Agriculture Reference
In-Depth Information
*
CH
2
*
CH
2
COO
-
CH
3
S
CH
NH
+
L
-Methionine (Met)
*
CH
2
*
CH
2
S
+
COO
-
CH
3
CH
NH
+
Adenine-ribose
S
-Adenosylmethionine (AdoMet)
NH
+
*
CH
2
C
1-Aminocyclopropane-1
-carboxylic acid (ACC)
*
CH
2
COO
*
CH
2
Ethylene (ethene)
*
CH
2
Fig. 10.1.
Biosynthesis of ethylene from methionine. The two CH
2
groups
of methionine, which form
ethylene, are marked with asterisks. Note that plants convert methionine into 1-amino-cyclopropane-1-
carboxylic acid to produce ethylene, whereas bacteria utilize 2-oxo-4-methylthiobutyric acid formed from
methionine as an intermediate in ethylene synthesis. From Primrose (1979) and Yang and Hoffman (1984).
The second reaction is catalysed by ACC
synthase (ACS; EC 4.4.1.14), which
requires pyridoxal phosphate:
The last reaction is catalysed by ACC
oxidase (ACO) (EC 1.14.17.4), which
requires oxygen, Fe
2+
, CO
2
(supplied as
bicarbonate) and ascorbate:
Methionine and
S
-adenosylmethionine
are important constituents of all living
cells. Reactions (2) and (3), however, are
specifi c for ethylene biosynthesis in plants,
and are catalysed by ACS and ACO. These
two enzymes require a number of factors
for activity, and ACO, for example, can
only function in the presence of O
2
. The
requirement for O
2
for ethylene synthesis
was exploited by Adams and Yang (1979)
to study the biosynthetic pathway. They
showed that when apple tissue was placed
under N
2
, ethylene synthesis was inhibited
and radioactive carbon tracer from
methionine accumulated in an unknown
compound. When O
2
was restored to the
tissue, ethylene production rapidly re-
sumed and the radioactive carbon
appeared in ethylene. This suggested that
the unknown compound, which Adams
and Yang later identifi ed as ACC, was a
precursor, and that its conversion to
ethylene required O
2
. The enzyme that
catalyses this reaction (ACO) could, at the
time, only be studied in tissue slices, and it
was originally referred to as 'ethylene-
forming enzyme' (EFE). Although Yang
studied the stereospecifi city of the
reaction, it was more than a decade before
the enzyme was fully characterized, as
discussed below, and named ACC oxidase
(ACO).
In addition to their roles in ethylene
synthesis, methionine is required for pro-
tein synthesis, and
S
-adenosylmethionine
functions as a methyl donor in general
metabolism. In order to maintain high rates
of ethylene production, without depleting
the cellular pools of these compounds,
the methylthioadenosine formed during
ethylene biosynthesis is recycled in a series
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