Native state fluctuations in a peroxiredoxin active site match motions needed for catalysis

Peroxiredoxins are ubiquitous enzymes that detoxify peroxides and regulate redox signaling. During catalysis, a "peroxidatic" cysteine (C_P) in the conserved active site reduces peroxide while being oxidized to a C_P-sulfenate, prompting a local unfolding event that enables formation of a disulfide with a second, "resolving" cysteine. Here, we use nuclear magnetic resonance spectroscopy to probe the dynamics of the C_P-thiolate and disulfide forms of Xanthomonas campestris peroxiredoxin Q. Chemical exchange saturation transfer behavior of the resting enzyme reveals 26 residues in and around the active site exchanging at a rate of 72 s^-1 with a locally unfolded, high-energy (2.5% of the population) state. This unequivocally establishes that a catalytically relevant local unfolding equilibrium exists in the enzyme's C_P-thiolate form. Also, faster motions imply an active site instability that could promote local unfolding and, based on other work, be exacerbated by C_P-sulfenate formation so as to direct the enzyme along a functional catalytic trajectory.