Key roles of the Escherichia coli AhpC C-terminus in assembly and catalysis of alkylhydroperoxide reductase, an enzyme essential for the alleviation of oxidative stress

2-Cys peroxiredoxins (Prxs) are a large family of peroxidases, responsible for antioxidant function and regulation in cell signaling, apoptosis and differentiation. The Escherichia coli alkylhydroperoxide reductase (AhpR) is a prototype of the Prxs-family, and is composed of an NADH-dependent AhpF reductase (57 kDa) and AhpC (21 kDa), catalyzing the reduction of H₂O₂. We show that the E. coli AhpC (EcAhpC, 187 residues) forms a decameric ring structure under reduced and close to physiological conditions, composed of five catalytic dimers. Single particle analysis of cryo-electron micrographs of C-terminal truncated (EcAhpC_{1 -172} and EcAhpC_{1 -182}) and mutated forms of EcAhpC reveals the loss of decamer formation, indicating the importance of the very C-terminus of AhpC in dimer to decamer transition. The crystallographic structures of the truncated EcAhpC_{1 -172} and EcAhpC_{1 -182} demonstrate for the first time that, in contrast to the reduced form, the very C-terminus of the oxidized EcAhpC is oriented away from the AhpC dimer interface and away from the catalytic redox-center, reflecting structural rearrangements during redox-modulation and -oligomerization. Furthermore, using an ensemble of different truncated and mutated EcAhpC protein constructs the importance of the very C-terminus in AhpC activity and in AhpC-AhpF assembly has been demonstrated.