PDBsum entry 1e5b

Hydrolase

PDB id: 1e5b

Contents

Protein chain

87 a.a. *

* Residue conservation analysis

References listed in PDB file

Key reference

Title

The structural basis for the ligand specificity of family 2 carbohydrate-Binding modules.

Authors

P.J.Simpson, H.Xie, D.N.Bolam, H.J.Gilbert, M.P.Williamson.

Ref.

J Biol Chem, 2000, 275, 41137-41142. [DOI no: 10.1074/jbc.M006948200]

PubMed id

10973978

Abstract

The interactions of proteins with polysaccharides play a key role in the microbial hydrolysis of cellulose and xylan, the most abundant organic molecules in the biosphere, and are thus pivotal to the recycling of photosynthetically fixed carbon. Enzymes that attack these recalcitrant polymers have a modular structure comprising catalytic modules and non-catalytic carbohydrate-binding modules (CBMs). The largest prokaryotic CBM family, CBM2, contains members that bind cellulose (CBM2a) and xylan (CBM2b), respectively. A possible explanation for the different ligand specificity of CBM2b is that one of the surface tryptophans involved in the protein-carbohydrate interaction is rotated by 90 degrees compared with its position in CBM2a (thus matching the structure of the binding site to the helical secondary structure of xylan), which may be promoted by a single amino acid difference between the two families. Here we show that by mutation of this single residue (Arg-262-->Gly), a CBM2b xylan-binding module completely loses its affinity for xylan and becomes a cellulose-binding module. The structural effect of the mutation has been revealed using NMR spectroscopy, which confirms that Trp-259 rotates 90 degrees to lie flat against the protein surface. Except for this one residue, the mutation only results in minor changes to the structure. The mutated protein interacts with cellulose using the same residues that the wild-type CBM2b uses to interact with xylan, suggesting that the recognition is of the secondary structure of the polysaccharide rather than any specific recognition of the absence or presence of functional groups.

Figure 2.
Fig. 2. The major functional difference between CBM Families 2a and 2b. MOLSCRIPT (28) depictions of the key surface tryptophan, and the residue (Gly or Arg) that determines its orientation.

Figure 4.
Fig. 4. Structure of R262G in a stereo view of the backbone for an ensemble of 33 structures, superimposed for best fit on the lowest energy structure. Trp-259 and Trp-291 sidechains are indicated.

The above figures are reprinted by permission from the ASBMB: J Biol Chem (2000, 275, 41137-41142) copyright 2000.