A comparison of the solution n.m.r. structures of barley serine protease inhibitor 2 (BSPI-2) with the X-ray structures of both subtilisin complexed and native BSPI-2 is presented. It is shown that the n.m.r. and X-ray structures are very similar in terms of overall shape, size, polypeptide fold and secondary structure. The average atomic rms difference between the 11 restrained dynamics structures on the one hand and the two X-ray structures on the other is 1.9 +/- 0.2 A for the backbone atoms and 3.0 +/- 0.3 A for all atoms. The corresponding values for the restrained energy minimized mean dynamics structure are 1.5 and 2.4 A, respectively.
The solution structure of the 64 residue structured domain (residues 20-83) of barley serine proteinase inhibitor 2 (BSPI-2) is determined on the basis of 403 interproton distance, 34 phi backbone torsion angle and 26 hydrogen bonding restraints derived from n.m.r. measurements. A total of 11 converged structures were computed using a metric matrix distance geometry algorithm and refined by restrained molecular dynamics. The average rms difference between the final 11 structures and the mean structure obtained by averaging their coordinates is 1.4 +/- 0.2 A for the backbone atoms and 2.1 +/- 0.1 A for all atoms. The overall structure, which is almost identical to that found by X-ray crystallography, is disc shaped and consists of a central four component mixed parallel and antiparallel beta-sheet flanked by a 13 residue alpha-helix on one side and the reactive site loop on the other.
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