The mechanisms described are from right to left

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The mechanisms described are, from right to left, concerted addition of oxygen to the double bond, reaction via a metallocylic intermediate, reaction via a ring-opened radical intermediate, and reaction proceeding via an initial electron-transfer step.91 oping charge separation that occurs in heterolytic cleavage (see Section VI). The crystal structure of cytochrome P-450 cam shows no such groups in the active- site cavity, nor does it give any clue to the source of a proton to protonate the peroxide ligand when it is produced. 8o Also, we have little experimental evi- dence concerning possible roles that the cysteinyl sulfur axial ligand might play in facilitating 0-0 bond cleavage. These issues remain areas of active interest for researchers interested in cytochrome P-450 mechanisms. 2. Other metal-containing monooxygenase enzymes As mentioned above, much less is known about the structural characteristics and mechanisms of the nonheme metal-containing monooxygenase enzymes. From the similarities of the overall stoichiometries of the reactions and the resem- blance of some of the enzymes to dioxygen-binding proteins, it is likely that the initial steps are the same as those for cytochrome P-450, i.e., dioxygen binding followed by reduction to form metal-peroxide or hydroperoxide complexes. It is not obvious that the next step is the same, however (i.e., 0-0 bond cleavage
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VI. CATALASE AND PEROXIDASE 295 to form a high-valent metal-oxo complex prior to attack on substrate). The prob- lem is that such a mechanism would generate metal-oxo complexes that appear to contain metal ions in chemically unreasonable high-oxidation states, e.g., Fe v, Cu III , or Cu IV (Reactions 5.79-5.81). (Felli - 00H)2+---> (Fe V O)3+ + OH (CUll - OOH) + ---> (CU IV O)2+ + OH ~ (CUll 00 CUll) 2+ ---> 2 (CuIllO) + (5.79) (5.80) (5.81) An alternative mechanism is for the peroxide or hydroperoxide ligand to attack the substrate directly; i.e., 0-0 bond cleavage could be concerted with attack on substrate. Another possibility is that the oxygen atom is inserted in a metal- ligand bond prior to transfer to the substrate. Neither of these alternative mech- anisms has been demonstrated experimentally. These various possibilities re- main to be considered as more information about the monooxygenase enzymes becomes available. VI. CATALASE AND PEROXIDASE A. Description of the Enzymes Catalase and peroxidase are heme enzymes that catalyze reactions of hydrogen peroxide. 94 ,95 In catalase, the enzymatic reaction is the disproportionation of hydrogen peroxide (Reaction 5.82) and the function of the enzyme appears to be prevention of any buildup of that potentially dangerous oxidant (see the dis- cussion of dioxygen toxicity in Section III). (5.82) Peroxidase reacts by mechanisms similar to catalase, but the reaction catalyzed is the oxidation of a wide variety of organic and inorganic substrates by hydro- gen peroxide (Reaction 5.83).
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