Galactose oxidase alcohol oxidation

These systems are also described as normal copper proteins due to their conventional ESR features. In the oxidized state, their color is light blue (almost undetectable) due to weak d-d transitions of the single Cu ion. The coordination sphere around Cu, which has either square planar or distorted tetrahedral geometry, contains four ligands with N and/or 0 donor atoms [ 12, 22]. Representative examples of proteins with this active site structure (see Fig. 1) and their respective catalytic function include galactose oxidase (1) (oxidation of primary alcohols) [23,24], phenylalanine hydroxylase (hydroxy-lation of aromatic substrates) [25,26], dopamine- 6-hydroxylase (C-Hbond activation of benzylic substrates) [27] and CuZn superoxide dismutase (disproportionation of 02 superoxide anion) [28,29]. 

The mechanism of action of galactose oxidase involves oxidation of the alcohol to the aldehyde by a copper(iii) form of the enzyme which is reduced to copper(i). Reoxidation involves a copper(ii)-superoxide intermediate. ... 

Copper(II) complexes with phenoxo ligands have attracted great interest, in order to develop basic coordination chemistry for their possible use as models for tyrosinase activity (dimeric complexes) and fungal enzyme galactose oxidase (GO) (monomeric complexes). The latter enzyme catalyzes the two-electron oxidation of primary alcohols with dioxygen to yield aldehyde and... 

Another oxidizing enzyme with very interesting synthetic potential is galactose oxidase [14]. This copper protein oxidizes primary hydroxy functions in polyols enantioselectively to the corresponding aldehydes. Thus, sugar alcohols may be transformed into the interesting non-natural L-configurated... 

The selective oxidation of alcohols to the corresponding aldehydes and ketones is of prime importance for organic synthesis, and various types of reagents have been described that achieve this transformation selectively and efficiently [141,142]. However, the number of sub-stoichiometric, nontoxic, non-hazardous oxidation systems has been relatively Umited. As copper enzymes such as galactose oxidase are known to catalyze this oxidation reaction, bioinspired homogeneous catalysts based on copper species have also been developed in recent years. 

In contrast to the active site of galactose oxidase, to pre-catalyst 13, and to the system reported by Stack et al., the proposed catalytic species 15 does not imdergo reduction to Cu intermediates, as the oxidation equivalents needed for the catalysis are provided for solely by the phenoxyl radical Hgands. Since the conversion of alcohols into aldehydes is a two-electron oxidation process, only a dinuclear Cu species with two phenoxyl ligands is thought to be active. Furthermore, concentrated H2O2 is formed as byproduct in the reaction instead of H2O, as in the system described by Marko et al. [159]. 

There is no valid interpretation for the activation by OJ and by hexacyano-ferrate(III), although they fitted nicely in a reaction scheme with Cu(III) as the active species In the oxidation of an alcohol to an aldehyde Cu(III) would be reduced to Cu(I). In the subsequent reaction of Cu(I) with Oj, Cu(II)Oj was considered an intermediate yielding Cu(III) and H O. This intermediate would be in a reversible equilibrium with OJ and with the resting Cu(II)-enzyme. This resting enzyme would be oxidized by hexacyanoferrate(III) to the active Cu(III) species. There was unfortunately no indication in X-ray absorption measurements for the formation of Cu(III) with hexacyanoferrate(III) and the resting enzyme . EPR measurements indicated that Cu(II) was present in the active enzyme It was not possible, moreover, to detect Oj by the reduction of Fe(III)-cytochrome c in a galactose oxidase — galactose system... 

Nonblue. These include galactose oxidase (GO) and amine oxidases (e.g., plasma amine oxidase, diamine oxidase, lysyl oxidase), which produce dihydrogen peroxide by the two-electron reduction of 02 [33], For GO (stereospecific primary alcohol oxidation), spectroscopic studies by Whittaker [70,71] suggest that the two-electron oxidation carried out by a mononuclear copper center is aided by a stabilized ligand-protein radical (i.e., (L)Cu(I) + 02 —> (L +)Cu(lI) + H202), obviating the need to get to Cu(III) in the catalytic cycle. Protein x-ray structures [33,72] reveal a novel copper protein cofactor, which would seem... 

Galactose oxidase is an extracellular enzyme secreted by the fungus Dactylium den-droides. It is monomeric (M = 68000), contains a single copper site and catalyses the oxidation of a wide range of primary alcohols to the corresponding aldehydes. The two-electron transfer reaction RCH2OH - RCHO + 2H+ + 2e does not utilise a Cu(III)/Cu(I) couple, but a second redox site, involving a tyrosine radical which mediates the transfer of the second electron. 

Figure 5. Proposed mechanism of alcohol oxidation for galactose oxidase.

An example of a one-pot, three-step catalytic cascade is shown in Fig. 1.51 [139]. In the first step galactose oxidase catalyses the selective oxidation of the primary alcohol group of galactose to the corresponding aldehyde. This is fol-... 

The alcohol dehydrogenases were already described in Chapter 3. These enzymes are cofactor dependent and in the active site hydrogen transfer takes place from NADH or NADPH. In the reverse way they can, however, be applied for the oxidation of alcohols in some cases (see below). Oxidases are very appealing for biocatalytic purposes, because they use oxygen as the only oxidant without the need for a cofactor. Oxidases usually have flavins (glucose oxidase, alcohol oxidase) or copper (examples galactose oxidase, laccase and tyrosinase) in the active site [18]. The mechanism for glucose oxidase (GOD) is denoted in... 

Galactose oxidase exhibits a mononuclear copper-active site, which is flanked by a tyrosinyl radical. In a single step, a two-electron oxidation of alcohols can be performed by this enzyme, where one electron is extracted by copper and the other by the tyrosine residue [19]. 

Copper would seem to be an appropriate choice of metal for the catalytic oxidation of alcohols with dioxygen since it comprises the catalytic centre in a variety of enzymes, e.g. galactose oxidase, which catalyze this conversion in vivo [188, 189]. Several catalytically active biomimetic models for these enzymes have been designed which are seminal examples in this area [190-193]. A complete overview of this field can be found in a review [194]. 

Chaudhuri, P., Hess, M., FTrke, U., and Wieghardt, K., 1998, From Structural Models of Galactose Oxidase to Homogeneous Catalysis Efficient Aerobic Oxidation of Alcohols, Angew. Chem., Int. Ed. Engl. 37 2217n2220. 

Galactose Oxidase (GO) from the filamentous wheat-root fungus Fusarium spp. is a mononuclear type 2 copper enzyme that catalyzes the two-electron oxidation of a large number of primary alcohols to their corresponding aldehydes, coupled with the reduction of dioxygen to hydrogen peroxide [1,18] ... 

As a mimic of the well-studied galactose oxidase [37], a copper(II) thiophenol complex catalyzes the oxidation of primary alcohols to aldehydes in the presence of (Scheme 12) [38]. The latter also promotes the oxidation of secondary alcohols to diols (Scheme 12). The catalytic cycle starts with the oxidation of copper by O, leading to a biradical species. The intermediate 39 is produced from 38 by coordination of two alkoxide substrates. The rate-limiting step is the formation of 40 from 39 by a hydrogen atom transfer from the secondary alcoholate to the oxygen-centered radicals of the aminophenols ligands. The cycle is then closed by radical dimerization which leads the formation of the diol [39]. 

Function. Galactose oxidase catalyses the oxidation of primary alcohols to aldehydes, reducing oxygen to hydrogen peroxide in a two-electron reduction [30] ... 

Scheme 14 Galactose oxidase cofactor-dependent alcohol oxidation.

Other examples of amperometric enzyme electrodes based on the measurement of oxygen or hydrogen peroxide include electrodes for the measurement of galactose in blood (galactose oxidase,enzyme), oxalate in urine (oxalate oxidase), and cholesterol in blood serum (cholesterol oxidase). Ethanol is determined by reacting with a cofactor, nicotinamide adenine dinucleotide (NAD" ) in the presence of the enzyme alcohol dehydrogenase to produce the reduced form of NAD", NADH, which is electrochemically oxidized. Lactate in blood is similarly determined (lactate dehydrogenase enzyme). 

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