Aromatic amino acid aminotransferases in plants

Aromatic amino acid aminotransferases (AAA-ATs) catalyze the reversible transamination reactions of proteinogenic and non-proteinogenic aromatic amino acids to corresponding keto acids and vice versa. The products of plant AAA-ATs serve as key precursors of many primary and secondary metabolites that are crucial for both plant and human metabolism and physiology. In most microbes, l-tyrosine (Tyr) and l-phenylalanine (Phe) aminotransferases (Tyr and Phe-ATs) catalyze the final steps of Phe and Tyr biosynthesis. On the other hand, plants use different pathways to synthesize Tyr and Phe via arogenate, in which prephenate-specific aminotransferases (PPA-ATs) catalyze the committed step in the plastids. Plant Tyr and Phe-ATs, unlike microbial counterparts, often prefer the reverse reactions and metabolize Tyr and Phe to their respective aromatic keto acids, which serve as precursors of various plant natural products (e.g. benzenoid volatiles, tocochromanols, plastoquinone, and tropane and benzylisoquinoline alkaloids). Unlike plastidic PPA-ATs, plant Tyr/Phe-ATs are localized outside of the plastids, have broad substrate specificity, and interlink Tyr and Phe metabolism. l-Tryptophan (Trp) aminotransferases (Trp-ATs) are involved in biosynthesis of the plant hormone auxin. Although significant advancement has been made on biochemical, molecular, and genetic characterizations of plant AAA-ATs, there are still many critical knowledge gaps, which are highlighted in the current review.