Thermal-induced dissociation and unfolding of homodimeric DsbC revealed by temperature-jump time-resolved infrared spectra.

The thermal stability of DsbC, a homodimeric protein disulfide isomerase in prokaryotic periplasm, has been studied by using temperature-dependent Fourier transformation infrared and time-resolved infrared spectroscopy coupled with temperature-jump initiation. The infrared absorbance thermal titration curves for thermal-induced unfolding of DsbC in D(2)O exhibit a three-state transition with the first transition midpoint temperature at 37.1 +/- 1.1 degrees C corresponding to dissociation, and the second at >74.5 degrees C corresponding to global unfolding and aggregation. The dissociation midpoint temperature of DsbC in phosphate buffer shifts to 49.2 +/- 0.7 degrees C. Temperature-jump time-resolved infrared spectra in D(2)O shows that DsbC dissociates into the corresponding germinate monomeric encounter pair with a time constant of 40 +/- 10 ns independent of the protein concentration and 77% of the newly formed monomeric encounter pair undergoes further coil to helix/loop transition with a time constant of 160 +/- 10 ns. The encounter pair is expected to proceed with further dissociation into monomers. The dissociation of DsbC is confirmed by size-exclusion chromatography and subunit hybridization. The in vivo oxidase activity of DsbC attributed to the monomer has also been observed by using cadmium sensitivity and the oxidative state of beta-lactamase.