“…This data enables a more extended comparison, evidencing differences in the contributions that underlie a very similar outcome in terms of free-energy barriers, probably attributable to the absence/presence of water molecules at each active site and to differences in the nature and strength of the HB networks established at the corresponding Michaelis complex (RC) and TS pairs. In the particular case of hPrx5, examined in depth by some of us [20], the catalytic efficiency displayed by this peroxiredoxin is ascribed to a combination of effects including: (a) differential electrostatic stabilization of the anionic TS by protonated Arg127, also responsible of lowering Cys47 pKa, (b) strengthening the H2O2 HB network in going from RC to TS, and (c) the pivotal role played by a well-conserved Thr44 at the active site, that rotates its side-chain to modulate Cys47 nucleophilicity by the progressive release of a Sγ⋯HOT44 H-bond formed at RC while driving the substrate (tethered through HB to OHT44 as acceptor) into the right place at the right time for the reaction to proceed in terms of activation of the thiolate. Most of these effects are not found in the reaction with HSA.…”