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134 a.a.
_NA
Key reference
DOI no: 10.1074/jbc.M410113200 J Biol Chem 280:530-537 (2005) PubMed id: 15501830
Family 6 carbohydrate binding modules recognize the non-reducing end of beta-1,3-linked glucans by presenting a unique ligand binding surface. A.L.van Bueren, C.Morland, H.J.Gilbert, A.B.Boraston. ABSTRACT
Enzymes that hydrolyze insoluble complex polysaccharide structures contain non-catalytic carbohydrate binding modules (CBMS) that play a pivotal role in the action of these enzymes against recalcitrant substrates. Family 6 CBMs (CBM6s) are distinct from other CBM families in that these protein modules contain multiple distinct ligand binding sites, a feature that makes CBM6s particularly appropriate receptors for the beta-1,3-glucan laminarin, which displays an extended U-shaped conformation. To investigate the mechanism by which family 6 CBMs recognize laminarin, we report the biochemical and structural properties of a CBM6 (designated BhCBM6) that is located in an enzyme, which is shown, in this work, to display beta-1,3-glucanase activity. BhCBM6 binds beta-1,3-glucooligosaccharides with affinities of approximately 1 x 10(5) m(-1). The x-ray crystal structure of this CBM in complex with laminarihexaose reveals similarity with the structures of other CBM6s but a unique binding mode. The binding cleft in this protein is sealed at one end, which prevents binding of linear polysaccharides such as cellulose, and the orientation of the sugar at this site prevents glycone extension of the ligand and thus conferring specificity for the non-reducing ends of glycans. The high affinity for extended beta-1,3-glucooligosaccharides is conferred by interactions with the surface of the protein located between the two binding sites common to CBM6s and thus reveals a third ligand binding site in family 6 CBMs. This study therefore demonstrates how the multiple binding clefts and highly unusual protein surface of family 6 CBMs confers the extensive range of specificities displayed by this protein family. This is in sharp contrast to other families of CBMs where variation in specificity between different members reflects differences in the topology of a single binding site.
Selected figure(s)
Figure 4.
FIG. 4. Observed electron density for xylobiose (A) and laminarihexaose (B) bound to BhCBM6. All maps are maximum-likelihood (25)/[A] (38) weighted 2F[obs] - F[calc] electron density maps contoured at 1
Figure 5.
FIG. 5. A schematic showing the interactions of BhCBM6 with xylobiose (A) and laminarihexaose (B). Binding subsites referred to in the text are shown above the schematics with brackets and are numbered in accordance with IUPAC nomenclature. The water molecule conserved in the cleft A binding site of CBM6s is indicated with an arrow.The above figures are reprinted by permission from the ASBMB: J Biol Chem (2005, 280, 530-537) copyright 2005. Figures were selected by an automated process.
Literature references that cite this PDB file's key reference
PubMed id Reference
19218457 C.Montanier, A.L.van Bueren, C.Dumon, J.E.Flint, M.A.Correia, J.A.Prates, S.J.Firbank, R.J.Lewis, G.G.Grondin, M.G.Ghinet, T.M.Gloster, C.Herve, J.P.Knox, B.G.Talbot, J.P.Turkenburg, J.Kerovuo, R.Brzezinski, C.M.Fontes, G.J.Davies, A.B.Boraston, and H.J.Gilbert (2009).
Evidence that family 35 carbohydrate binding modules display conserved specificity but divergent function.Proc Natl Acad Sci U S A, 106, 3065-3070.
PDB codes: 2vzp 2vzq 2vzr 2w1w 2w3j 2w46 2w47 2w87
19240276 G.Michel, T.Barbeyron, B.Kloareg, and M.Czjzek (2009).
The family 6 carbohydrate-binding modules have coevolved with their appended catalytic modules toward similar substrate specificity.Glycobiology, 19, 615-623.
18074341 A.D.Hill, and P.J.Reilly (2008).
A Gibbs free energy correlation for automated docking of carbohydrates.J Comput Chem, 29, 1131-1141.
17473009 J.Brown, C.A.O'Callaghan, A.S.Marshall, R.J.Gilbert, C.Siebold, S.Gordon, G.D.Brown, and E.Y.Jones (2007).
Structure of the fungal beta-glucan-binding immune receptor dectin-1: implications for function.Protein Sci, 16, 1042-1052.
PDB codes: 2bpd 2bpe 2bph 2cl8
18032609 L.Cuthbertson, M.S.Kimber, and C.Whitfield (2007).
Substrate binding by a bacterial ABC transporter involved in polysaccharide export.Proc Natl Acad Sci U S A, 104, 19529-19534.
PDB code: 2r5o
16537424 L.McCartney, A.W.Blake, J.Flint, D.N.Bolam, A.B.Boraston, H.J.Gilbert, and J.P.Knox (2006).
Differential recognition of plant cell walls by microbial xylan-specific carbohydrate-binding modules.Proc Natl Acad Sci U S A, 103, 4765-4770.
17005007 M.S.Centeno, A.Goyal, J.A.Prates, L.M.Ferreira, H.J.Gilbert, and C.M.Fontes (2006).
Novel modular enzymes encoded by a cellulase gene cluster in Cellvibrio mixtus.FEMS Microbiol Lett, 265, 26-34. The most recent references are shown first. Citation data come partly from CiteXplore and partly from an automated harvesting procedure. Note that this is likely to be only a partial list as not all journals are covered by either method. However, we are continually building up the citation data so more and more references will be included with time. Where a reference describes a PDB structure, the PDB codes are shown on the right.
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