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PDBsum entry 2bvd
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References listed in PDB file
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Key reference
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Title
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How family 26 glycoside hydrolases orchestrate catalysis on different polysaccharides: structure and activity of a clostridium thermocellum lichenase, Ctlic26a.
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Authors
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E.J.Taylor,
A.Goyal,
C.I.Guerreiro,
J.A.Prates,
V.A.Money,
N.Ferry,
C.Morland,
A.Planas,
J.A.Macdonald,
R.V.Stick,
H.J.Gilbert,
C.M.Fontes,
G.J.Davies.
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Ref.
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J Biol Chem, 2005,
280,
32761-32767.
[DOI no: ]
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PubMed id
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Abstract
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One of the most intriguing features of the 90 glycoside hydrolase families (GHs)
is the range of specificities displayed by different members of the same family,
whereas the catalytic apparatus and mechanism are often invariant. Family GH26
predominantly comprises beta-1,4 mannanases; however, a bifunctional Clostridium
thermocellum GH26 member (hereafter CtLic26A) displays a markedly different
specificity. We show that CtLic26A is a lichenase, specific for mixed
(Glcbeta1,4Glcbeta1,4Glcbeta1,3)n oligo- and polysaccharides, and displays no
activity on manno-configured substrates or beta-1,4-linked homopolymers of
glucose or xylose. The three-dimensional structure of the native form of
CtLic26A has been solved at 1.50-A resolution, revealing a characteristic
(beta/alpha)8 barrel with Glu-109 and Glu-222 acting as the catalytic acid/base
and nucleophile in a double-displacement mechanism. The complex with the
competitive inhibitor, Glc-beta-1,3-isofagomine (Ki 1 microm), at 1.60 A sheds
light on substrate recognition in the -2 and -1 subsites and illuminates why the
enzyme is specific for lichenan-based substrates. Hydrolysis of beta-mannosides
by GH26 members is thought to proceed through transition states in the B2,5
(boat) conformation in which structural distinction of glucosides versus
mannosides reflects not the configuration at C2 but the recognition of the
pseudoaxial O3 of the B2,5 conformation. We suggest a different conformational
itinerary for the GH26 enzymes active on gluco-configured substrates.
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Figure 1.
FIGURE 1. Schematic of the molecular architecture of
CtLic26A-Cel5E. The modules encoded by the defined recombinant
plasmids are indicated with the gray and black boxes
representing the signal peptide and linker sequences,
respectively.
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Figure 4.
FIGURE 4. Schematic diagram of the interactions of the C.
thermocellum CtLic26A with Glc-IsoF.
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2005,
280,
32761-32767)
copyright 2005.
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