STRUCTURAL CONSEQUENCES OF THE NATURAL SUBSTITUTION, E9K, ON REACTIVE-SITE-HYDROLYZED SQUASH (CUCURBITA-MAXIMA) TRYPSIN-INHIBITOR (CMTI), AS STUDIED BY 2-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., & Wilusz, T. (1989) J. Mol. Biol. 210,635-6481 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., & 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 CMTI-III: 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-degrees-C.