5.1 Methionine and Tryptophan Interconversion by VAS1
and TAA Aminotransferases
For a long time, it was assumed that the unique role of methionine as a molecule
in the cell was to produce proteins via its utilization in methionyl-tRNA formation as
a substrate of aminoacyl-tRNA synthetase. However, it was demonstrated recently
that the cytosolic PLP-dependent aminotransferase called VAS1/ISS1 (for reversal
of sav3 phenotype and indole severe sensitive1) catalyzes the conversion of methionine as amino acid donor and indole-3-pyruvic acid (IPA) as amino acceptor into
L-tryptophan and 2-keto-4-methythiobutyrate (Zheng et al. 2013a; Pieck et al.
2015). IPA is the precursor of IAA (Fig. 4) that is involved in the most important
route of plant IAA biosynthesis belonging to the tryptophan-dependent pathways
(Stepanova et al. 2008, 2011; Mashiguchi et al. 2011; Won et al. 2011; Zhao 2014).
Indeed, among 19 of naturally occurring amino acids examined, methionine was the
most catalytically preferred amino acid donor followed by Phe with only 21%
specificity relative to Met (Pieck et al. 2015). Therefore, VAS1/ISS1 catalyzes the
opposing reaction of tryptophan aminotransferases TAA1 (Fig. 4) and tryptophan
aminotransferases-related TAR1 and TAR2 (Stepanova et al. 2008). TAA PLPdependent enzymes convert Trp by using α-ketoglutarate or pyruvate as organic
acid donors, into IPA and glutamate or alanine, respectively (Fig. 4). In Arabidopsis,
a vas1-2 single mutant and vas1-2sav3 double mutant showed a significant increase
in IAA levels and a fivefold increase in the level of the ethylene precursor ACC
(Zheng et al. 2013a). However, free L-methionine levels did not differ between
mutant and WT, demonstrating that regulation of the methionine biosynthesis
is controlled downstream by methionine synthase (MS) and methionine adenosyltransferase (SAM) (Bürstenbinder et al. 2007). Since the VAS1/ISS1 and TAA
aminotransferases catalyze opposite reactions, it is assumed that VAS1 could counteract the production of IPA and methionine by subtracting a part of the IPA and
methionine from IAA and methionine pathways, respectively (Zheng et al. 2013a).
However, some evidence suggests that SAV1/ISS1 could be mainly involved either
in Trp catabolism or in metabolism of Phe and Tyr (Pieck et al. 2015).
Indeed, protein sequence analyses of SAV1/ISS1 with other plant aromatic aminotransferases (AroAT) showed that SAV1/ISS1 is a new subgroup of aromatic PLPdependent enzymes belonging to the Iα aspartate family. This subgroup is distinct
from Trp ATs and Tyr ATs previously described and could be involved in Phe
and/or Tyr biosynthesis (Pieck et al. 2015). Unfortunately, these studies did not test
whether SAV1/ISS1 could also catalyze the conversion of Met and phenylpyruvate
into KMTB and Phe, or the conversion of Met and 4-hydroxyphenyl pyruvate into
KTMB and Tyr (Fig. 4). The latter reactions would explain the role of SAV1/ISS1 in
Phe and Tyr biosynthesis from methionine.
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E. Le Deunff
and TAA Aminotransferases
For a long time, it was assumed that the unique role of methionine as a molecule
in the cell was to produce proteins via its utilization in methionyl-tRNA formation as
a substrate of aminoacyl-tRNA synthetase. However, it was demonstrated recently
that the cytosolic PLP-dependent aminotransferase called VAS1/ISS1 (for reversal
of sav3 phenotype and indole severe sensitive1) catalyzes the conversion of methionine as amino acid donor and indole-3-pyruvic acid (IPA) as amino acceptor into
L-tryptophan and 2-keto-4-methythiobutyrate (Zheng et al. 2013a; Pieck et al.
2015). IPA is the precursor of IAA (Fig. 4) that is involved in the most important
route of plant IAA biosynthesis belonging to the tryptophan-dependent pathways
(Stepanova et al. 2008, 2011; Mashiguchi et al. 2011; Won et al. 2011; Zhao 2014).
Indeed, among 19 of naturally occurring amino acids examined, methionine was the
most catalytically preferred amino acid donor followed by Phe with only 21%
specificity relative to Met (Pieck et al. 2015). Therefore, VAS1/ISS1 catalyzes the
opposing reaction of tryptophan aminotransferases TAA1 (Fig. 4) and tryptophan
aminotransferases-related TAR1 and TAR2 (Stepanova et al. 2008). TAA PLPdependent enzymes convert Trp by using α-ketoglutarate or pyruvate as organic
acid donors, into IPA and glutamate or alanine, respectively (Fig. 4). In Arabidopsis,
a vas1-2 single mutant and vas1-2sav3 double mutant showed a significant increase
in IAA levels and a fivefold increase in the level of the ethylene precursor ACC
(Zheng et al. 2013a). However, free L-methionine levels did not differ between
mutant and WT, demonstrating that regulation of the methionine biosynthesis
is controlled downstream by methionine synthase (MS) and methionine adenosyltransferase (SAM) (Bürstenbinder et al. 2007). Since the VAS1/ISS1 and TAA
aminotransferases catalyze opposite reactions, it is assumed that VAS1 could counteract the production of IPA and methionine by subtracting a part of the IPA and
methionine from IAA and methionine pathways, respectively (Zheng et al. 2013a).
However, some evidence suggests that SAV1/ISS1 could be mainly involved either
in Trp catabolism or in metabolism of Phe and Tyr (Pieck et al. 2015).
Indeed, protein sequence analyses of SAV1/ISS1 with other plant aromatic aminotransferases (AroAT) showed that SAV1/ISS1 is a new subgroup of aromatic PLPdependent enzymes belonging to the Iα aspartate family. This subgroup is distinct
from Trp ATs and Tyr ATs previously described and could be involved in Phe
and/or Tyr biosynthesis (Pieck et al. 2015). Unfortunately, these studies did not test
whether SAV1/ISS1 could also catalyze the conversion of Met and phenylpyruvate
into KMTB and Phe, or the conversion of Met and 4-hydroxyphenyl pyruvate into
KTMB and Tyr (Fig. 4). The latter reactions would explain the role of SAV1/ISS1 in
Phe and Tyr biosynthesis from methionine.
272
E. Le Deunff