Peroxiredoxins (Prxs), some of nature's dominant peroxidases, use a conserved Cys residue to reduce peroxides. They are highly expressed in organisms from all kingdoms, and in eukaryotes they participate in hydrogen peroxide signaling. Seventy-two Prx structures have been determined that cover much of the diversity of the family. We review here the current knowledge and show that Prxs can be effectively classified by a structural/evolutionary organization into six subfamilies followed by specification of a 1-Cys or 2-Cys mechanism, and for 2-Cys Prxs, the structural location of the resolving Cys. We visualize the varied catalytic structural transitions and highlight how they differ depending on the location of the resolving Cys. We also review new insights into the question of how Prxs are such effective catalysts: the enzyme activates not only the conserved Cys thiolate but also the peroxide substrate. Moreover, the hydrogen-bonding network created by the four residues conserved in all Prx active sites stabilizes the transition state of the peroxidatic S(N)2 displacement reaction. Strict conservation of the peroxidatic active site along with the variation in structural transitions provides a fascinating picture of how the diverse Prxs function to break down peroxide substrates rapidly.

Peroxiredoxins are abundant cellular antioxidant proteins that help to control intracellular peroxide levels. These proteins may also function, in part, through an evolved sensitivity of some peroxiredoxins towards peroxide‐mediated inactivation in hydrogen peroxide signaling in eukaryotes. This review summarizes recent progress in our understanding of the catalytic and regulatory mechanisms of ‘typical 2‐Cys’ peroxiredoxins and of the biological roles played by these important enzymes in oxidative stress and nonstress‐related cellular signaling. New evidence suggests localized peroxide buildup plays a role in nonstress‐related signaling.

Some bacterial species are able to utilize extracellular mineral forms of iron and manganese as respiratory electron acceptors. In Shewanella oneidensis this involves decaheme cytochromes that are located on the bacterial cell surface at the termini of transouter-membrane electron transfer conduits. The cell surface cytochromes can potentially play multiple roles in mediating electron transfer directly to insoluble electron sinks, catalyzing electron exchange with flavin electron shuttles or participating in extracellular intercytochrome electron exchange along "nanowire" appendages. We present a 3.2-Å crystal structure of one of these decaheme cytochromes, MtrF, that allows the spatial organization of the 10 hemes to be visualized for the first time. The hemes are organized across four domains in a unique crossed conformation, in which a staggered 65-Å octaheme chain transects the length of the protein and is bisected by a planar 45-Å tetraheme chain that connects two extended Greek key split β-barrel domains. The structure provides molecular insight into how reduction of insoluble substrate (e.g., minerals), soluble substrates (e.g., flavins), and cytochrome redox partners might be possible in tandem at different termini of a trifurcated electron transport chain on the cell surface.c-type cytochromes | iron respiration | MtrC | multiheme

Peroxiredoxins (Prxs) are important peroxidases associated with both antioxidant protection and redox signaling. They use a conserved Cys residue to reduce peroxide substrates. The Prxs have a remarkably high catalytic efficiency that makes them a dominant player in cell-wide peroxide reduction, but the origins of their high activity have been mysterious. We present here a novel structure of human PrxV at 1.45 Å resolution that has a dithiothreitol bound in the active site with its diol-moiety mimicking the two oxygens of a peroxide substrate. This suggests diols and similar di-oxygen compounds as a novel class of competitive inhibitors for the Prxs. Common features of this and other structures containing peroxide, peroxide-mimicking ligands or peroxide-mimicking water molecules reveal hydrogen bonding and steric factors that promote its high reactivity by creating an oxygen track along which the peroxide oxygens move as the reaction proceeds. Key insights include how the active site microenvironment activates both the peroxidatic cysteine side chain and the peroxide substrate, and is exquisitely well-suited to stabilize the transition state of the in-line S N 2 substitution reaction that is peroxidation.

Salmonella typhimurium AhpC is a founding member of the peroxiredoxin family, a ubiquitous group of cysteine-based peroxidases with high reactivity toward hydrogen peroxide, organic hydroperoxides and peroxynitrite. For all of the peroxiredoxins, the catalytic cysteine, referred to as the peroxidatic cysteine (CP), acts as a nucleophile in attacking the peroxide substrate, forming a cysteine sulfenic acid at the active site. Because thiolates are far stronger nucleophiles than thiol groups, it is generally accepted that cysteine-based peroxidases should exhibit pKa values lower than an unperturbed value of 8.3 – 8.5. In this investigation, several independent approaches were used to assess the pKa of the two cysteinyl residues of AhpC. Methods using two different iodoacetamide derivatives yielded unperturbed pKa values (7.9 – 8.7) for both cysteines, apparently due to reactivity with the “wrong” conformation of CP (i.e. locally unfolded and flipped out of the active site), as supported by X-ray crystallographic analyses. A functional pKa of 5.94 ± 0.10 presumably reflecting titration of CP within the fully folded active site was obtained by measuring AhpC competition with horseradish peroxidase for hydrogen peroxide; this value is quite similar to that obtained by analyzing the pH dependence of the ε240 of wild-type AhpC (5.84 ± 0.02), and similar to those obtained for two typical 2-cysteine peroxiredoxins from Saccharomyces cerevisiae (5.4 and 6.0). Thus, the pKa value of AhpC balances the need for a deprotonated thiol (at pH 7, ∼90% of the CP would be deprotonated) with the fact that thiolates with higher pKa values are stronger nucleophiles.