The molecular structures of the acetate complexes of wild-type human carbonic anhydrase II (HCAII) and of E106Q mutant human carbonic anhydrase II were solved with high completeness (89-91%) to 2.1 and 1.9 A resolution, respectively. Both wild-type and mutant enzyme crystallize in space group P2(1) with cell dimensions a = 42.7, b = 41.7, c = 73.0 A and beta = 104.6 degrees. The altered active-site hydrogen-bond network caused by the mutation results in a different binding of the inhibitor in the two complexes. In the mutant, but not in the wild-type complex, a carboxylate O atom is within hydrogen-bond distance of Thr199 Ogamma1. In the wild-type enzyme ligand hydrogen bonding to this atom is normally only found for hydrogen-bond donors. The importance of this discrimination on catalysis by the enzyme is discussed briefly.