Biochemical properties and subcellular localization of tyrosine aminotransferases in Arabidopsis thaliana.

Plants produce various L-tyrosine (Tyr)-derived compounds that are of pharmaceutical or nutritional importance to humans. Tyr aminotransferase (TAT) catalyzes the reversible transamination between Tyr and 4-hydroxyphenylpyruvate (HPP), the initial step in the biosynthesis of many Tyr-derived plant natural products. Herein reported is the biochemical characterization and subcellular localization of TAT enzymes from the model plant Arabidopsis thaliana. Phylogenetic analysis showed that Arabidopsis has at least two homologous TAT genes, At5g53970 (AtTAT1) and At5g36160 (AtTAT2). Their recombinant enzymes showed distinct biochemical properties: AtTAT1 had the highest activity towards Tyr, while AtTAT2 exhibited a broad substrate specificity for both amino and keto acid substrates. Also, AtTAT1 favored the direction of Tyr deamination to HPP, whereas AtTAT2 preferred transamination of HPP to Tyr. Subcellular localization analysis using GFP-fusion proteins and confocal microscopy showed that AtTAT1, AtTAT2, and HPP dioxygenase (HPPD), which catalyzes the subsequent step of TAT, are localized in the cytosol, unlike plastid-localized Tyr and tocopherol biosynthetic enzymes. Furthermore, subcellular fractionation indicated that, while HPPD activity is restricted to the cytosol, TAT activity is detected in both cytosolic and plastidic fractions of Arabidopsis leaf tissue, suggesting that an unknown aminotransferase(s) having TAT activity is also present in the plastids. Biochemical and cellular analyses of Arabidopsis TATs provide a fundamental basis for future in vivo studies and metabolic engineering for enhanced production of Tyr-derived phytochemicals in plants.