Structure of concanavalin A at 4.25-ångström resolution

An electron density map produced by x-ray diffraction analysis of concanavalin A has been calculated to 4.25 A from data of three isomorphous heavy atom derivatives. The crystals are orthorhombic, with unit-cell dimensions of 63.1, 87.0, and 89.2 A for a, b, and c, respectively. The space group is I222, with eight asymmetric units per unit cell. The crystal asymmetric unit contains 27,000 daltons of protein and reflects the chemically unique component (protomer) within the oligomer. Separate chemical studies indicate that the protomer consists of two different polypeptide chains. Four protomers cluster around the intersection of three mutually perpendicular two-fold rotation axes to form a molecule of 108,000 daltons. The molecule can also be subdivided into two-protomer units of 54,000 daltons. Within the two-protomer unit, there are significantly more contacts joining the protomers than there are between adjacent two-protomer units that form the total molecule. These results provide a possible explanation for disagreement in molecular weights obtained in previous ultracentrifugal studies.