Theoretical and experimental analysis of ionization equilibria in ovomucoid third domain.

2D-NMR experiments were used to determine the pKa values ranging from 8.0 to >/=11.1 of seven basic residues in turkey ovomucoid third domain (OMTKY3) and were compared to values predicted as described by Antosiewicz et al. [(1996) Biochemistry 35, 7819-7833]. Lys 13, 29, and 34 were previously attributed with increasing the acidity of numerous acidic residues [Schaller, W., and Robertson, A. D. (1995) Biochemistry 34, 4714-4723]. These interactions were expected to raise the pKa values of those basic groups; however, the pKa values of Lys 13 and 34 are less than the model compound values. The pKa values of the other basic residues are greater than the model compound values and, unlike the acidic residues, all are surprisingly insensitive to salt. While the calculations properly predict the direction of most of the pKa shifts and provide valuable insight into the possible molecular origins of the interactions that perturb pKa values, there is a tendency to overestimate the magnitude of the shifts and their salt dependence. Interestingly, the shapes of both the calculated and observed transitions are often more complex than expected for a simple titration, suggesting that pKa values at many sites are changing during the transition. Differences between predicted and experimental pKa values and titration profiles for some residues may be due to as yet uncharacterized structural changes at the extremes of pH.