Theoretical insights into the mechanism of redox switch in heat shock protein Hsp33
Résumé
Heat Shock Protein 33 (Hsp33) is activated in the presence of H2O2 by a very
interesting redox switch based on a tetra-coordinated zinc-cysteine complex present in the
fully reduced and inactive protein form. The oxidation of this zinc centre by H2O2 induces
formation of two S-S bridges and the zinc release followed by the protein unfolding. We report here a
theoretical study of the step by step sequence of the overall process starting with the oxidation of the
first cysteine residue and ending with the zinc release. Each reaction step is characterized by its Gibbs
free energy barrier (∆G‡ ). It is predicted that the first reaction step consists in the oxidation of Cys263
by H2O2 which is by far the most reactive cysteine (∆G‡=15.4 kcal mol-1). The next two reaction steps
are the formation of the first S-S bridge between Cys263 and Cys266 (∆G‡=13.6 kcal mol-1) and the
oxidation of Cys231 by H2O2 (∆G‡ =20.4 kcal mol-1). It is then shown that the formation of the second
S-S bridge (Cys231-Cys233) before the zinc release is most unlikely (∆G‡=34.8 kcal mol-1). Instead,
the release of zinc just after the oxidation of the third cysteine (Cys231) is shown to be
thermodynamically (dissociation Gibss free energy ∆Gd=6.0 kcal mol-1) and kinetically (reaction rate
constant kd≈106 s-1) favored. This result is in good agreement with the experimental data on the
oxidation mechanism of Hsp33 zinc centre available to date.