Activators of Molecular Oxygen

The first illustration of this class of enzymes is the haem-copper cytochrome c oxidase (CcO), the terminal component of the respiratory chain in aerobic organism. These membrane-bound enzymes catalyse the reduction of molecular dioxygen to water (Reaction (4)) at the rate of up to 250 molecules of O2 per second  

FIGURE 13.8 Complexes of the respiratory chain. These include NADH dehydrogenase, succinate dehydrogenase (PDB code INEN), bci complex (PDB code 1PP9), cytochrome c oxidase (PDB code 1V54), and cytochrome c (PDB code IHRC). (From Hosier et at, 2006. Copyright 2006 with permission from Annual Reviews.) 

CcOs from bovine heart mitochondria and bacteria Rhodobacter sphaeroides and Paracoccus denitrificans) have been determined, culminating in the 2.0 A resolution structure of CcO from Rhodobacter sphaeroides which contains only the two catalytic subunits — subunit I with 3 redox-active centres, haem a, and the catalytic site made up of haem ag and Cub, and subunit II with the Cua redox centre made up of 2 copper ions, together with two other subunits (Qin, Hiser, Mulichak, Garavito, Ferguson-Miller, 2006). 

CcO catalyses the oxidation of four molecules of cytochrome and uses these electrons to reduce molecular 

FIGURE 13.9 (a) The structure of cytochrome c oxidase from R. sphaeroides (PDB code 1M56). The four subunits of the enzyme are 

---

Activation of molecular oxygen. G. Henrici-Olive and S. Olive, Angew. Chem., Int. Ed. Engl., 1974,13,29-38 (74). 

Activation of molecular oxygen on interaction with transition metal complexes. A. V. Savitskii and V. I. Nelyubin, Russ. Chem. Rev. (Engl. Transl.), 1975,44,110-121 (124). 

Figure 28. Possible mechanistic model for the activation of molecular oxygen over Au/Ti-Si02 catalysts [92].

The activation of molecular oxygen by copper plays a central role in synthetically useful stoichiometric and catalytic oxidative conversions of organic molecules and in biological systems.4"26... 

Although there are indeed only few reported methods of direct activation of molecular oxygen via transition metals, there are many reports of indirect oxidation. The majority of this research is based on palladium-based oxidation as summarized in equation 32. The palladium complex catalyzed oxidation reactions have been reviewed previously186 and also only very recently187 and in this book the palladium catalyzed oxidation of dienes and polyenes will be discussed separately and therefore will not be discussed... 

Figure 13.4 Proposed catalytic cycle for activation of molecular oxygen and substrate by the ruthenium-substituted sandwich-type [WZnRu23 (XW90 34)2]11 (X=Zn2+ or Co2 ). (From Neumann, R., and Dahan, M., Nature, 388, 353, 1997 and Neumann, R., and Dahan, M., J. Am. Chem. Soc., 120, 11969, 1998.)...

A dinuclear Fe(II) centre is bridged by two carboxylate and one hydroxide group. It is thought that the activation of molecular oxygen is a consequence of the ability of the active site to shuttle the FeuFen/ FeIIFeIII/FeIIIFeIn redox changes. 

One pecuhar oxygen buffer reaction is the well-known thermobarometric equation of Buddington and Lindsley (1964), which furnishes information on both T and the activity of molecular oxygen. The reaction uses the equilibrium among titanomagnetite (or ulvospinel ), magnetite, hematite, and ilmenite components in the hemo-ilmenite and spinel phases ... 

A study of electro-assisted biomimetic activation of molecular oxygen by a chiral Mn(salen) complex in [BMIMJPFs showed that a highly reactive oxomanganese(V) intermediate could transfer its oxygen to an alkene (229). 

The binding and activation of molecular oxygen is an important process in nature and is achieved with the help of metallo-enzymes, e.g. dicopper enzymes... 

Massey, Y (1994) Activation of molecular oxygen by flavins and flavoproteins. J. Biol. Chem. 269, 22,459-22,462. 

The biochemical importance of flavin coenzymes ap-pears to be their versatility in mediating a variety of redox processes, including electron transfer and the activation of molecular oxygen for oxygenation reactions. An especially important manifestation of their redox versatility is their ability to serve as the switch point from the two-electron processes, which predominate in cytosolic carbon metabo-lism, to the one-electron transfer processes, which predomi-nate in membrane-associated terminal electron-transfer pathways. In mammalian cells, for example, the end products of the aerobic metabolism of glucose are C02 and NADH (see chapter 13). The terminal electron-transfer pathway is a membrane-bound system of cytochromes, nonheme iron proteins, and copper-heme proteins—all one-electron acceptors that transfer electrons ultimately to 02 to produce H20 and NAD+ with the concomitant production of ATP from ADP and P . The interaction of NADH with this pathway is mediated by NADH dehydrogenase, a flavoprotein that couples the two-electron oxidation of NADH with the one-electron reductive processes of the membrane. 

As shown by Table 3, most of the Group VIII metal-peroxo complexes are obtained from the direct interaction of dioxygen with the corresponding reduced forms. A considerable effort has been devoted to this subject in the last decade with the hope that selective oxidations of hydrocarbons could be achieved by the activation of molecular oxygen under mild conditions12,56 133,184 and several such examples have actually been shown to occur. 

The dynamic XAFS study of the catalytically active structures under the reaction conditions should enable us to further develop novel catalysts for direct phenol synthesis from benzene and the efficient activation of molecular oxygen. 

D. J. Kosman and J. J. Driscoll, Solvent exchangeable protons and the activation of molecular oxygen the galactose oxidase reaction, Prog. Clin. Biol. Res., 274 (1988) 251-267. 

---