5.2 Methionine and Phenylalanine Interconversion
by PPY-AT in the New Phenylpyruvate Pathway
Until now, the arogenate pathway was considered as the exclusive route for Tyr
and Phe biosynthesis in plants (Cho et al. 2007; Maeda and Dudareva 2012).
However, it was recently demonstrated that Phe could also be synthesized in cytosol
via phenylpyruvate alternative pathway (Yoo et al. 2013; El-Azaz et al. 2016).
This functional phenylpyruvate pathway was similar to most microorganisms and
required two reaction steps. In the first reaction step, prephenate is converted to
phenylpyruvate in plastids by arogenate dehydratases (ADT) that retain prephenate
dehydratase activity (PDT) from a 22-amino acid region called PAC domain (for
PDT activity conferring Domain) conferring PDT activity to ADTs (El-Azaz et al.
2016). In the second reaction step, a cytosolic phenylpyruvate-aminotransferase
(PPY-AT) catalyzes the interconversion of phenylpyruvate and tyrosine to 4hydroxyphenylpyruvate and Phe (Yoo et al. 2013). As demonstrated PPY-AT
could also convert phenylpyruvate and Met into Phe and 2-keto-4-methylthiobutyrate
(Fig. 4) since PPY-AT is able to use Met as amino donor with only 13% specificity
relative to tyrosine (Yoo et al. 2013). Therefore, the discovery of phenylpyruvate
pathway links biosynthesis of phenylalanine with catabolism of tyrosine but also
increases the complexity of the transaminases network involved in the regulation of
aromatic amino acids and methionine (Fig. 4).
5.3 In Search of the Last Missing Aminotransferase
to Complete the Yang Cycle
In plants and many microorganisms, due to their promiscuity, various aspartate
aminotransferases (AAT) are presumed to be involved in the last step of the Yang
cycle catalyzing the transamination of KMTB into methionine (Berger et al. 2003;
Sekowska et al. 2004; Pommerrenig et al. 2011). In Arabidopsis, plastids contain
two different AAT: a prokaryotic-type (PT-pAAT) and a eukaryotic-type (ETpAAT) that are involved in the first step of the aspartate-derived amino acid pathway
(de la Torre et al. 2006, 2014a, b). It is strongly suspected from analysis sequences
that ET-pATT (ASP5) and PT-AAT could be implied in the last step of the Yang
cycle (Pommerrenig et al. 2011). Moreover, it was recently discovered that plant
PT-pAAT is bispecific since it also displays a prephenate aminotransferase activity
(PAT) and catalyzes the conversion of glutamate and prephenate into arogenate
and 2-oxoglutarate (Graindorge et al. 2010; Maeda et al. 2011). Therefore, it is not
excluded that methylthio-group salvage of AdoMet by the Yang cycle in plastids
could compete in its last step with the biosynthesis of Phe and Tyr (lignin and
phenylpropanoids precursors) in the arogenate pathway catalyzed by the PAT
activity of PT-pAAT (Figs. 1 and 4). In this respect, the PT-pAAT/PAT silenced
plants show a severe reduction in growth as well as a decrease in chlorophyll and
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by PPY-AT in the New Phenylpyruvate Pathway
Until now, the arogenate pathway was considered as the exclusive route for Tyr
and Phe biosynthesis in plants (Cho et al. 2007; Maeda and Dudareva 2012).
However, it was recently demonstrated that Phe could also be synthesized in cytosol
via phenylpyruvate alternative pathway (Yoo et al. 2013; El-Azaz et al. 2016).
This functional phenylpyruvate pathway was similar to most microorganisms and
required two reaction steps. In the first reaction step, prephenate is converted to
phenylpyruvate in plastids by arogenate dehydratases (ADT) that retain prephenate
dehydratase activity (PDT) from a 22-amino acid region called PAC domain (for
PDT activity conferring Domain) conferring PDT activity to ADTs (El-Azaz et al.
2016). In the second reaction step, a cytosolic phenylpyruvate-aminotransferase
(PPY-AT) catalyzes the interconversion of phenylpyruvate and tyrosine to 4hydroxyphenylpyruvate and Phe (Yoo et al. 2013). As demonstrated PPY-AT
could also convert phenylpyruvate and Met into Phe and 2-keto-4-methylthiobutyrate
(Fig. 4) since PPY-AT is able to use Met as amino donor with only 13% specificity
relative to tyrosine (Yoo et al. 2013). Therefore, the discovery of phenylpyruvate
pathway links biosynthesis of phenylalanine with catabolism of tyrosine but also
increases the complexity of the transaminases network involved in the regulation of
aromatic amino acids and methionine (Fig. 4).
5.3 In Search of the Last Missing Aminotransferase
to Complete the Yang Cycle
In plants and many microorganisms, due to their promiscuity, various aspartate
aminotransferases (AAT) are presumed to be involved in the last step of the Yang
cycle catalyzing the transamination of KMTB into methionine (Berger et al. 2003;
Sekowska et al. 2004; Pommerrenig et al. 2011). In Arabidopsis, plastids contain
two different AAT: a prokaryotic-type (PT-pAAT) and a eukaryotic-type (ETpAAT) that are involved in the first step of the aspartate-derived amino acid pathway
(de la Torre et al. 2006, 2014a, b). It is strongly suspected from analysis sequences
that ET-pATT (ASP5) and PT-AAT could be implied in the last step of the Yang
cycle (Pommerrenig et al. 2011). Moreover, it was recently discovered that plant
PT-pAAT is bispecific since it also displays a prephenate aminotransferase activity
(PAT) and catalyzes the conversion of glutamate and prephenate into arogenate
and 2-oxoglutarate (Graindorge et al. 2010; Maeda et al. 2011). Therefore, it is not
excluded that methylthio-group salvage of AdoMet by the Yang cycle in plastids
could compete in its last step with the biosynthesis of Phe and Tyr (lignin and
phenylpropanoids precursors) in the arogenate pathway catalyzed by the PAT
activity of PT-pAAT (Figs. 1 and 4). In this respect, the PT-pAAT/PAT silenced
plants show a severe reduction in growth as well as a decrease in chlorophyll and
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
273