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Structural effects induced by mutagenesis affected by crystal packing factors: The structure of a 30–51 disulfide mutant of basic pancreatic trypsin inhibitor

The X‐ray structure of the C30V/C51A disulfide mutant of basic pancreatic trypsin inhibitor (BPTI) has been analyzed at 1.6 Å resolution. The mutant crystallizes in a cell having two molecules in the asymmetric unit. The packing environments of these two molecules are quite different, allowing for an assessment of which among the observed structural changes result from the mutation and which are produced by lattice packing considerations. The removal of the 30–51 disulfide bridge has little apparent affect on the B‐factors of segments of adjacent polypeptide chain, although there are distinct differences in the structure compared to wild‐type BPTI crystal structures. Both of the two C30V/C51A molecules show differences at the mutation site when compared to another 30–51 disulfide mutant, C30A/C51A, presumably due to the larger steric bulk of a valine versus an alanine at residue 30. A comparison of the two independent C30V/C51A molecules indicates that there are significant differences between them even at the site of mutation. The description of the specific structural differences of each molecule differs in detail and suggests different conclusions about the nature of structural perturbation near 30–51. In addition, when these two molecules are compared to two different wild‐type structures, which had been determined from different space groups, a somewhat different pattern of changes is observed. These findings indicate that crystal packing can influence the observed perturbations in mutant structures. © 1992 Wiley‐Liss, Inc.