Abstract
Despite the fact that complex saccharides play an important role in many biological recognition processes, molecular level descriptions of protein-carbohydrate interactions are sparse. The legume lectin concanavalin A (con A), from Canavalia ensiformis, specifically recognizes the trimannoside core of many complex glycans. We have determined the crystal structure of a con A-trimannoside complex at 2.3-A resolution now describe the trimannoside interaction with conA. All three sugar residues are in well defined difference electron density. The 1,6-linked mannose residue is bound at the previously reported monosaccharide binding site; the other two sugars bind in an extended cleft formed by residues Tyr-12, Pro-13, Asn-14, Thr-15, and Asp-16. Hydrogen bonds are formed between the protein and all three sugar residues. In particular, the 1,3-linked mannose residue makes a strong hydrogen bond with the main chain of the protein. In addition, a water molecule, which is conserved in other con A structures, plays an important role in anchoring the reducing sugar unit to the protein. The complex is further stabilized by van der Waals interactions. The structure provides a rationale for the high affinity of con A for N-linked glycans.
Highlights
The con A-trimannoside structure reveals a highly specific interaction based on hydrogen bonds to the main chain and/or side chain of the protein from all three sugar residues
A structurally conserved water molecule appears to play a crucial role in recognition
A comparison between the native and mannoside complexes of con A at 2.0-Å resolution shows that trimannoside binds to what appears to be a relatively rigid and preformed extended binding site from which water molecules are displaced
Summary
From the Centre for Biomolecular Sciences, Purdie Building, The University, St. Andrews, Fife KY16 9ST, Scotland, United Kingdom. Despite the fact that complex saccharides play an important role in many biological recognition processes, molecular level descriptions of protein-carbohydrate interactions are sparse. A water molecule, which is conserved in other con A structures, plays an important role in anchoring the reducing sugar unit to the protein. The atomic coordinates and structure factors (codes 5CNA, 1 CVN, and R1CVNSF) have been deposited in the Protein Data Bank, Brookhaven National Laboratory, Upton, NY Their interaction with saccharides has proved a valuable source of fundamental information. The sugar was reported to be anchored to the protein by several direct hydrogen bonds and by van der Waals interactions. The precise contributions of hydrogen bonding, van der Waals interactions, and rearrangement of bound and bulk water to the specificity of the lectin-oligosaccharide interactions continues to be a subject of interest (20). We report the 2.3-Å resolution structure of con A bound to the N-linked glycan core trimannoside Man␣1– 6(Man␣1–3)Man
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