2.1 Glutamine Involvement in the Methionine Pathway
Is Hidden Behind the PLP Coenzyme Used
by Aminotransferases
The three steps of methionine biosynthesis in the plastids from O-phosphohomoserine require the action of cystathionine γ-synthase (CgS), cystathionine
β-lyase (CbL), and methionine synthase (MS), respectively (Fig. 3). Cystathionine
γ-synthase and β-lyase are two enzymes that belong to the γ- and β-lyase subfamilies
of the α family of aminotransferases (Christen and Mehta 2001). For their catalytic
activity, they need pyridoxal-5
0 -phosphate (PLP), a derivative form of vitamin B 6
that is a versatile coenzyme. De novo PLP biosynthesis in the cytosol is catalyzed by
pyridoxine synthase enzymes (PDX1 and PDX2) from glutamine, produced by the
GS/GOGAT cycle after reduction of nitrate and glyceraldehyde 3-phosphate (G3P)
and ribose 5-phosphate (5RP) provided by the glycolysis and pentose phosphate
pathways, respectively (Tambasco-Studart et al. 2005; Fitzpatrick 2011; Colinas
et al. 2016). PLP is also synthesized in chloroplasts from pyridoxamine 5
0 -phosphate
(PMP) or pyridoxine 5
0 -phosphate (PNP) by pyridoxamine 5
0 -phosphate oxidase
enzyme (Fitzpatrick 2011). It is not known how de novo PLP synthesis in cytosol
modulates the PLP pool in the plastid and which plastidial carriers are involved
in the transport of different forms of vitamin B 6 (Gerdes et al. 2012). Two olefinic
compounds, DL-propargylglycine (PAG) and L-aminoethoxyvinylglycine (AVG),
that act as suicide inhibitors of PLP-enzymes of the α family of aminotransferases
(Lieberman 1979; Satoh and Yang 1989a) can inhibit in vitro and in vivo activities
of CgS and CbL enzymes, respectively (Ravanel et al. 1998).
2.2 Serine Is Doubly Implicated in Methionine Biosynthesis
Serine is directly involved in the methionine pathway through cysteine biosynthesis
(Romero et al. 2014). Cysteine provides a sulfur atom to methionine through the
action of cystathionine γ-synthase (CgS) and comes from the reduction and assimilatory pathway of inorganic sulfate (SO 4
2À ). The biosynthesis of cysteine requires
serine as the amino acid skeleton-donor and bisulfide from sulfate reduction as a
sulfur donor to form cysteine (Takahashi et al. 2011; Romero et al. 2014). In plants,
the reduction of sulfate takes place in the chloroplast and produces sulfide, an
inorganic anion of sulfur with the chemical formula S 2
À
. However, in aqueous
solution most sulfide ions are neutralized under the conjugate acid form of bisulfide:
SH
À
. The two consecutive reactions catalyzed by serine acyltransferase (SAT) and
O-acetylserine(thiol)lyase (OSATL) produce cysteine (Fig. 3). SAT catalyzes the
conversion of serine and acetyl-CoA into O-acetylserine (OAS) and acetyl-CoA-SH
whereas OSATL catalyzes the final PLP-dependent conversion of bisulfide and OAS
into cysteine and acetate (Bonner et al. 2005). These two enzymes can assemble in a
hetero-oligomeric cysteine synthase complex formed by one SAT hexamer and two
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
259
Is Hidden Behind the PLP Coenzyme Used
by Aminotransferases
The three steps of methionine biosynthesis in the plastids from O-phosphohomoserine require the action of cystathionine γ-synthase (CgS), cystathionine
β-lyase (CbL), and methionine synthase (MS), respectively (Fig. 3). Cystathionine
γ-synthase and β-lyase are two enzymes that belong to the γ- and β-lyase subfamilies
of the α family of aminotransferases (Christen and Mehta 2001). For their catalytic
activity, they need pyridoxal-5
0 -phosphate (PLP), a derivative form of vitamin B 6
that is a versatile coenzyme. De novo PLP biosynthesis in the cytosol is catalyzed by
pyridoxine synthase enzymes (PDX1 and PDX2) from glutamine, produced by the
GS/GOGAT cycle after reduction of nitrate and glyceraldehyde 3-phosphate (G3P)
and ribose 5-phosphate (5RP) provided by the glycolysis and pentose phosphate
pathways, respectively (Tambasco-Studart et al. 2005; Fitzpatrick 2011; Colinas
et al. 2016). PLP is also synthesized in chloroplasts from pyridoxamine 5
0 -phosphate
(PMP) or pyridoxine 5
0 -phosphate (PNP) by pyridoxamine 5
0 -phosphate oxidase
enzyme (Fitzpatrick 2011). It is not known how de novo PLP synthesis in cytosol
modulates the PLP pool in the plastid and which plastidial carriers are involved
in the transport of different forms of vitamin B 6 (Gerdes et al. 2012). Two olefinic
compounds, DL-propargylglycine (PAG) and L-aminoethoxyvinylglycine (AVG),
that act as suicide inhibitors of PLP-enzymes of the α family of aminotransferases
(Lieberman 1979; Satoh and Yang 1989a) can inhibit in vitro and in vivo activities
of CgS and CbL enzymes, respectively (Ravanel et al. 1998).
2.2 Serine Is Doubly Implicated in Methionine Biosynthesis
Serine is directly involved in the methionine pathway through cysteine biosynthesis
(Romero et al. 2014). Cysteine provides a sulfur atom to methionine through the
action of cystathionine γ-synthase (CgS) and comes from the reduction and assimilatory pathway of inorganic sulfate (SO 4
2À ). The biosynthesis of cysteine requires
serine as the amino acid skeleton-donor and bisulfide from sulfate reduction as a
sulfur donor to form cysteine (Takahashi et al. 2011; Romero et al. 2014). In plants,
the reduction of sulfate takes place in the chloroplast and produces sulfide, an
inorganic anion of sulfur with the chemical formula S 2
À
. However, in aqueous
solution most sulfide ions are neutralized under the conjugate acid form of bisulfide:
SH
À
. The two consecutive reactions catalyzed by serine acyltransferase (SAT) and
O-acetylserine(thiol)lyase (OSATL) produce cysteine (Fig. 3). SAT catalyzes the
conversion of serine and acetyl-CoA into O-acetylserine (OAS) and acetyl-CoA-SH
whereas OSATL catalyzes the final PLP-dependent conversion of bisulfide and OAS
into cysteine and acetate (Bonner et al. 2005). These two enzymes can assemble in a
hetero-oligomeric cysteine synthase complex formed by one SAT hexamer and two
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
259