Galactose copper site

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. 

The classification introduced in this review (type I-type IV) should cover all structural types of copper sites known to date. For instance, based on this nomenclature, ascorbate oxidase contains type I and type IV, and nitrite reductase contains type I and type II (more precisely, type IIA). Galactose oxidase has a type IIB site. 

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. 

Domain 1 contains the first 8 -strands as well as a binding site for a sodium ion and for D-galactose. It is yet unknown whether the sodium ion is of importance for the enzyme s function. Domain 1 is of some distance from the copper center and its function is not clearly defined. It is possibly a chaperone for the correct folding of the enzyme, in which domain 1 is utilized as a blueprint for the complicated folding of domain 2. The D-galactose binding site of domain 1 may possibly be needed to attach the enzyme to the cell walls of trees, the natural habitat of Dactylium dendroides [30]. 

The crystal structure of galactose oxidase from the fungus Dactylium dendroides has been determined. Accordingly, galactose oxidase (639 amino acid residues) consists of three domains predominantly formed from /3-structures (Figure 14). The first domain (residues 1-155) has a /3-sandwich structure. The catalytic domain (residues 156-532) comprises a seven-fold /3-propeller based on the kelch structural motif The copper lies on the solvent-accessible surface of this domain close to the pseudo seven-fold axis. The third domain (residues 533-639) is comprised of seven /3-strands. The copper site on the second domain lies in a region extremely rich... 

Galactose oxidase (GO) catalyses the two-electron oxidation of primary alcohols to aldehydes. It contains a single type II copper centre. The enzyme employs the metal and a protein radical cofactor to effect the chemistry. The crystal structure shows a square pyramidal five-coordinate copper site with the metal coordinated by two histidines, two tyrosines and a water or acetate ligand. The equatorial tyrosine, Tyr272, has an interesting crosslink to a cysteine group ortho to the tyrosine oxygen. 

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. 

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]. 

So-called blue multinuclear copper oxidase enzymes, such as laccase and ascorbate oxidase, catalyze the stepwise oxidation of organic substrates (most likely in successive one-electron steps) in tandem with the four-electron reduction of O2 to water, i.e. no oxygen atom(s) from O2 are incorporated into the substrate (Eq. 4) [15]. Catechol oxidase, containing a type 3 center, mediates a two-electron substrate oxidation (o-diphenols to o-chinones), and turnover of two substrate molecules is coupled to the reduction of O2 to water [34,35]. The non-blue copper oxidases, e.g. galactose oxidase and amine oxidases [27,56-59], perform similar oxidation catalysis at a mononuclear type 2 Cu site, but H2O2 is produced from O2 instead of H2O, in a two-electron reduction. 

There are numerous reports on the chemical synthesis of models for the active site of galactose oxidase both in the reduced Cu(l) and the oxidized Cu(II) form. We mention only a selection in which EPR is at least used to characterize the complex either on the phenoxy radical or on the copper part, typically in conjunction with X-ray data.48,49 50 A review on structural, spectroscopic and redox aspects of galactose oxidase models is available.51 More important with respect to EPR is the report on the 3-tensor calculation of the thioether substituted tyrosyl radical by ab initio methods but this is borderline to the aspects treated in this review since the copper ion is not involved.52... 

The CuA center has an unusual structure.130-132 It was thought to be a single atom of copper until the three-dimensional structure revealed a dimetal center, whose structure follows. The CuB-cytochrome a3 center is also unusual. A histidine ring is covalently attached to tyrosine.133-1353 Like the tyrosine in the active site of galactose oxidase (Figs. 16-29,16-30), which carries a covalently joined cysteine, that of cytochrome oxidase may be a site of tyrosyl radical formation.135... 

---