Structural comparison of Escherichia coli L-asparaginase in two monoclinic space groups

The functional L-asparaginase from Escherichia coli is a homotetramer with a molecular weight of about 142 kDa. The X-ray structure of the enzyme, crystallized in a new form (space group C2) and refined to 1.95 Å resolution, is compared with that of the previously determined crystal form (space group P2₁). The asymmetric unit of the new crystal form contains an L-asparaginase dimer instead of the tetramer found in the previous crystal form. It is found that crystal contacts practically do not affect the conformation of the protein. It is shown that subunit C of the tetrameric form is in a conformation which is systematically different from that of all other subunits in both crystal forms. Major conformational differences are confined to the lid loop (residues 14-27). In addition, the stability of this globular protein is analyzed in terms of the interactions between hydrophobic parts of the subunits.