Figure 2.

Model of Hsp33’s activation. Under nonstress conditions, Hsp33 is monomeric and inactive. All four invariant cysteines are reduced and bind one zinc ion (red), forming a compactly folded zinc center (orange). (1) Exposure to oxidants causes the formation of the first disulfide bond connecting Cys₂₆₅ with Cys₂₆₈, which triggers zinc release and the unfolding of the zinc binding domain. (2) Unfolding of the zinc binding domain destabilizes the adjacent linker region (green), which is now in a dynamic equilibrium between a folded and partially unfolded conformation. At nonstress temperatures, the equilibrium favors the folded conformation and kinetically slow oxidants, like H₂O₂, cannot access the cysteines or activate Hsp33. (3) Mild denaturing conditions (e.g., elevated temperatures) shift the equilibrium and allow H₂O₂ to induce formation of the second disulfide bond between Cys₂₃₂ and Cys₂₃₄. (4) This disulfide bond apparently locks the linker region in an unfolded conformation. It causes the exposure of large hydrophobic surfaces on the N-terminal Hsp33 domain (blue), the proposed binding sites for unfolded proteins, and causes the formation of highly chaperone active Hsp33 dimers.