Catalase-peroxidases are bifunctional heme enzymes with a high structural homology to peroxidases from prokaryotic origin and a catalatic activity comparable to monofunctional catalases. These unique features of catalase-peroxidases make them good systems to study and understand the role of alternative electron pathways both in catalases and peroxidases. In particular, it is of interest to study the poorly understood role of tyrosyl and tryptophanyl radicals as alternative cofactors in the catalytic cycle of catalases and peroxidases. In this work, we have used a powerful combination of multifrequency EPR spectroscopy, isotopic labeling of tryptophan and tyrosine residues, and site-directed mutagenesis to unequivocally identify the reactive intermediates formed by the wild-type Synechocystis PCC6803 catalase-peroxidase. Selected variants of the heme distal and proximal sides of the Synechocystis enzyme were investigated. Variants on the aromatic residues of the short stretch located relatively close to the heme and spanning the distal and proximal sides were also investigated. In the wild-type enzyme, the EPR signal of the catalases and peroxidases (typical) Compound I intermediate [Fe(IV)=O por.+] was observed. Two protein-based radical intermediates were also detected and identified as a Tyr. and a Trp. . The site of Trp. is proposed to be Trp 106, a residue belonging to the conserved short stretch in catalase-peroxidases and located at a 7-8 A distance to the heme propionate groups. An extensive hydrogen-bonding network on the heme distal side, involving Trp122, His123, Arg119, seven structural waters, the heme 6-propionate group, and Trp106, is proposed to have a key role on the formation of the tryptophanyl radical. We used high-field EPR spectroscopy (95-285 GHz) to resolve the g-anisotropy of the protein-based radicals in Synechocystis catalase-peroxidase. The broad gx component of the HF EPR spectrum of the Tyr. in Synechocystis catalase-peroxidase was consistent with a distributed electropositive protein environment to the tyrosyl radical.

؊1 at pH 7 and 15°C) were determined by using the sequential stopped-flow technique. The relevance of these findings is discussed with respect to the bifunctional activity of KatGs and the recently published first crystal structure.

Structural and biochemical characterization of aspartate 152 at the distal heme side of catalase-peroxidase (KatG) from Synechocystis PCC 6803 reveals an important functional role for this residue. In the wild-type protein, the side chain carboxyl group of Asp152 is 7.8 A apart from the heme iron and is hydrogen-bonded to two water molecules and a KatG-specific large loop. We have prepared the site-specific variants Asp152Asn, Asp152Ser, Asp152Trp, and Pro151Ala. Exchange of Asp152 exhibited dramatic consequences on the bifunctional activity of this unique peroxidase. The turnover number of catalase activity of Asp152Asn is 2.7%, Asp152Ser 5.7%, and Asp152Trp is 0.6% of wild-type activity. By contrast, the peroxidase activity of the Asp152 variants was 2-7 times higher than that of wild-type KatG or Pro151Ala. The KatG-specific pH profile of the catalase activity was completely different in these variants and exchange of Asp152 made it possible to follow the transition of the ferric enzyme to the redox intermediate compound I by hydrogen peroxide spectroscopically and to determine the corresponding bimolecular rate constant to be 7.5 x 10(6) M(-1) s(-1) (pH 7 and 15 degrees C). The reactivity of compound I toward aromatic one-electron donors was enhanced in the Asp152 variants compared with the wild-type protein, whereas the reactivity toward hydrogen peroxide was dramatically decreased. A mechanism for the hydrogen peroxide oxidation, which is different from monofunctional catalases and involves the distal residues Trp122 and Asp152, is proposed.

Background: First analysis was made of bacterial ancestor of peroxidases from mammalian innate immune system. Results: Highly stable heme enzyme possesses high bromination activity and covalently bound prosthetic group. Conclusion: Post-translational autocatalytic (peroxide-driven) heme modification is found in prokaryotic and eukaryotic halogenating peroxidases. Significance: Peroxidase-mediated production of antimicrobial hypohalous acids was developed early in evolution.

Catalase-peroxidases (KatGs) are unique in exhibiting an overwhelming catalase activity and a peroxidase activity of broad specificity. Similar to other peroxidases the distal histidine in KatGs forms a hydrogen bond with an adjacent conserved asparagine. To investigate the catalytic role(s) of this potential hydrogen bond in the bifunctional activity of KatGs, Asn153 in Synechocystis KatG was replaced with either Ala (Asn153fiAla) or Asp (Asn153fiAsp). Both variants exhibit an overall peroxidase activity similar with wild-type KatG. Cyanide binding is monophasic, however, the second-order binding rates are reduced to 5.4% (Asn153fiAla) and 9.5% (Asn153fiAsp) of the value of native KatG [(4.8 ± 0.4) · 10 5 M )1 AEs )1 at pH 7 and 15°C]. The turnover number of catalase activity of Asn153fiAla is 6% and that of Asn153fiAsp is 16.5% of wild-type activity. Stopped-flow analysis of the reaction of the ferric forms with H 2 O 2 suggest that exchange of Asn did not shift significantly the ratio of rates of H 2 O 2 -mediated compound I formation and reduction. Both rates seem to be reduced most probably because (a) the lower basicity of His123 hampers its function as acid-base catalyst and (b) Asn153 is part of an extended KatG-typical H-bond network, the integrity of which seems to be essential to provide optimal conditions for binding and oxidation of the second H 2 O 2 molecule necessary in the catalase reaction.Keywords: catalase-peroxidase; Synechocystis PCC 6803; catalase activity; peroxidase activity; compound I.On the basis of sequence similarities with yeast cytochrome c peroxidase (CCP) and plant ascorbate peroxidases (APXs), catalase-peroxidases (KatGs) have been shown to be members of class I of the superfamily of plant, fungal and bacterial heme peroxidases [1]. KatGs have been found in prokaryotes (archaebacteria and eubacteria) and fungi and are homomultimeric proteins with monomers being twice as large as CCP or APXs adding up to about 79-85 kDa, which is ascribed to gene duplication [2]. From both CCP and APX the crystal structures have been solved [3,4] and, quite recently, the 2.0 Å crystal structure of the homodimeric KatG from Haloarcula marismortui has been published [5]. This structure and sequence alignments suggest that all class I peroxidases have conserved the amino-acid triad His, Asp and Trp in the proximal pocket and the triad Trp, Arg and His in the distal pocket (Fig. 1). Despite this homology, class I peroxidases dramatically differ in their reactivities towards hydrogen peroxide and one-electron donors. Catalase-peroxidases have a predominant catalase activity but differ from monofunctional catalases in also exhibiting a substantial peroxidatic activity with broad specificity. However, no substantial catalase activity has ever been reported for either CCP or APX. Cytochrome c peroxidase (CCP) is unusual in that it prefers another protein (cytochrome c) as a redox partner, whereas ascorbate peroxidases (APXs) prefer the anion ascorbate as electron donor. But both cytochrome c and ascorba...