Galactose-6-oxidase enzyme

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... 

Interest in this class of coordination compounds was sparked and fueled by the discovery that radical cofactors such as tyrosyl radicals play an important role in a rapidly growing number of metalloproteins. Thus, in 1972 Ehrenberg and Reichard (1) discovered that the R2 subunit of ribonucleotide reductase, a non-heme metal-loprotein, contains an uncoordinated, very stable tyrosyl radical in its active site. In contrast, Whittaker and Whittaker (2) showed that the active site of the copper containing enzyme galactose oxidase (GO) contains a radical cofactor where a Cu(II) ion is coordinated to a tyrosyl radical. 

The function of the metal site in the oxygen-dependent radical enzymes galactose oxidase, amine oxidases, ribonucleotide reductase, and cytochrome c oxidase is inter alia to bind 02 in their reduced forms and undergo the appropriate redox chemistry to generate a metal-bound, activated oxygen species of variable nature. 

Recently, Kogel et al. (98) showed that the injection of galactosebinding lectins or the enzyme galactose oxidase into resistant wheat leaves prevented the hypersensitive cell death of penetrated host cells and concomitantly led to increased fungal growth, suggesting a causal relationship between hypersensitive cell death and resistance. 

Galactose is determined by immobilizing the enzyme galactose oxidase at an oxygen electrode (190) or at a micro-platinum electrode operating anodically to monitor H2O2 formation (191, 192). The enzyme is immobilized on collagen membranes (193) or cellulose acetate membranes, and the probe is applied selectively to plasma d whole blood determinations (191, 194). 

Coordinated tyrosyl radicals have been discovered in the active form of the Cun-containing fungal enzyme, galactose oxidase. The unpaired electron on the ligand is strongly anti-ferromag-netically coupled to the unpaired d electron. Phenoxyl radical ligands have been confirmed in the successive one-electron oxidation of [Ga(L )3] by spectroelectrochemistry of the Ga model complex (Li shown in Scheme 2). 

Copper(m)—A copper(iii) complex of the enzyme galactose oxidase has been postulated as the active intermediate in the oxidation of D-galactose. Autoxidation of copper(ii)-peptide complexes leads to relatively long-lived copper(iii)-peptide species. ... 

The selective aerobic oxidation of primary alcohols to aldehydes, but not secondary alcohols to ketones, is reminiscient of the chemistry catalyzed by the Cu-dependent enzyme, galactose oxidase (39). Similarly, the Cu-binding P-amyloid protein relevant to Alzheimer s disease promotes aerobic oxidation of cholesterol, a primary alcohol (cholesterol oxidase activity) (40). The Cu-dependent amine oxidases catalyze the aerobic oxidation of amines to aldehydes (41), the hydration products of imines. Each of these enzymes that promotes aerobic oxidation of primary alcohols and amines to the same products as Ni(TRISOX) catalyze the net reaction in Equation 1. If the net reactions... 

The fungal enzyme galactose oxidase (GAO) catalyzes the reaction shown in Equation (1) at a monocopper active sited ... 

The Cu/TEMPO catalyst system has been the subject of considerable mechanistic investigation. Initial reports of the Cu/TEMPO catalyst drew mechanistic analogies to the enzyme galactose oxidase, in which a coordinated tyrosyl radical ligand mediates H-atom abstraction from a Cu-bound alkoxide (Figure 6.7) [26]. Early mechanistic studies [27] showed that kinetic isotope effect (KIE) values with Cu/TEMPO catalysts and galactose oxidase are similar and led to the proposal that Cu/TEMPO-mediated alcohol oxidation proceeds via intramolecular abstraction of an H atom by and of a / -coordinated TEMPO. 

We have shown, in stoichiometric experiments, that reaction of copper(I) with TEMPO affords a piperidinyloxyl copper(II) complex. Reaction of the latter with a molecule of alcohol afforded the alkoxycopper(II) complex and TEMPOH. Reaction of the alkoxycopper(II) complex with a second molecule of TEMPO gave the carbonyl compound, copper(I), and TEMPOH. This mechanism resembles that proposed for the aerobic oxidation of alcohols catalyzed by the copper-dependent enzyme, galactose oxidase, and mimics thereof. Finally, TEMPOH is reoxidized to TEMPO by oxygen. We have also shown that copper in combination with PIPO affords an active and recyclable catalyst for alcohol oxidation [18]. 

Two phenoxyl radical complexes [Cu (2 )N03] and [Zn (2 )N03] oxidize benzyl alcohol to benzaldehyde and have been studied as models for the enzyme galactose oxidase (GO). GO contains a dipeptide unit (3) in which a tyrosine residue is covalently bound to an adjacent cysteine residue and which is similar to (2), the tyrosyl radical in (3) also being bound to the Cu centre (see Figure 1). Second-order kinetics were observed with respect to [Zn°(2 )N03]+ and there was no evidence of redox reaction at the zinc site, suggesting that a dimeric form of the complex is active however, the reaction of [Cu H2 )N03]+ with benzyl alcohol is first order in the metal complex and [Cu (2H)]+ is identified as a product, suggesting a formal 2e /2H+ mechanism in which the monomeric form coordinates the alcohol in the manner believed to operate for G0. 2... 

Galactose can be measured in body fluids, using the enzym< galactose oxidase in a reaction analogous to the glucose oxidast reaction for the determination of glucose. 

Recently, some low-molecular-weight catalysts have been found that mimic enzymes and have potential practical applications. Reactions of limited complexity, mediated by metalloenzymes, have been reported. One such example is a mimic of the copper enzyme galactose oxidase, which catalyzes the oxidation of primary alcohols to aldehydes in the presence of air. 

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