Crystal structure of yeast cytochrome c peroxidase
(from SWISS-PROT
P00431,
EC
1.11.1.5)
The peroxidase family of proteins includes mammalian (e.g., myelo-, lacto- and thyroid peroxidases), fungal (e.g., lignin and cytochrome c peroxidases), and plant (horseradish peroxidase) enzymes. There is strong sequence identity among the mammalian peroxidases but less so between the mammalian and plant/fungal enzymes. Nevertheless, similar if not identical catalytic residues are found in the active sites of all the classical peroxidases and their catalytic mechanisms appear to be similar. In each instance, the ferric resting state of the enzyme is oxidized to a two-electron oxidized species known as Compound I in which the iron is oxidized to a ferryl (FeIV=O) species. The second oxidizing equivalent is stored as either a porphyrin (e.g. horseradish peroxidase) or protein (cytochrome c peroxidase) radical cation, or in some instances appears to exist in both forms (lactoperoxidase).
A unique feature of the mammalian peroxidases is the presence of covalent links between the heme group and the protein. The links common to myeloperoxidase, lactoperoxidase, eosinophil peroxidase, and probably thyroid peroxidase are two ester bonds between aspartic or glutamic acid residues and hydroxyl groups on the 1- and 5-methyl groups of the heme (Figure 1). Myeloperoxidase has a unique third covalent bond between one of its vinyl groups and the methionine residue. We have demonstrated that the covalent bonds in heterologously expressed lactoperoxidase are formed by an autocatalytic process involving reaction of the apoprotein-heme complex with H2O2. The reaction is proposed to involve the formation of a Compound I species lacking the heme covalent bonds. This species undergoes a self-catalyzed reaction that converts one of the heme methyl groups to a methylene cation that is then trapped by the carboxyl group of an aspartic or glutamic acid residue. Formation of the methylene cation can be rationalized by radical or acid base mechanisms, but the detailed mechanism continues under investigation. Subsequent studies in other laboratories have provided evidence that similar mechanisms probably operate in the other mammalian peroxidases. Ongoing studies have shown, at least for lactoperoxidase, that a single covalent bond suffices for full catalytic activity, whereas a mutated enzyme complex unable to form the covalent links is unstable and inactive.
Figure 1. The prosthetic heme group in lactoperoxidase is bound to the protein through ester bonds between two of its methyl groups and Glu375 and Asp225 of the protein.
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