The stability and folding process of amyloidogenic mutant human lysozymes

Amyloid deposits are frequently formed by mutant proteins that have a lower stability than the wild-type proteins. Some reports, however, have shown t hat mutant-induced thermodynamic destabilization is not always a general me chanism of amyloid formation. To obtain a better understanding of the mecha nism of amyloid fibril formation, we show in this study that equilibrium an d kinetic refolding-unfolding reaction experiments with two amyloidogenic m utant human lysozymes (I56T and D67H) yield folding pathways that can be dr awn as Gibbs energy diagrams. The equilibrium stabilities between the nativ e and denatured states of both mutant proteins were decreased, but the degr ees of instability were different. The Gibbs energy diagrams of the folding process reveal that the Gibbs energy change between the native and folding intermediate states was similar for both proteins, and also that the activ ation Gibbs energy change from the native state to the transition state dec reased. Our results confirm that the tendency to favor the intermediate of denaturation facilitates amyloid formation by the mutant human lysozymes mo re than equilibrium destabilization between the native and completely denat ured states does.