Equilibrium and kinetics of the unfolding of α-lactalbumin by guanidine hydrochloride (II)

Abstract

The reversible unfolding of α-lactalbumin by guanidine hydrochloride, was studied at 25.0°C in a relatively low concentration range of the denaturant (0.80–2.00 mol/1) by means of difference spectra and pH-jump measurements. The unfolding was shown to occur between two states, N and D, because apparent rate-constants of the unfolding and the refolding reactions depended only on pH. All curves plotted as the logarithmical equilibrium constant log K D against pH could fall on the same base curve by shifting each curve along the log K D axis. From the dependence of the logarithmic rate constant on pH, master curves could also be made for the forward and the backward reactions. The dependence of these master curves on pH indicates that the groups affecting the pH dependence of the unfolding are three residues with p K N  3.3 and p K A  4.4, one residue with p K N  p K A  3.8 and p K D  4.4, and one residue with p K N  5.8 and p K A  p K D  6.3, where A indicates the activated state. On the other hand, from the denaturant activity dependence of the shift factors required for making the master curves, the value of the intrinsic binding constant of the denaturant to the protein was found to be similar to that obtained from previous measurements at pH 5.5. Differences between the numbers of the binding sites of the denaturant on the denaturated and the native proteins, and between those on the activated and the native proteins were shown to be 5.3 and 2.1, respectively. The free energy of stabilization in the native-like environment also shows that the protein in the native state is more unstable than lysozyme.