Solanum lycopersicum ACC
Amanda He '16 and Andrew Pearlman '15
1-aminoacyclopropane 1-carboxylate synthase, also referred to as
ACC synthase (ACS), is an enzyme that plays a significant role in
synthesizing ethylene, a plant hormone produced in a variety of higher
plants . Ethylene is an
important hormone for ripening and germination processes. Ethylene
biosynthesis consists of three main steps. The rate-limiting
step in ethylene synthesis is the second step, which involves ACS .
In the second step, ACS catalyzes the cyclization of
(SAM), which produces 5'-methylthioadenosine and ACC .
In order for ACS to function, it requires the cofactor, pyridoxal-5'-phosphate
(PLP) . PLP
has been found to activate catalysis in enzymes, including ACS, by
binding to the substrate active site .
As the rate-limiting step of ethylene
biosynthesis, ACS provides a number of opportunities to inhibit the
process. A competitive inhibitor for ACS is aminoethoxyvinylglycine
(AVG) . AVG
binds to the substrate active site, preventing SAM from interacting
with the site . AVG
is applied to various fruit-bearing plants to control the rate at
which ethylene is produced .
Establishing an understanding of how the ACS and the factors it
interacts with operate on a molecular level can help identify
potential risks of our current agricultural industry.
II. General Structure
The tomato ACC synthase
has two domains, an alpha-beta-alpha sandwich
domain and an alpha-beta domain .
is comprised of seven beta strands,
which flank nine alpha-helices. The
consists of five beta strands and five
ACS typically appears in a
as do most PLP-dependent enzymes. However, there have been
no results indicating that monomeric ACS is not catalytically
active. So both the monomer and dimer forms are functional. In the
dimeric formation, the N-terminal residue in
alpha-beta domain makes contact with the
The contact could have a potential role in conformation
stabilization or catalysis .
Oftentimes, ACS forms complexes with PLP
and/or AVG. The ACS-PLP
complex uses the dimer form, while ACS-PLP-AVG
complex uses the monomer form. When comparing the structural
superposition, there was not a significant difference between ACS-PLP
indicating that AVG binding does not
significantly change ACS formation .
Although it appears more frequently in dimeric form, there is
not a substantial difference between the functionality of the
monomer and dimer forms because there were not a significant
difference in conformational change between the two when the
inhibitor, AVG, binds .
are the main catalytic domains.
III. Cofactor PLP Binding
forms a complex with cofactor, pyridoxal-5'-phosphate
, an active form of vitamin B6,
in order to be able to catalysis its substrate, S-adenosylmethionine
(SAM) . In the dimeric
form of ACS, both PLP
and SAM are bound to the active site. The PLP
ligands interact with the residues of the second ACS
monomer, which helps to stabilize the active dimeric form of the
forms bonds with ACS with four specific structural components :
Click reset protein between each button for the list
of the pyridine forms a covalent bond with amine of Lys278
of the pyridine forms a H-bond with Asp237
of pyridine forms a H-bond with oxygen (O) of Tyr240
and nitrogen (N) of Asn209
Each of these PLP-ACS
bonds are a means to stabilizing the structure. For example,
in the N1-Asp237
hydrogen bond, the aspartate's negative charge is able to
stabilize the protonation state of the pyridine nitrogen.
It should be noted that while PLP
binding and interactions are largely conserved among most
ACC synthase-carrying plant species, the specific amino acid
sites where the interactions take place may vary; the
information above applies most accurately to tomato ACC
IV. Substrate SAM Binding
(SAM) is the substrate that catalyzes the reaction. As
substrate binding proceeds, SAM
interacts with the PLP-ACS
This interaction relies on the participation of a
variety of structures and residues. As PLP
and SAM both bind to the active sites, proper
positioning is crucial. SAM was not crystallized in
this pdb file, but all the residues of ACS involved in
interactions were highlighted. This is promoted by Ala127,
Ser277, and Arg286,
in part due to H-bonding with
. There are a
number of residue interactions between SAM and ACS
that allow for its binding, which include :
between the positively-charged sulfonium group
within SAM and Glu55
from the alpha-carboxylate of SAM to nitrogen (N) of Ala54
and the guanidinium group (g-group) of Arg412
between the O2' and O3' of SAM and the guanidinium group
(g-group) of Arg157
between the adenine ring portion of SAM and a hydrophobic
pocket resulting from Pro26,
V. Inhibitor AVG Binding
(AVG) is a competitive inhibitor of ACC synthase, it
is used in the agricultural industry to reduce pre-harvest
drop, delay fruit ripening, and reduce ethylene production
AVG is becoming more
commonly used, it is important to identify how AVG
interacts with ACC synthase and other parts of the
Click reset complex between each button.
AVG binds to a location
neighboring the cofactor, PLP.
The alpha-carboxylate group of AVG
forms three hydrogen bonds with PLP
, and water. Both Ala54
and Arg412 are highly
conserved amino acids in ACS because they are also binding
sites for SAM. Previous studies identified that substitution
at these sites reduces the rate of catalysis. The
substitution of the apple ACC synthase equivalent to Arg412
to lysine results in an increase in KM .
KM is the measurement of the substrate
concentration required for catalysis to occur. The increase
in KM when substituting Arg407 for Lys
indicates there needs to be a higher concentration of
substrate in order for catalysis to occur. As apple and
tomato ACC synthase share many similarities, it is likely to
assume that the replacement of Arg412
in tomato ACC synthase would have a similar effect.
Additionally, AVG forms a van
der Waals contact with
. As mentioned earlier, Tyr152
is parallel to the ring of PLP, which allows it to stack,
stabilizing the structure.
While the ethylene biosynthesis inhibition effects of AVG
has been studied, there is very limited research on how it
affects the plant in other manners. A previous study found
that AVG presence is correlated
to reduced protein synthesis .
However, there are not any results correlating AVG
use to any consumer harm, so it is continued to be used in
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