Characterization of the unfolding pathway of hen egg white lysozyme.

After the recent discovery of a ribonuclease A unfolding intermediate [Kiefhaber, T., et al. (1995) Nature 375, 513-515], we investigated the unfolding pathway of hen egg white lysozyme. At pH* 4.00 with D2O at 10 degrees C and 6 M guanidinium chloride, unfolding shows a single, slow kinetic phase, with a relaxation time of 3300 s when monitored by circular dichroism (CD). Exchange of the tryptophan indole nitrogen protons shows that buried Trp residues 123, 111, and 108 lose tight packing and become solvent-exposed simultaneously, with a mean relaxation time of 3300 s, similar to the CD-monitored unfolding rate. Unfolding monitored by Trp fluorescence shows, moreover, that 90% of the amplitude change occurs in a slow phase, with a relaxation time of 2400 s. Faster-unfolding phases with minor amplitudes are detected by Trp indole hydrogen exchange and by fluorescence. It is likely that these changes are caused by Trp 62 and Trp 63, active site residues which are not buried in the hydrophobic core. Lysozyme unfolding was further monitored by the histidine 15 C epsilon1 proton, which gives resolved lines for the native and unfolded species in one-dimensional 1H-NMR spectra. The majority of the unfolding reaction, 70%, occurs in a slow phase with a relaxation time of 3600 s, but there is also a rapid unfolding phase; 30% of the His 15 C epsilon1 proton resonance intensity is found at the unfolded chemical shift within tens of seconds after the start of unfolding. The amplitude of the rapid unfolding phase increases proportionally with the concentration of GdmCl denaturant present. These results show that a partially buried residue of lysozyme, histidine 15, takes part in forming an unfolding intermediate similar to the one observed earlier for valine 63 in ribonuclease A. The tryptophan side chains buried in the hydrophobic core of lysoyzme, in contrast, do not participate in forming the unfolding intermediate, as judged by proton chemical shifts. The buried tryptophan residues of dihydrofolate reductase, monitored by 19F-NMR, do participate in forming an unfolding intermediate [Hoeltzli, S. D., & Frieden, C. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9318-9322]; the difference between that study and ours may reside in the greater sensitivity of 19F to the detection of motional differences.