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PDBsum entry 2qw8
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Oxidoreductase
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PDB id
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2qw8
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References listed in PDB file
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Key reference
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Title
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Structure and reaction mechanism of basil eugenol synthase.
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Authors
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G.V.Louie,
T.J.Baiga,
M.E.Bowman,
T.Koeduka,
J.H.Taylor,
S.M.Spassova,
E.Pichersky,
J.P.Noel.
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Ref.
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Plos One, 2007,
2,
e993.
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PubMed id
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Abstract
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Phenylpropenes, a large group of plant volatile compounds that serve in multiple
roles in defense and pollinator attraction, contain a propenyl side chain.
Eugenol synthase (EGS) catalyzes the reductive displacement of acetate from the
propenyl side chain of the substrate coniferyl acetate to produce the
allyl-phenylpropene eugenol. We report here the structure determination of EGS
from basil (Ocimum basilicum) by protein x-ray crystallography. EGS is
structurally related to the short-chain dehydrogenase/reductases (SDRs), and in
particular, enzymes in the isoflavone-reductase-like subfamily. The structure of
a ternary complex of EGS bound to the cofactor NADP(H) and a mixed competitive
inhibitor EMDF ((7S,8S)-ethyl (7,8-methylene)-dihydroferulate) provides a
detailed view of the binding interactions within the EGS active site and a
starting point for mutagenic examination of the unusual reductive mechanism of
EGS. The key interactions between EMDF and the EGS-holoenzyme include stacking
of the phenyl ring of EMDF against the cofactor's nicotinamide ring and a
water-mediated hydrogen-bonding interaction between the EMDF 4-hydroxy group and
the side-chain amino moiety of a conserved lysine residue, Lys132. The C4 carbon
of nicotinamide resides immediately adjacent to the site of hydride addition,
the C7 carbon of cinnamyl acetate substrates. The inhibitor-bound EGS structure
suggests a two-step reaction mechanism involving the formation of a
quinone-methide prior to reduction. The formation of this intermediate is
promoted by a hydrogen-bonding network that favors deprotonation of the
substrate's 4-hydroxyl group and disfavors binding of the acetate moiety, akin
to a push-pull catalytic mechanism. Notably, the catalytic involvement in EGS of
the conserved Lys132 in preparing the phenolic substrate for quinone methide
formation through the proton-relay network appears to be an adaptation of the
analogous role in hydrogen bonding played by the equivalent lysine residue in
other enzymes of the SDR family.
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