Biochemical Properties and Physiological Roles of Tyrosine Aminotransferases in Olive (Olea europaea L.)

Increasing the tocopherol content in plant-derived foods not only improves their nutritional quality but may also enhance plant resilience against abiotic stress factors. Tyrosine catabolism is the origin of homogentisic acid, which constitutes the core aromatic ring in the structure of tocopherols. Two olive tyrosine aminotransferase genes, OeTAT1 and OeTAT2, have been cloned, and the corresponding recombinant proteins have been functionally validated. Both proteins showed significant differences in substrate specificity and catalytic efficiency. OeTAT1 protein exhibited a marked substrate preference for tyrosine (Km 0.57 mM and Vmax of 47.61 U mg−1), forming 4-hydroxyphenyl pyruvic acid, which is the direct precursor of homogentisic acid. On the contrary, OeTAT2, with significantly lower catalytic activity, displayed broad substrate specificity and was less efficient with tyrosine (Km 6.80 mM and Vmax 5.72 U mg−1). The expression analysis of OeTAT1 and OeTAT2 during the development and ripening of fruits in seven olive cultivars, as well as in olive trees grown under water stress, suggests different roles for both genes. The data suggest that OeTAT1 seems to play a key role in tocopherol biosynthesis during olive fruit development, while OeTAT2 may be more closely associated with other metabolic pathways unrelated to tocopherol biosynthesis. Furthermore, both genes were found to be upregulated under water stress conditions in olive.