Two-dimensional NMR studies of Kazal proteinase inhibitors. 2. Sequence-specific assignments and secondary structure of reactive site modified turkey ovomucoid third domain.

The solution structure of modified turkey ovomucoid third domain (OMTKY3*) was investigated by high-resolution proton NMR techniques. OMTKY3* was obtained by enzymatic hydrolysis of the scissile reactive site peptide bond (Leu18-Glu19) in turkey ovomucoid third domain (OMTKY3). All of the backbone proton resonances were assigned to sequence-specific residues except the NH's of Leu1 and Glu19, which were not observed. Over 80% of the side-chain protons also were assigned. The secondary structure of OMTKY3*, as determined from assigned NOESY cross-peaks and identification of slowly exchanging amide protons, contains antiparallel beta-sheet consisting of three strands (residues 21-25, 28-32, and 49-54), one alpha-helix (residues 33-44), and one reverse turn (residues 26-28). This secondary structure closely resembles that of OMTKY3 in solution [Robertson, A. D., Westler, W. M., & Markley, J. L. (1988) Biochemistry (preceding paper in this issue)]. On the other hand, changes in the tertiary structure of the protein near to and remote from the cleavage site are indicated by differences in the chemical shifts of numerous backbone protons of OMTKY3 and OMTKY3*.