A family IIb xylan-binding domain has a similar secondary structure to a homologous family IIa cellulose-binding domain but different ligand specificity.
BACKGROUND
Many enzymes that digest polysaccharides contain separate polysaccharide-binding domains. Structures have been previously determined for a number of cellulose-binding domains (CBDs) from cellulases.


RESULTS
The family IIb xylan-binding domain 1 (XBD1) from Cellulomonas fimi xylanase D is shown to bind xylan but not cellulose. Its structure is similar to that of the homologous family IIa CBD from C. fimi Cex, consisting of two four-stranded beta sheets that form a twisted 'beta sandwich'. The xylan-binding site is a groove made from two tryptophan residues that stack against the faces of the sugar rings, plus several hydrogen-bonding polar residues.


CONCLUSIONS
The biggest difference between the family IIa and IIb domains is that in the former the solvent-exposed tryptophan sidechains are coplanar, whereas in the latter they are perpendicular, forming a twisted binding site. The binding sites are therefore complementary to the secondary structures of the ligands cellulose and xylan. XBD1 and CexCBD represent a striking example of two proteins that have high sequence similarity but a different function.
Simpson
P J
PJ
Department of Molecular Biology and Biotechnology, Krebs Institute, University of Sheffield, UK.
Bolam
D N
DN
Cooper
A
A
Ciruela
A
A
Hazlewood
G P
GP
Gilbert
H J
HJ
Williamson
M P
MP
D000595:Amino Acid Sequence; D001483:Base Sequence; D002482:Cellulose / Q000378:metabolism*; D018360:Crystallography, X-Ray; D017931:DNA Primers; D043364:Endo-1,4-beta Xylanases; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D013816:Thermodynamics; D014990:Xylans / Q000378:metabolism*; D014995:Xylosidases / Q000737:chemistry* / Q000378:metabolism; D001617:beta-Glucosidase / Q000737:chemistry* / Q000378:metabolism
D017931:DNA Primers; D008024:Ligands; D014990:Xylans; D002482:Cellulose; D014995:Xylosidases; D001617:beta-Glucosidase; D043364:Endo-1,4-beta Xylanases