Analysis of the transition state in the unfolding of hen lysozyme by introduction of Gly-Pro and Pro-Gly sequences at the same site

We developed a sensitive method for analyzing the conformation of the transition state in the unfolding of hen lysozyme. The activation free energy changes of mutant lysozymes with Gly-Pro and Pro-Gly sequences at the same sites (Gly47Pro47', Pro47Gly47', Gly101-Pro102, Pro101Gly102, Gly117Pro118, Pro117Gly118, Gly121Pro122, and Pro121Gly122 lysozymes) were obtained for the unfolding in aqueous solution at pH 5.5 and 35°C. Since we had shown that the difference of energies of the unfolded state in lysozymes having an introduced Gly-Pro or Pro-Gly sequence at the same site was much smaller than the difference of energies of the folded states we could estimate the difference of energies of the folded and the transition states unequivocally. We defined the φ-value as the ratio of the difference in the free energy change in the transition state to that in the free energy change in the folded state between lysozymes with Gly-Pro and Pro-Gly sequences at the same site. The φ-values gave information on how much the mutated sites retained the folded structure in the transition state. These values were 0.45 around position 47, which is located in the β-sheet structure, 0.12 at position 101-102, which is located in the loop at the upper part of the active site, 0.17 at position 117-118, which is located in the β-turn and 0.64 at position 121-122, which is located in the 3₁₀-helix. Therefore, in the transition state in the unfolding of lysozyme, it was found that the 3₁₀-helical region had a similar structure to the intact region, while both the β-turn and the loop at the upper part of the active site were considerably unfolded. The β-sheet structure was also moderately disrupted in the transition state.