Protein inhibitors of serine proteinases: role of backbone structure and dynamics in controlling the hydrolysis constant

Standard mechanism protein inhibitors of serine proteinases bind as substrates and are cleaved by cognate proteinases at their reactive sites. The hydrolysis constant for this cleavage reaction at the P(1)-P(1)' peptide bond (K(hyd)) is determined by the relative concentrations at equilibrium of the "intact" (uncleaved, I) and "modified" (reactive site cleaved, I*) forms of the inhibitor. The pH dependence of K(hyd) can be explained in terms of a pH-independent term, K(hyd) degrees, plus the proton dissociation constants of the newly formed amino and carboxylate groups at the cleavage site. Two protein inhibitors that differ from one another by a single residue substitution have been found to have K(hyd) degrees values that differ by a factor of 5 [Ardelt, W., and Laskowski, M., Jr. (1991) J. Mol. Biol. 220, 1041-1052]: turkey ovomucoid third domain (OMTKY3) has K(hyd) degrees = 1.0, and Indian peafowl ovomucoid third domain (OMIPF3), which differs from OMTKY3 by the substitution P(2)'-Tyr(20)His, has K(hyd) degrees = 5.15. What mechanism is responsible for this small difference? Is it structural (enthalpic) or dynamic (entropic)? Does the mutation affect the free energy of the I state, the I* state, or both? We have addressed these questions through NMR investigations of the I and I forms of OMTKY3 and OMIPF3. Information about structure was derived from measurements of NMR chemical shift changes and trans-hydrogen-bond J-couplings; information about dynamics was obtained through measurements of (15)N relaxation rates and (1)H-(15)N heteronuclear NOEs with model-free analysis of the results. Although the I forms of each variant are more dynamic than the corresponding I forms, the study revealed no appreciable difference in the backbone dynamics of either intact inhibitor (OMIPF3 vs OMTKY3) or modified inhibitor (OMIPF3* vs OMTKY3*). Instead, changes in chemical shifts and trans-hydrogen-bond J-couplings suggested that the K(hyd) degrees difference arises from differential intramolecular interactions within the intact inhibitors (OMIPF3 vs OMTKY3) in a region of each protein that becomes disordered upon reactive site cleavage (to OMIPF3* and OMTKY3*).