Structure of anionic salmon trypsin in a second crystal form

Anionic salmon trypsin in a second crystal form (ST-IIB) has been refined at 1.83 Å, resolution. The crystals are orthorhombic and belong to space group P2₁2₁2 with lattice parameters a = 77.09, b = 82.33 and c = 31.16 Å. The present structure has been compared to salmon trypsin as it appears in a previously reported crystal form (ST-IIA) with cell dimensions a = 61.95, b = 84.33 and c = 39.11 Å [Smalås & Hordvik (1993). Acta Cryst. D49, 318-330]. The presence of a sulfate group involved in several hydrogen bonds to active-site residues, and the location of an additional benzamidine site in the crystal lattice, are the most striking differences between the present and the previous structure. Superposition of main-chain atoms in the two structures give an overall r.m.s. difference of 0.26 Å, with the main differences located to areas with different molecular packing. The overall coordinate error is estimated to be between 0.20 and 0.25 Å, by the method of Luzzati.