Ultrahigh-resolution structure of a BPTI mutant

The crystal structure of a mutant of bovine pancreatic trypsin inhibitor ha s been refined to 0.86 Angstrom resolution using low-temperature synchrotro n data. The variant contains three mutations in the binding loop (Thr11Ala, Pro13Ala, Lys15Arg) and an unrelated Met52Leu substitution. Refinement wit h anisotropic displacement parameters and with removal of main-chain stereo chemical restraints converged with R = 0.1035. The use of full-matrix refin ement provided an estimate of the variances in the derived parameters. Some stereochemical parameters, such as the planarity of the peptide group and the value of the N-C-alpha-C angle, show a wide spread, suggesting that the standard values used as restraints in protein structure refinements may no t always be entirely appropriate. Comparison with the recently determined r oom-temperature structure of the same mutant at 1.42 Angstrom resolution co nfirms the previous observations and provides new details, such as a double conformation of the main chain at Leu29 and at Gly56-Gly57, a high proport ion (over 20%) of residues in double conformations, correlation of disorder through lattice contacts and the positions of H atoms, including those in water molecules, and their involvement in C-H . . .O and N-H . . . pi hydro gen bonds.