The structure of the Gla‐domainless form of the human anticoagulant enzyme activated protein C has been solved at 2.8 A resolution. The light chain is composed of two domains: an epidermal growth factor (EGF)‐like domain modified by a large insert containing an additional disulfide, followed by a typical EGF‐like domain. The arrangement of the long axis of these domains describes an angle of approximately 80 degrees. Disulfide linked to the light chain is the catalytic domain, which is generally trypsin‐like but contains a large insertion loop at the edge of the active site, a third helical segment, a prominent cationic patch analogous to the anion binding exosite I of thrombin and a trypsin‐like Ca[II] binding site. The arrangement of loops around the active site partially restricts access to the cleft. The S2 and S4 subsites are much more polar than in factor Xa and thrombin, and the S2 site is unrestricted. While quite open and exposed, the active site contains a prominent groove, the surface of which is very polar with evidence for binding sites on the primed side, in addition to those typical of the trypsin class found on the non‐primed side.

Crystalline holo inorganic pyrophosphatase from Escherichia coli was grown in the presence of 250 mM MgCl2. The crystal structure has been solved by Patterson search techniques and refined to an R-factor of 17.6% at 1.9 A resolution. The upper estimate of the root-mean-square error in atomic positions is 0.26 A. These crystals belong to space group P3(2)21 with unit cell dimensions a = b = 110.27 A and c = 78.17 A. The asymmetric unit contains a trimer of subunits, i.e., half of the hexameric molecule. In the central cavity of the enzyme molecule, three Mg2+ ions, each shared by two subunits of the hexamer, are found. In the active sites of two crystallographically independent subunits, two Mg2+ ions are bound. The second active site Mg2+ ion is missing in the third subunit. A mechanism of catalysis is proposed whereby a water molecule activated by a Mg2+ ion and Tyr 55 play essential roles.