Ligands galactose oxidase

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... 

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]. 

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]. 

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... 

Ligands to Metal in Galactose Oxidase ESR and Model Studies... 

However, the Schiff base complex lacks the stability towards reduction by CN" that characterizes the Cu( II) in galactose oxidase. While the enzyme binds a single CN" even at large CN" excess (22), the Cu(II) in the model is reduced by the ligand. To assess the underlying structural components which stabilize the enzymic Cu(II) towards reduction by CN", a five-coordinate model (Figure 2) having square bipyramidal symmetry was prepared (23). (The conditions and system procedures... 

Given some notion of the nature of the endogenous ligands, we can next ask how exogenous ligands perturb the copper atom and what this can tell us about the electronic transitions exhibited by the Cu(II) atom in galactose oxidase. 

It is interesting to consider the effect of exogenous ligands (which have previously been shown to bind to the Cu(II) atom inner sphere by ESR studies (22)) on the optical spectrum of galactose oxidase. (While... 

Galactose oxidase can illustrate how ligands, geometry, and active site groups together provide the basis for the structure-function properties of a metal active site. Figure 10 summarizes mutual interactions... 

Oxidation of this tryptophan in galactose oxidase also prevents alkylation of the histidine residue. Alkylation of the histidine residue in turn markedly affects the fluorescence quantum yield of this tryptophan (43) and nearly abolishes the absorbance of the copper atom. The copper atom itself is also essential to the reactivity of this histidine. Thus, we appear to have a consistent set of highly interdependent components. Not unexpectedly, the copper site cannot be fully understood without considering its interactions with non-ligand protein groups. 

This approach was used to examine the redox chemistry of the Cu site in galactose oxidase (41), which had been proposed to contain an unusual Cu(III) center (52). The lack of a significant Cu K-edge energy shift between the oxidized and reduced forms of the protein demonstrated that the redox chemistry was not metal-centered and implicated another redox active site. The crystal structure of the protein subsequently revealed a novel thioether composed of a cysteine and a tyro-sinate ligand of the Cu site that is likely to be involved in the redox process (53). 

Transition metal ions with organic radicals exist in the active sites of metalloproteins. The best understood example is galactose oxidase, which features a single Cu(II) ion coordinated to a modified tyrosyl radical. Many combined experimental and theoretical studies have focused on electronic properties of metal complexes with redox active ligands, yet reactivity beyond characterization has been limited. We will demonstrate the influence of the metal complex redox state on H2 activation by anilino-phenolate noninnocent ligands. 

Fig. 7. Inner sphere of the galactose oxidase copper-binding site. Geometric details of the ligand arrangement in the aquo complex are indicated in the figure. (Based on protein coordinates PDB ID IGOG.)...

Whittaker, M. M., and Whittaker, J. W., 1993, Ligand interactions with galactose oxidase mechanistic insights, Biophys. J. 64 7629772. 

The copper complex of the diaminodiol (98) functions as a model foT galactose oxidase.A related diamidodiol (99) has been reported binding to high-valent Os and Ru, as well as in mixed [Cu L M(bipy)2] (M = Co, Ni, Zn) compounds, some of the latter being antiferromagnetically coupled. A number of diamine-diacid ligands have appeared, such as (100), ... 

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