Superoxide dismutase catalytic cycle

Strangely, these small molecules are also used industrially as oxidation catalysts, however, small changes in their structures lead to dramatically different behaviour. In these model compounds, unlike SOD2, the metal is bound by two oxygen and two nitrogen atoms in a square planar orientation with axial positions occupied by water or reactive oxygen species. The superoxide dismutase catalytic cycle has been proposed to occur by a one electron mechanism ... 

Fig. 6. Suggested changes in the coordination at tte copper (xntre upon anion binding and during the catalytic cycle of Cu—Za superoxide dismutase...

Fig. 16 Catalytic cycle of manganese superoxide dismutase X is a dead-end peroxide complex (Reprinted with permission from Ref 103, Copyright 2000, Marcel Dekker).

The reaction of nitric oxide with superoxide dismutase is a simple reversible equilibrium, whereas the catalytic cycle with superoxide involves a two step sequence. Consequently, superoxide dismutase may be reduced by superoxide and then react with nitric oxide to form nitroxyl anion. Nitroxyl anion may react with molecular oxygen to form peroxynitrite anion (ONOO"). 

Finally, Zn also plays an important role in Cu,Zn-SOD (copper-zinc superoxide dismutase) for which the structure is shown in Figure 8 ". Although it is well-known that Zn(II) is not involved in electron-transfer chemistry, it is generally believed that the essential role of Zn(II) ion in SODs is to accelerate both the oxidation and reduction of superoxide by controlhng the redox potential of the Cu(II) ion and superoxide ion in the catalytic cycle. ... 

Other than in prokaryotic cells which lack mitochondria and chloroplasts, manganese superoxide dismutases are apparently restricted to the above two organelles in eukaryotic cells (51, 52) this forms strong support for the symbiotic hypothesis for the origin of mitochondria and chloroplasts (53, 54). Kinetic studies of superoxide dismutation by these enzymes indicate three oxidation states of Mn (presumably divalent, trivalent, and tetravalent) are involved in the catalytic cycle (57, 58). They also show that a Mn-02 complex may conceivably be formed. Well-characterized Mn-dioxygen (i.e., 02,02 , 022 ) adducts are extremely rare, the first structurally characterized example being reported only in 1987 (60). 

Fig. 4 Reaction pathways for superoxide dismutases and superoxide reductases. Shown in black are the two possible pathways leading to oxidation of the active site by superoxide, producing peroxide this half of the catalytic cycle is common to SOR and SOD. Shown in gray is the re-reductive path specific to SOD, and which in SOR is replaced by a single-electron transfer from the specialized redox protein, rubredoxin. M may be Fe (n = 2, in SOR, or in SOD), Ni, Mn (n = 2, in SOD), or Cu (n = 1, in SOD)...

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