A tyrosine aminotransferase involved in tocopherol synthesis in Arabidopsis

Abstract

SUMMARYThe metabolic function of the predicted Arabidopsis tyrosine aminotransferase (TAT) encoded by the At5g53970 gene was studied using two independent knock-out mutants. Gas chromatography-mass spectrometry based metabolic profiling revealed a specific increase in tyrosine levels, supporting the proposed function of At5g53970 as a tyrosine-specific aminotransferase not involved in tyrosine biosynthesis, but rather in utilization of tyrosine for other metabolic pathways. The TAT activity of the At5g53970-encoded protein was verified by complementation of the Escherichia coli tyrosine auxotrophic mutant DL39, and in vitro activity of recombinantly expressed and purified At5g53970 was found to be specific for tyrosine. To investigate the physiological role of At5g53970, the consequences of reduction in tyrosine utilization on metabolic pathways having tyrosine as a substrate were analysed. We found that tocopherols were substantially reduced in the mutants and we conclude that At5g53970 encodes a TAT important for the synthesis of tocopherols in Arabidopsis.

A Root-Expressed l-Phenylalanine:4-Hydroxyphenylpyruvate Aminotransferase Is Required for Tropane Alkaloid Biosynthesis in Atropa belladonna

“…Asterisks denote significant differences (**P < 0.01) as determined by Student's t test. the kinetic properties of ArATs previously determined to exhibit maximal activity with 2-oxoglutarate as the cosubstrate to assess their activities with aromatic keto-acids (Prabhu and Hudson, 2010;Lee and Facchini, 2011;Riewe et al, 2012).…”

“…The reported activity of several ArATs indicates they can use L-Tyr, L-Phe, and L-Trp to varying degrees as amino donors, with 2-oxoglutarate generally being the preferential amino acceptor (Prabhu and Hudson, 2010;Lee and Facchini, 2011;Riewe et al, 2012). More recently, Ph-PPY-AT and Cm-ArAT1 were shown to preferentially catalyze the formation of L-Phe using L-Tyr as the amino donor and phenylpyruvate as the amino acceptor (Yoo et al, 2013).…”

“…Full-length transcripts were obtained for Ab-ArAT1 to 5, which encode proteins of between 419 and 441 amino acids (46 to 48 kD) that share between 50 and 72% amino acid identity. The majority of putative plant ArATs remain uncharacterized but the closely related Arabidopsis proteins encoded by At5g36160 and At5g53970, Ps-TyrAT from Papaver somniferum, Ph-PPY-AT from petunia, and Cm-ArAT1 from melon (Cucumis melo) all utilize L-Phe and L-Tyr as amino donors (Prabhu and Hudson, 2010;Lee and Facchini, 2011;Riewe et al, 2012;Yoo et al, 2013). An amino acid alignment of the predicted protein sequence of the Ab-ArAT proteins and selected homologs indicates that each contains a conserved catalytic lysine that corresponds to residue 270 in Ab-ArAT1, together with conserved residues required for the binding of the pyridoxal-59-phosphate cofactor (Supplemental Figure 3).…”

An alternative pathway contributes to phenylalanine biosynthesis in plants via a cytosolic tyrosine:phenylpyruvate aminotransferase

“…Indeed, a Blastp search against Arabidopsis using PhPPY-AT as query retrieves as the best hit At5g53970, a recently characterized tyrosine aminotransferase demonstrated to be involved in tocopherol biosynthesis 23 . The discovery that the microbial-like phenylpyruvate pathway does in fact operate in plants raises many questions about the control of carbon flux towards phenylalanine and its downstream metabolites.…”

“…Nevertheless, overexpression of a bacterial bifunctional chorismate mutase/prephenate dehydratase (PheA) in Arabidopsis resulted in significantly increased phenylalanine production 19 , implicating the presence of an aminotransferase capable of converting phenylpyruvate to phenylalanine in plants, as it occurs in the final step of the microbial phenylpyruvate pathway. Although several plant aminotransferases, mainly participating in aromatic amino-acid catabolism, have recently been described [20][21][22][23] , none of them have been demonstrated to function in phenylalanine biosynthesis.…”