Dioxygen binding/cleavage

Another gradient-corrected density functional study of reversible dioxygen binding and reversible 0-0 bond cleavage has been carried out for Cu2(p-T 2 r 2... 

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 resemblance 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., O—O bond cleavage... 

The dissociation of water produces a five-coordinate Fe(III) complex (B) that can be reduced to an Fe(II) analog. Ferrous P450cam (C) readily binds dioxygen to form the oxy form of the enzyme. Dioxygen binds in a bent end-on mode (t/ -Oz) with the iron center formally oxidized to Fe(III) and the O2 reduced by one electron to superoxide (D). Addition of a second electron from putidaredox-in follows to further reduce the complex to an Fe(III)-peroxo species (E). Proton transfer is required before the cleavage of the 0-0 bond an Fe(III)-hydroperoxo... 

In the first reaction, the two-electron reduction of molecular oxygen is followed by protonation of the resulting anionic species to yield hydrogen peroxide. On the other hand, the second reaction requires cleavage of the dioxygen bond, followed ultimately by protonation of hydroxide ions to afford water this process has not been observed unless each of the oxygen atoms is able to bind to a unique metal center. 

The presently accepted model1555 for the reaction of bleomycin with DNA involves the binding of an Fen-bleomycin complex to DNA, and the reduction of iron-bound dioxygen. Cleavage of DNA is known to occur by the action of the 1,10-phenanthroline complex of Cu1 in the presence of hydrogen peroxide, and this reaction is sustained by the presence of NADH.1556 This provides a useful parallel for the bleomycin reaction. 

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