Agriculture Reference
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of reactions to form methionine (Fig. 10.2).
In addition, the cyanide produced during
the conversion of ACC to ethylene is
detoxifi ed by
E
-cyanoalanine synthase,
forming
E
-cyanoalanine. Without this, the
consumption of ripening fruits and cut
vegetables would be potentially hazardous
(Fig. 10.2)! ACC can also be converted to
malonyl-ACC, which is stored in the
vacuole, where it is thought to be
unavailable for ethylene synthesis. Recent
developments
signalling and responses have been
reviewed by Lin
et al.
(2009). For a full
appreciation of how the ethylene pathway
fi ts into the general metabolism, including
the synthesis of another group of important
growth compounds, the polyamines,
readers are recommended to consult
Urstenbinder and Sauter (2012) and
Harpaz-Saad
et al.
(2012).
The ACC pathway is believed to be the
source of most, if not all, ethylene
produced by plants and its control holds
in
ethylene
synthesis,
HCOO
-
HPO
4
2-
[O]
CH
3
S
CH
2
OPO
3
H
-
O
CH
2
CH
2
CO COO
-
CH
3
S
+
Keto-methylthiobutyric acid
NH
3
R-CH-COO
-
OH OH
ADP
AT P
5-Methyl thioribose-
1-phosphate
R-CO-COO
-
+
NH
3
CH
3
S
CH
2
CH
2
CH COO
-
CH
3
S
CH
2
OH
O
Methionine
AT P
OH
OH
5-Methylthioribose-1
phosphate
PP
i
+ P
i
+
NH
3
+
COO
-
Ade
CH
3
S
CH
3
S
CH
2
CH
2
CH
CH
2
CH
2
S
-Adenosyl-
L
-methionine
Ade
Ade
O
O
ACC
synthase
OH OH
OH OH
5-Methylthioadenosine
+
CH
2
=CH
2
Ethylene (ethene)
H
2
C
ACC oxidase
NH
3
C
COO
-
H
2
C
1-Aminocyclopropane-
1-carboxylic acid (ACC)
½ O
2
CO
2
+ HCN
Fig. 10.2.
The ethylene biosynthesis cycle. The reactions and enzymes involved in ethylene synthesis,
methionine resynthesis, cyanide detoxifi cation and malonyl-ACC production surrounding a photograph of
the late Shang Fa Yang (Yang and Hoffman, 1984; Bradford, 2008). Enzymes and substrates/products
referred to in the text are in boxes. Modifi ed from Bradford (2008) and reproduced with permission.
S
-Adenosyl-
L
-methionine
o
1-aminocyclopropane-1-carboxylate + methylthioadenosine
(2)
1-Aminocyclopropane-1-carboxylate + ascorbate + O
2
o
ethylene + HCN +
dehydroascorbate + CO
2
+ 2H
2
O
(3)
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