs from mammals and insects accept a broa d range of aromatic L-amino acids, whereas TDC and TYDC from plants exhibit exclusive substrate specificity for L-amino acids with either indole or ph enol side chains, but not both. Recent biochemical and kinetic studies on a nimal AADCs support basic features of the classic AADC reaction mechanism. The catalytic mechanism involves the formation of a Schiff base between PLP and an invariable lysine residue, followed by a transaldimination reaction with an aromatic L-amino acid substrate. Both TDC and TYDC are primarily r egulated at the transcriptional level by developmental and environmental fa ctors. However, the putative post-translational regulation of TDC via the u biquitin pathway, by an ATP-dependent proteolytic process, has also been su ggested. Isolated TDC and TYDC genes have been used to genetically alter th e regulation of secondary metabolic pathways derived from aromatic amino ac ids in several plant species. The metabolic modifications include increased serotonin levers, reduced indole glucosinolate levels, redirected shikimat e metabolism, increased indole alkaloid levels, and increased cell wall-bou nd tyramine levels. (C) 2000 Elsevier Science Ltd. All rights reserved.