Denaturation of an extremely stable hyperthermophilic protein occurs via a dimeric intermediate

摘要

To elucidate determinants of thermostability and folding pathways of the intrinsically stable proteins from extremophilic organisms, we are studying β-glucosidase from Pyrococcus furiosus. Using fluorescence and circular dichroism spectroscopy, we have characterized the thermostability of β-glucosidase at 90°C, the lowest temperature where full unfolding is achieved with urea. The chemical denaturation profile reveals that this homotetrameric protein unfolds at 90°C with an overall ΔG° of ～ 20 kcal mol?1. The high temperatures needed to chemically denature P. furiosus β-glucosidase and the large ΔG° of unfolding at high temperatures shows this to be one of the most stable proteins yet characterized. Unfolding proceeds via a three-state pathway that includes a stable intermediate species. Stability of the native and intermediate forms is concentration dependent, and we have identified a dimeric assembly intermediate using high temperature native gel electrophoresis. Based on this data, we have developed a model for the denaturation of β-glucosidase in which the tetramer dissociates to partially folded dimers, followed by the coupled dissociation and denaturation of the dimers to unfolded monomers. The extremely high stability is thus derived from a combination of oligomeric interactions and subunit folding.