Disulfide bond isomerization in basic pancreatic trypsin inhibitor: multisite chemical exchange quantified by CPMG relaxation dispersion and chemical shift modeling.

Conformational changes occurring on the microsecond-millisecond time scale in basic pancreatic trypsin inhibitor (BPTI) are investigated using nuclear magnetic resonance spectroscopy. The rczz CPMG experiment (Wang, C.; Grey, M. J.; Palmer, A. G. J. Biomol. NMR 2001, 21, 361-366) is used to record (15)N spin relaxation dispersion data, R(ex)(1/tau(cp)), in which 1/tau(cp) is the pulsing rate in the CPMG sequence, at two static magnetic fields, 11.7 and 14.1 T, and three temperatures, 280, 290, and 300 K. These data are used to characterize the kinetics and mechanism of chemical exchange line broadening of the backbone (15)N spins of Cys 14, Lys 15, Cys 38, and Arg 39 in BPTI. Line broadening is found to result from two processes: the previously identified isomerization of the Cys 38 side chain between chi(1) rotamers (Otting, G.; Liepinsh, E.; Wüthrich, K. Biochemistry 1993, 32, 3571-3582) and a previously uncharacterized process on a faster time scale. At 300 K, both processes contribute significantly to the relaxation dispersion for Cys 14 and an analytical expression for a linear three-site exchange model is used to analyze the data. At 280 K, isomerization of the Cys 38 side chain is negligibly slow and the faster process dominates the relaxation dispersion for all four spins. Global analysis of the temperature and static field dependence of R(ex)(1/tau(cp)) for Cys 14 and Lys 15 is used to determine the activation parameters and chemical shift changes for the previously uncharacterized chemical exchange process. Through an analysis of a database of chemical shifts, (15)N chemical shift changes for Cys 14 and Lys 15 are interpreted to result from a chi(1) rotamer transition of Cys 14 that converts the Cys 14-Cys 38 disulfide bond between right- and left-handed conformations. At 290 K, isomerization of Cys 14 occurs with a forward and reverse rate constant of 35 s(-1) and 2500 s(-1), respectively, a time scale more than 30-fold faster than the Cys 38 chi(1) isomerization. A comparison of the kinetics and thermodynamics for the transitions between the two alternative Cys 14-Cys 38 conformations highlights the factors that affect the contribution of disulfide bonds to protein stability.