Heme-binding catalase-peroxidase

Halophilic proteins whose solution structures are currently under study include glyceraldehyde-3-phosphate dehydrogenase from Hal-oarcula vallismortis (Krishnan and Altekar, 1990) and a heme-binding catalase-peroxidase from H.marismortui (F. Cendrin, H. Jouvre, and G. Zaccai, private communication). 

Let us now consider the questions of the biological oxidation of various substrates with hydrogen peroxide in the presence of catalases and peroxidases. As Pratt notes [82], this group of enzymes is unique in that it is the only one in which intermediates are detected, and all stages of catalytic process are determined, identified and studied. Of special value is the characteristic that catalase consists of four subunits, whereas peroxidase possesses only one subunit. Using special technique, it is also shown that every iron atom (heme) binds one H202 molecule [83 ]. 

These are difficult enzymes to work with and only recently have crystal structures become available for two catalase-peroxidases Haloarcula marismortui (HMCP) and Burkholderia pseudomallei (BpKatG). A typical subunit is approximately 80 kDa in molecular mass, with a single heme b prosthetic group. The primary structure of each subunit can be divided into two distinct domains, N terminal and C terminal. The N-terminal domain contains the heme and active site, while the C-terminal domain does not contain a heme binding motif and its function remains unclear. The clear sequence similarity between the two domains suggests gene duplication and fusion. Curiously, despite many years of study, the actual in vivo peroxidatic substrate of the catalase-peroxidases has not been identified. 

Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Cytochrome Oxidase Iron Heme Proteins Dioxygen Transport Storage Iron Heme Proteins Electron Transport Iron Heme Proteins, Mono- Dioxygenases Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases Iron Proteins with Dinuclear Active Sites Iron Proteins with Mononuclear Active Sites Iron-Sulfur Proteins Peptide Metal Interactions Zinc DNA-binding Proteins Zinc Enzymes. 

Catalases and peroxidases. Many iron and copper proteins do not bind 02 reversibly but "activate" it for further reaction. We will look at such metallo-protein oxidases in Chapter 18. Here we will consider heme enzymes that react not with 02 but with peroxides. The peroxidases,1943 which occur in plants, animals, and fungi, catalyze the following reactions (Eq. 16-6,16-7) ... 

Aside from the classical examples of hemoglobin and myoglobin, reaction of ferrous heme iron with O2 in hemeperoxidases has been reported for myeloperoxidase [60], horseradish peroxidase C [62], bovine liver catalase [68], lignin peroxidase [46], and lactoperoxidase [61]. With the exception of lactoperoxidase, the binding of O2 is irreversible and CIII engages in one or more of the decay pathways described below. 

The reaction sequence at the heme active site starts with the binding of unactivated triplet dioxygen forming the so-called oxy-heme complexes. The iron center in 02-activating heme enz5maes is then thought to be converted into a peroxo anion species. It can be protonated to form a ferric hydroperoxo intermediate usually termed compormd 0 (183), which is a crucial reactive species in catalase and peroxidase enz5nne catalysis (Fig. 21). These hydroperoxo intermediates of hemoproteins are important... 

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