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PDBsum entry 2ipf
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Oxidoreductase
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PDB id
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2ipf
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References listed in PDB file
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Key reference
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Title
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Mouse 17alpha-Hydroxysteroid dehydrogenase (akr1c21) binds steroids differently from other aldo-Keto reductases: identification and characterization of amino acid residues critical for substrate binding.
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Authors
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F.Faucher,
L.Cantin,
K.Pereira de jésus-Tran,
M.Lemieux,
V.Luu-The,
F.Labrie,
R.Breton.
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Ref.
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J Mol Biol, 2007,
369,
525-540.
[DOI no: ]
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PubMed id
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Abstract
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The mouse 17alpha-hydroxysteroid dehydrogenase (m17alpha-HSD) is the unique
known member of the aldo-keto reductase (AKR) superfamily able to catalyze
efficiently and in a stereospecific manner the conversion of androstenedione
(Delta4) into epi-testosterone (epi-T), the 17alpha-epimer of testosterone.
Structural and mutagenic studies had already identified one of the residues
delineating the steroid-binding cavity, A24, as the major molecular determinant
for the stereospecificity of m17alpha-HSD. We report here a ternary complex
crystal structure (m17alpha-HSD:NADP(+):epi-T) determined at 1.85 A resolution
that confirms this and reveals a unique steroid-binding mode for an AKR enzyme.
Indeed, in addition to the interactions found in all other AKRs (van der Waals
contacts stabilizing the core of the steroid and the hydrogen bonds established
at the catalytic site by the Y55 and H117 residues with the oxygen atom of the
ketone group to be reduced), m17alpha-HSD establishes with the other extremity
of the steroid nucleus an additional interaction involving K31. By combining
direct mutagenesis and kinetic studies, we found that the elimination of this
hydrogen bond did not affect the affinity of the enzyme for its steroid
substrate but led to a slight but significant increase of its catalytic
efficiency (k(cat)/K(m)), suggesting a role for K31 in the release of the
steroidal product at the end of the reaction. This previously unobserved
steroid-binding mode for an AKR is similar to that adopted by other
steroid-binding proteins, the hydroxysteroid dehydrogenases of the short-chain
dehydrogenases/reductases (SDR) family and the steroid hormone nuclear
receptors. Mutagenesis and structural studies made on the human type 3
3alpha-HSD, a closely related enzyme that shares 73% amino acids identity with
the m17alpha-HSD, also revealed that the residue at position 24 of these two
enzymes directly affects the binding and/or the release of NADPH, in addition to
its role in their 17alpha/17beta stereospecificity.
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Figure 1.
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Figure 6.
Figure 6. Comparison of the steroid-binding cavity of the
m17α-HSD with the more spacious one of the h3α-HSD3.
Superposition of the residues making the hydrophobic contacts
between the enzyme and the steroid (m17α-HSD with epi-T (in
standard CPK color set) and h3α-HSD3 with T (in orange)).
Except for the position 24, the side-chain of the residues in
the m17α-HSD are generally much bulkier than residues at the
same position in the h3α-HSD3 structure. Residues of the
m17α-HSD enzyme are identified with the one letter code and the
corresponding residue in the h3α-HSD3 sequence is in
parenthesis when it differs. The Figure was generated with Pymol
(DeLano Scientific).
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The above figures are
reprinted
by permission from Elsevier:
J Mol Biol
(2007,
369,
525-540)
copyright 2007.
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