A Pseudohypervalent Sulfur Intermediate as an Oxidative Protective Mechanism in the Archaea Peroxiredoxin Enzyme ApTPx.

Peroxiredoxins (Prxs) are a ubiquitous class of enzymes that have central roles in a number of important physiological processes. Using a multiscale computational approach, we have investigated the mechanism by which the active-site cysteine (Cys50) in the typical 2-Cys Prx from Archaea (ApTPx) is oxidized by HO to sulfenic acid. In addition, its further oxidation to give a sulfinic acid and its possible alternate intramolecular reaction to form an experimentally proposed hypervalent sulfurane were examined. Oxidation of Cys50 by HO to give the sulfenic acid intermediate occurs in one step with a barrier of 82.1 kJ mol. A two-step pathway is proposed with a very low barrier of 16.5 kJ mol by which it can subsequently react with an adjacent histidyl (His42) to form the pseudohypervalent sulfurane. This pathway also involves an adjacent aspartyl (Asp45), which helps alternate the protonation state of His42. The sulfurane's S···Nδ interaction was characterized using QTAIM, NCI, and NBO analyses and found to be a noncovalent interaction. Notably, this bond helps orient the SOH moiety such that it is less susceptible to oxidation by HO to sulfinic acid. Significantly, sulfurane formation is energetically favored to further HO oxidation of SOH to a sulfinic acid, providing a mechanism by which the active-site Cys50 is protected against overoxidation.