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Contents |
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* Residue conservation analysis
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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 of penicillium citrinum alpha 1,2-Mannosidase reveals the basis for differences in specificity of the endoplasmic reticulum and golgi class i enzymes.
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Authors
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Y.D.Lobsanov,
F.Vallée,
A.Imberty,
T.Yoshida,
P.Yip,
A.Herscovics,
P.L.Howell.
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Ref.
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J Biol Chem, 2002,
277,
5620-5630.
[DOI no: ]
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PubMed id
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Abstract
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Class I alpha1,2-mannosidases (glycosylhydrolase family 47) are key enzymes in
the maturation of N-glycans. This protein family includes two distinct
enzymatically active subgroups. Subgroup 1 includes the yeast and human
endoplasmic reticulum (ER) alpha1,2-mannosidases that primarily trim
Man(9)GlcNAc(2) to Man(8)GlcNAc(2) isomer B whereas subgroup 2 includes
mammalian Golgi alpha1,2-mannosidases IA, IB, and IC that trim Man(9)GlcNAc(2)
to Man(5)GlcNAc(2) via Man(8)GlcNAc(2) isomers A and C. The structure of the
catalytic domain of the subgroup 2 alpha1,2-mannosidase from Penicillium
citrinum has been determined by molecular replacement at 2.2-A resolution. The
fungal alpha1,2-mannosidase is an (alphaalpha)(7)-helix barrel, very similar to
the subgroup 1 yeast (Vallée, F., Lipari, F., Yip, P., Sleno, B., Herscovics,
A., and Howell, P. L. (2000) EMBO J. 19, 581-588) and human (Vallée, F.,
Karaveg, K., Herscovics, A., Moremen, K. W., and Howell, P. L. (2000) J. Biol.
Chem. 275, 41287-41298) ER enzymes. The location of the conserved acidic
residues of the catalytic site and the binding of the inhibitors, kifunensine
and 1-deoxymannojirimycin, to the essential calcium ion are conserved in the
fungal enzyme. However, there are major structural differences in the
oligosaccharide binding site between the two alpha1,2-mannosidase subgroups. In
the subgroup 1 enzymes, an arginine residue plays a critical role in stabilizing
the oligosaccharide substrate. In the fungal alpha1,2-mannosidase this arginine
is replaced by glycine. This replacement and other sequence variations result in
a more spacious carbohydrate binding site. Modeling studies of interactions
between the yeast, human and fungal enzymes with different Man(8)GlcNAc(2)
isomers indicate that there is a greater degree of freedom to bind the
oligosaccharide in the active site of the fungal enzyme than in the yeast and
human ER alpha1,2-mannosidases.
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Figure 1.
Fig. 1. Schematic representation of the Man[9]GlcNAc[2].
The mannose residues are labeled M3 to M11. Removal of mannose
residues M11, M10, and M9 results in the formation of
Man[8]GlcNAc[2] isomers A, B, and C, respectively. Residues on
each of the A, B, and C branches are colored purple, red, and
orange, respectively. The torsion angles varied in the energy
calculations, and the corresponding glycosidic linkages are
labeled according to the type of saccharide unit (M, mannose;
GN, N-acetylglucosamine), the chemical linkage (12,  1,2; 13,
1,3; 14,
1,4; 16,
1,6), and
the branch they belong to (A, B, C). To distinguish the 1,2
linkages between M11 and M8 from that between M8 and M5 on
branch A, the linkage between M11 and M8 is designated M12M-AA.
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Figure 4.
Fig. 4. A, structural superposition of the conserved
acidic residues and calcium ions in the active site region of
the fungal (red), yeast (yellow), and human (blue) enzymes. Only
a subset of the 11 catalytic residues is shown for clarity.
Superposition was done with LSQMAN (34) using all C[  ]atoms.
B, structural superposition of the FM·dMNJ ( pink),
FM·KIF (purple), HM·dMNJ (yellow), and
HM·KIF (green). Only residues implicated in catalysis are
shown. The following fungal residues are shown (the equivalent
yeast numbering is in parentheses): Glu122 (Glu132), Asp267
(Asp275), Ser268 (Ser276), Glu271 (Glu279), Glu409 (Glu435),
Glu412 (Glu438), Glu472 (Glu503), Thr501 (Thr525), Glu502
(Glu526).
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2002,
277,
5620-5630)
copyright 2002.
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