Structural consequences of the natural substitution, E9K, on reactive-site-hydrolyzed squash (Cucurbita maxima) trypsin inhibitor (CMTI), as studied by two-dimensional NMR.

Sequence-specific hydrogen-1 NMR assignments were made to all of the 29 amino acid residues of reactive-site-hydrolyzed Cucurbita maxima trypsin inhibitor I (CMTI-I) by the application of two-dimensional NMR (2D NMR) techniques, and its secondary structural elements (two tight turns, a 3(10)-helix, and a triple-stranded beta-sheet) were identified on the basis of short-range NOESY cross peaks and deuterium-exchange kinetics. These secondary structural elements are present in the intact inhibitor [Holak, T. A., Gondol, D., Otlewski, J., and Wilusz, T. (1989) J. Mol. Biol. 210, 635-648] and are unaffected by the hydrolysis of the reactive-site peptide bond between Arg5 and Ile6, in accordance with the earlier conclusion reached for CMTI-III [Krishnamoorthi, R., Gong, Y.-X., Lin, C. S., and VanderVelde, D. (1992) Biochemistry 31, 898-904]. Chemical shifts of backbone hydrogen atoms, peptide NH's, and C alpha H's, of CMTI-I were compared with those of the intact inhibitor, CMTI-I, and of the reactive-site-hydrolyzed, natural, E9K variant, CMTI-III. Cleavage of the Arg5-Ile6 peptide bond resulted in changes of chemical shifts of most of the backbone atoms of CMTI-I, in agreement with the earlier results obtained for CMTI-III. Comparison of chemical shifts of backbone hydrogen atoms of CMTI-I and CMTI-III revealed no changes, except for residues Glu9 and His25. However, the intact forms of the same two proteins, CMTI-I and CMTI-III, showed small but significant perturbations of chemical shifts of residues that made up the secondary structural elements of the inhibitors. Apparently, CMTI-I loses its conformational sensitivity to the E9K substitution upon conversion to CMTI-I. In order to gain an insight into the relative stabilities of the intact and reactive-site-hydrolyzed forms, the equilibrium between the intact and modified inhibitors was investigated by high-performance liquid chromatography (HPLC), and the following thermodynamic data were obtained for CM II: delta H degrees = -3.16 +/- 0.69 kcal/mol; delta S degrees = -8.89 +/- 0.37 eu. delta G degrees was determined to be -607 +/- 18 cal/mol for the equilibrium between CMTI-III and CMTI-III and -669 +/- 19 cal/mol for the equilibrium between CMTI-I and CMTI-I at 30 deg C.