The crystal structure of the complex formed by bovine trypsin and bovine pancreatic trypsin inhibitor has been refined with data to 1.9 Å resolution, using a procedure described by Deisenhofer & Steigemann (1974) in their refinement of the crystal structure of the free inhibitor. This procedure involves cycles consisting of phase calculation using the current atomic model, Fourier synthesis using these phases and the observed structure factor amplitudes and Diamond's real-space refinement (Diamond, 1971,1974). At various stages, difference Fourier syntheses are calculated to detect and correct gross errors in the model and to localize solvent molecules. The refinement progressed smoothly, starting with the model obtained from the isomorphous Fourier map at 2.6 Å resolution. The R-factor is 0.23 for 20,500 significantly measured reflections to 1.9 Å resolution, using an over-all temperature factor of 20 Å2. The estimated standard deviation of atomic positions is 0.09 Å. An objective assessment of the upper limit of the error in the atomic coordinates of the final model is possible by comparing the inhibitor component in the model of the complex with the refined structure of the free inhibitor (Deisenhofer & Steigemann, 1974). The mean deviation of main-chain atoms of the two molecular models in internal segments is 0.25 Å, of main-chain dihedral angles 5.1 ° and side-chain dihedral angles 6.5 °. A comparison of the trypsin component with α-chymotrypsin (Birktoft & Blow, 1972) showed a mean deviation of main-chain atoms of 0.75 Å. The structures are closely similar and the various deletions and insertions cause local structural differences only.
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