Figure 10. The proposed transition state(s) during the catalytic cycle of Prxs.

(A) (I) The reduced C_P adopts FF conformation and is essential to activate and stabilize the C_P-thiolate for peroxidation. (II) The oxidative modification of C_P to C_P-SOH leads to localized structural alterations around the conserved active site pocket as seen from the “FF_like” conformation of the reduced EcAhpC_1-186-YFSKHN. (III) The “LU_like” conformation in the reduce EcAhpC_1-186-YFSKHN structure, reveals a severe clash between the active site residues with the C-terminal tail. The entire C-terminal arm (aa 166–187) is needed to undergo alterations and to enable the disulphide formation between C_R’ and C_P (solid arrow). (IV) The disulphide bonded active site is stabilized in the LU conformation, wherein the C-terminal tail is disordered (denoted by asterisk). (V) The C-terminal tail of AhpC binds to its reducing partner, the N-terminal domain (NTD) of AhpF, to regenerate for another catalytic cycle. (VI) In human Prxs, the extra C-terminal YFSKHN-helix is folded across the active site, thereby delaying the FF to LU transition during the intermediate C_P-SOH state. In such case, the C_P-SOH form can further react with peroxide and leads to enzyme inactivation (VII). (B) The cartoon representation shows that the C_P and the first turn of the α2-helix is stabilized by the weak interactions with the I187 and C-terminal tail, which is hold by residues S86 and T88. Disruption of weak interactions of the C-terminal tail, like between I187 and the α2-helix or between I187 with S86A and T88A, leads to unfolding of the C-terminal tail from the active site region.