Graphical abstract

Background:The impact of autocatalytically formed covalent heme to protein bonds on formation and reactivity of redox intermediates of a peroxidase was analyzed. Results: Posttranslational modification significantly changes the reactivity of the ferric protein and of compounds I and II. Conclusion: Upon covalent attachment, the capacity of compound I to oxidize one-and two-electron donors is enhanced. Significance: First direct evidence about the relation between H 2 O 2 -triggered covalent heme to protein bond formation and enzyme reactivity.

Four heme peroxidase superfamilies arose independently in evolution. Only in the peroxidase-cyclooxygenase superfamily the prosthetic group is posttranslationally modified (PTM). As a consequence these peroxidases can form one, two or three covalent bonds between heme substituents and the protein. This may include ester bonds between heme 1- and 5-methyl groups and glutamate and aspartate residues as well as a sulfonium ion link between the heme 2-vinyl substituent and a methionine. Here the phylogeny and physiological roles of representatives of this superfamily, their occurrence in all kingdoms of life, the relevant sequence motifs for definite identification and the available crystal structures are presented. We demonstrate the autocatalytic posttranslational maturation process and the impact of the covalent links on spectral and redox properties as well as on catalysis, including Compound I formation and reduction by one- and two-electron donors. Finally, we discuss the evolutionary advantage of these PTMs with respect to the proposed physiological functions of the metalloenzymes that range from antimicrobial defence in innate immunity to extracellular matrix formation and hormone biosynthesis.

Recently, it was demonstrated that bifunctional catalase-peroxidases (KatGs) are found not only in archaea and bacteria but also in lower eukaryotes. Structural studies and preliminary biochemical data of the secreted KatG from the rice pathogen Magnaporthe grisea (MagKatG2) suggested both similar and novel features when compared to those of the prokaryotic counterparts studied so far. In this work, we demonstrate the role of the autocatalytically formed redox-active Trp140-Tyr273-Met299 adduct of MagKatG2 in (i) the maintenance of the active site architecture, (ii) the catalysis of hydrogen peroxide dismutation, and (iii) the protein stability by comparing wild-type MagKatG2 with the single mutants Trp140Phe, Tyr273Phe, and Met299Ala. The impact of disruption of the covalent bonds between the adduct residues on the spectral signatures and heme cavity architecture was small. By contrast, loss of its integrity converts bifunctional MagKatG2 to a monofunctional peroxidase of significantly reduced thermal stability. It increases the accessibility of ligands due to the increased flexibility of the KatG-typical large loop 1 (LL1), which contributes to the substrate access channel and anchors at the adduct Tyr. We discuss these data with respect to those known from prokaryotic KatGs and in addition present a high-resolution structure of an oxoiron compound of MagKatG2.

Oxidation of halides and thiocyanate by heme peroxidases to antimicrobial oxidants is an important cornerstone in the innate immune system of mammals. Interestingly, phylogenetic and physiological studies suggest that homologous peroxidases are already present in mycetozoan eukaryotes such as Dictyostelium discoideum. This social amoeba kills bacteria via phagocytosis for nutrient acquisition at its single-cell stage and for antibacterial defense at its multicellular stages. Here, we demonstrate that peroxidase A from D. discoideum (DdPoxA) is a stable, monomeric, glycosylated, and secreted heme peroxidase with homology to mammalian peroxidases. The first crystal structure (2.5 Å resolution) of a mycetozoan peroxidase of this superfamily shows the presence of a post-translationally-modified heme with one single covalent ester bond between the 1-methyl heme substituent and Glu-236. The metalloprotein follows the halogenation cycle, whereby compound I oxidizes iodide and thiocyanate at high rates (>108 m−1 s−1) and bromide at very low rates. It is demonstrated that DdPoxA is up-regulated and likely secreted at late multicellular development stages of D. discoideum when migrating slugs differentiate into fruiting bodies that contain persistent spores on top of a cellular stalk. Expression of DdPoxA is shown to restrict bacterial contamination of fruiting bodies. Structure and function of DdPoxA are compared with evolutionary-related mammalian peroxidases in the context of non-specific immune defense.