THE STRUCTURE OF CONCANAVALIN-A AND ITS BOUND SOLVENT DETERMINED WITHSMALL-MOLECULE ACCURACY AT 0.94 ANGSTROM RESOLUTION

The structure of concanavalin A, a saccharide-binding metalloprotein o f subunit molecular weight 25 000 Da , is reported with precision and accuracy equivalent to those of small-molecule crystal structures. An X-ray data-to-parameter ratio of 6.1:1 has allowed the first unrestrai ned anisotropic refinement of a protein structure and, indeed, one of large molecular weight. This has been achieved by the combined power o f synchrotron radiation, a highly sensitive CCD-based area detector an d cryogenic techniques on crystals of exceptional quality. The high pr ecision of the atomic coordinates and the direct observation of many h ydrogen atom positions, on both the protein and bound solvent, provide new insights into the protein structure, including precise geometry o f the metal-binding sites, the role of water molecules in the extended saccharide-binding site, the structure and dynamics of bound and buri ed water, and direct evidence that proximal carboxylic acid side chain s can exist as carboxyl-carboxylate pairs. The biophysical chemistry o f this protein is thereby elucidated in detail in this and the compani on paper.