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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.
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