Copper dioxygen activation

Under conditions of copper deficiency, some methanotrophs can express a cytosolic, soluble form of MMO (sMMO) (20-23), the properties of which form the focus of the present review. The sMMO system comprises three separate protein components which have all been purified to homogeneity (24,25). The hydroxylase component, a 251 kD protein, contains two copies each of three subunits in an a 82y2 configuration. The a subunit of the hydroxylase houses the dinuclear iron center (26) responsible for dioxygen activation and for substrate hydroxylation (27). The 38.6 kD reductase contains flavin adenine dinucleotide (FAD) and Fe2S2 cofactors (28), which enable it to relay electrons from reduced nicotinamide adenine dinucleotide (NADH) to the diiron center in the... 

Dioxygenases often have broad substrate specificity and require only a minimal characteristic structure for substrate recognition [310], Transition metal or an organic cofactor mediates dioxygen activation needed by the oxygenases action. Iron and copper, in their lower oxidation states are the metals most commonly used, but also organic co-factors like dihydroflavin and tetrahydropterin are able to activate the oxygen molecule. 

There has been enormous activity in the field of copper(I)-dioxygen chemistry in the last 25 years, with our information coming from both biochemical-biophysical studies and to a very important extent from coordination chemistry. This has resulted in the structural and spectroscopic characterization of a large number of copper dioxygen complexes, some of which are represented in Figure 14.2. The complex F, first characterized in a synthetic system was subsequently established to be present in oxy-haemocyanin, and is found in derivatives of tyrosinase and catechol oxidase, implying its involvement in aromatic hydroxylations in both enzymes and chemical systems. 

At present, we do not completely understand why only some of these very similar m-xylyl dicopper(I) complexes systems described above undergo ligand oxygenation reactions. However, based on the results outlined above, we can speculate on a number of aspects of this 02-activation process. Our studies implicate the presence of a copper-dioxygen (peroxo dicopper(II)) adduct as an intermediate in the oxygenation reaction and more recent kinetic studies (51) further support this conclusion. This adduct then either directly or via some further intermediate undergoes an electrophilic attack of the arene. The unique nature of this very fast reaction 2->3, and the observed inability to intercept the active... 

The enzymes of this type that have been characterized contain some type of redox-active cofactor, such as a flavin (3), or a metal ion (heme iron, non-heme iron, or copper), or both (4-6). Our understanding of the mechanism of these enzymes is most advanced in the case of the heme-containing enzyme cytochrome P450. But in spite of the availability of a crystal structure of an enzyme-substrate complex (7) and extensive information about related reactions of low molecular weight synthetic analogues of cytochrome P450 (8), a detailed picture of the molecular events that are referred to as "dioxygen activation" continues to elude us. 

The main relevant characteristic properties of oxygenases are that they mostly involve transition metals (particularly iron and copper) as active centers and that they activate dioxygen and transfer it selectively to organic substrates. Oxygenases can be conveniently classified into two main families. 

As the focus of this review is on copper-dioxygen chemistry, we shall briefly summarize major aspects of the active site chemistry of those proteins involved in 02 processing. The active site structure and chemistry of hemocyanin (He, 02 carrier) and tyrosinase (Tyr, monooxygenase) will be emphasized, since the chemical studies described herein are most relevant to their function. The major classes of these proteins and their origins, primary functions, and leading references are provided in Table 1. Other classes of copper proteins not included here are blue electron carriers [13], copper-thiolate proteins (metallothioneines) [17], and NO reductases (e.g., nitrite [NIR] [18] or nitrous oxide [19]). 

Mimicking and understanding tyrosinase activity (o-hydroxylation of phenols) have been of longtime interest because this was one of the earliest copper monooxygenases described and the significance of elucidating dioxygen activation mechanism(s) has widespread implications and potential applications. 

Mills S. A. Klinman J. P. Evidence against reduction of Cu2+ to Cu+ during dioxygen activation in a copper amine oxidase from yeast. J. Am. Chem. Soc. 2000, 122, 9897-9904. 

In light of the accepted mechanism for cytochrome P-450 (97-100), a superoxo-Cu(II) intermediate is further reduced, leading to dioxygen activation. Accordingly, a monomeric peroxo or hydroperoxo copper(II) complex serves as a synthetic model for these intermediates of copper-containing monooxygenases. However, no well-characterized complexes of these types are available to date. Formation of a monomeric hydroperoxo or acylperoxo complex was reported to occur when a trans-/u-l,2-peroxo complex, [(Cu(TPA))2(02)]z+, was treated with H+ or RC(O)+, but no details of the structures and properties of the complexes were provided (101). A related complex, a monomeric acylperoxo cop-per(II) complex, was synthesized (102). Low-temperature reaction of a dimeric copper(II) hydroxide complex, [Cu(HB(3,5-iPr2pz)3)]2(OH)2, with 2 equivalents of m-CPBA (3-chloroperoxybenzoic acid) yielded a monomeric acylperoxo complex whose structure was characterized by... 

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