Deconstructing the catalytic efficiency of peroxiredoxin-5 peroxidatic cysteine.

Human peroxiredoxin-5 (PRDX5) is a thiol peroxidase that reduces H2O2 10(5) times faster than free cysteine. To assess the influence of two conserved residues on the reactivity of the critical cysteine (C47), we determined the reaction rate constants of PRDX5, wild type (WT), T44V and R127Q with one substrate electrophile (H2O2) and a nonspecific electrophile (monobromobimane). We also studied the corresponding reactions of low molecular weight (LMW) thiolates in order to construct a framework against which we could compare our proteins. To obtain a detailed analysis of the structural and energetic changes involved in the reaction between WT PRDX5 and H2O2, we performed ONIOM quantum mechanics/molecular mechanics (QM/MM) calculations with a QM region including 60 atoms of substrate and active site described by the B3LYP density functional and the 6-31+G(d,p) basis set; the rest of the protein was included in the MM region. Brønsted correlations reveal that the absence of T44 can increase the general nucleophilicity of the C47 but decreases the specific reactivity toward H2O2 by a factor of 10(3). The R127Q mutation causes C47 to behave like a LMW thiolate in the two studied reactions. QM/MM results with WT PRDX5 showed that hydrogen bonds in the active site are the cornerstone of two effects that make catalysis possible: the enhancement of thiolate nucleophilicity upon substrate ingress and the stabilization of the transition state. In both effects, T44 has a central role. These effects occur in a precise temporal sequence that ensures that the selective nucleophilicity of C47 is available only for peroxide substrates.