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Ascorbate peroxidase (APX), cytochrome c peroxidase (CcP), and the catalase-peroxidases (KatG) share very similar active site structures and are distinguished from other peroxidases by the presence of a distal tryptophan residue. In KatG, this distal tryptophan forms a covalent link to an adjacent tyrosine residue, which in turn links to a methionine residue. We have previously shown [ Pipirou, Z. et al. ( 2007 ) Biochemistry 46 , 2174 - 2180 ] that reaction of APX with peroxide leads, over long time scales, to formation of a covalent link with the distal tryptophan (Trp41) in a mechanism that proceeds through initial formation of a compound I species bearing a porphyrin pi-cation radical followed by radical formation on Trp41, as implicated in the KatG enzymes. Formation of such a covalent link in CcP has never been reported, and we proposed that this could be because compound I in CcP uses Trp191 instead of a porphyrin pi-cation radical. To test this, we have examined the reactivity of the W191F variant of CcP with H(2)O(2), in which formation of a porphyrin pi-cation radical occurs. We show, using electronic spectroscopy, HPLC, and mass spectroscopy, that in W191F partial formation of a covalent link from Trp51 to the heme is observed, as in APX. Radical formation on Trp51, as seen for KatG and APX, is implicated; this is supported by QM/MM calculations. Collectively, the data show that all three members of the class I heme peroxidases can support radical formation on the distal tryptophan and that the reactivity of this radical can be controlled either by the protein structure or by the nature of the compound I intermediate.

Electronic spectroscopy, HPLC analyses, and mass spectrometry (MALDI-TOF and MS/MS) have been used to show that a covalent link from the heme to the distal Trp41 can occur on exposure of ascorbate peroxidase (APX) to H2O2 under noncatalytic conditions. Parallel analyses with the W41A variant and with APX reconstituted with deuteroheme clearly indicate that the covalent link does not form in the absence of either Trp41 or the heme vinyl groups. The presence of substrate also precludes formation of the link. Formation of a protein radical at Trp41 is implicated, in a reaction mechanism that is analogous to that proposed [Ghiladi, R. A., et al. (2005) Biochemistry 44, 15093−15105] for formation of a covalent Trp-Tyr-Met link in the closely related catalase peroxidase (KatG) enzymes. Collectively, the data suggest that radical formation at the distal tryptophan position is not an exclusive feature of the KatG enzymes and may be used more widely across other members of the class I heme peroxidase family.

The heme peroxidase and heme oxygenase enzymes share a common heme prosthetic group but catalyze fundamentally different reactions, the first being H(2)O(2)-dependent oxidation of substrate using an oxidized Compound I intermediate, and the second O(2)-dependent degradation of heme. It has been proposed that these enzymes utilize a common reaction intermediate, a ferric hydroperoxide species, that sits at a crossroads in the mechanism and beyond which there are two mutually exclusive mechanistic pathways. Here, we present evidence to support this proposal in a heme peroxidase. Hence, we describe kinetic data for a variant of ascorbate peroxidase (W41A) which reacts slowly with tert-butyl hydroperoxide and does not form the usual peroxidase Compound I intermediate; instead, structural data show that a product is formed in which the heme has been cleaved at the alpha-meso position, analogous to the heme oxygenase mechanism. We interpret this to mean that the Compound I (peroxidase) pathway is shut down, so that instead the reaction intermediate diverts through the alternative (heme oxygenase) route. A mechanism for formation of the product is proposed and discussed in the light of what is known about the heme oxygenase reaction mechanism.

We have previously shown that introduction of an engineered Met160 residue in ascorbate peroxidase (S160M variant) leads to the formation of a covalent link between Met160 and the heme vinyl group [Metcalfe, C. L., et al. (2004) J. Am. Chem. Soc. 126, 16242-16248]. In this work, we have used electronic spectroscopy, HPLC, and mass spectrometry to show that the introduction of a tyrosine residue at the same position (S160Y variant) leads, similarly, to the formation of a heme-tyrosine covalent link in an autocatalytic reaction that also leads to formation of a second covalent link from the heme to Trp41 [Pipirou, Z., et al. (2007) Biochemistry 46, 2174-2180]. Stopped-flow and EPR data implicate the involvement of a tyrosyl radical in the reaction mechanism. The results indicate that the heme can support the formation of different types of covalent links under appropriate conditions. The generality of this idea is discussed in the context of other heme enzymes.