4.2 ACS Aminotransferase Is the Rate-Limiting Step
for Ethylene Biosynthesis
In the ethylene pathway, AdoMet is converted by ACC synthase (ACS) into MTA and
ACC, a non-proteinogenic amino acid, precursor of ethylene (Figs. 3 and 4). ACS is
a PLP-dependent enzyme that belongs to the subgroup I of the α family of aminotransferases (Mehta et al. 1993; Christen and Mehta 2001). This subgroup contains
major enzymes involved in carbon, nitrogen, and sulfur assimilation and shuttling
such as ACC, aspartate, alanine, tyrosine, and aromatic aminotransferases (Mehta
et al. 1993). X-ray structure analysis of the ACS protein revealed that amino acids
surrounding the catalytic site share the same function as the aspartate aminotransferase (AAT) counterpart (Capitani et al. 1999; Yamagami et al. 2003). The ACS
isoforms form homodimers and heterodimers and the active site of the dimers results
Flavin monooxygenase
YUCCA
L-Tryptophan
AdoMet
ACC
Ethylene
ACC synthase
PLP-dependent
ACS
ACC oxidase
ACO
AIA IPyA pathway
Ethylene pathway
Methionine
SAM synthetase
SAM
Main effects on the roots and shoots morphogenetic programmes
KMTB
Yang Cycle
Oxaloacetate?
-KG
(Pyruvate)
Glutamate
(Alanine)
Aspartate ?
?
L-Tryptophan
catabolism?
L-Phenylalanine
Phenylpyruvic acid
(PPA)
Flavin monooxygenase
YUCCA
Indole-3-acetic acid
(IAA)
Phenylacetic acid
(PAA)
Lignin and
phenylpropanoids
PAA- PPA pathway
4-hydroxyphenyl
pyruvate
(KTMB)
Tyrosine
(Methionine)
IPyA/PPA
aminotransferase
PLP-dependent
VAS1/ISS1
Aspartate
aminotransferase
PLP-dependent
AAT/PAT?
Tryptophan
aminotransferase
PLP-dependent
TAA1 and TARs
Tryptophan
aminotransferase
PLP-dependent
TAA1 and TARs
Phenylpyruvate
aminotransferase
PLP-dependent
PPY-AT
2
2
1
4
3
Indole-3-pyruvic
acid (IPyA)
L-Phenylalanine
IPyA/PPA
aminotransferase
PLP-dependent
VAS1/ISS1
3
KMTB ?
Lignin and
phenylpropanoids
PAA
IAA
α
Fig. 4 Interconversion between aminotransferases belonging to the subgroup I of the α family
involved in the metabolic pathways of auxins, ethylene, and aromatic amino acids (Phe, Tyr, Trp).
1, phenylpyruvate aminotransferase (PPY-AT); 2, tryptophan aminotransferase of Arabidopsis
(TAA/TAR); 3, IPyA/PPA aminotransferase SAV1/ISS1; 4, eukaryotic aspartate aminotransferase
(ET-AAT) and prokaryotic type aspartate/prephenate aminotransferase (PT-AAT/PAT). Abbreviations used for metabolite are: KTMB, 2-keto-4-methylthio butyrate; 2-KG, α-ketoglutarate
(adapted from Zheng et al. 2013a; Yoo et al. 2013; Le Deunff et al. 2016; Pieck et al. 2015)
266
E. Le Deunff