ABSTRACT
The mechanism of complex unfolding process of Pseudomonas aeruginosa apoazurin is an arguing problem. Recent published results indicated that this problem could be resolved by hypothesizing two native conformations coexisting in solution.Urea-induced unfolding of apoazurin was investigated further using intrinsinc fluorescence and CD spectra. Equilibrium unfolding curves in urea could be depicted with an apparent two-state transition, but a biphase kinetic process. The fast unfolding process is finished within a few seconds as monitored by stopped-flow fluorescence intensity, whereas the slow process requires several hours for as well as the circular dichroism spectra just after manual mixing of protein and denaturant could be simulated by superposition of the spectra of the native and fully unfolded protein with the same coefficient of 0.37. This strongly suggests the three-state mechanism with a partially unfolded intermediate on its pathway is inadequate. The hypothesis of two native conformations coexistence is a reasonable selection.