Galactose oxidase function

E S R.. Fenton. Transferrin Complexes with Non-Physiological and Toxic Metals, David M. Taylor. Transferrins, Edward N. Baker. Galactose Oxidase, Peter Knowles and Nobutoshi Ito. Chemistry of Aqua Ions of Biological Importance, David T. Richens. From a Structural Perspective Structure and Function of Manganese - Containing Biomolecules, David C. Weatherburn, Index. Volume 3,1996,304 pp. 109.50/ 70.00 ISBN 1-55938-642-8... 

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 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. 

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]. 

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... 

The generation, stability, and function of tyrosyl radicals in ribonucleotide reductase, PGH synthase, and galactose oxidase continue to be active areas of research. The difficulties encountered in preparing and handling these proteins, as well as in probing the physical properties and reactivity of their metal-phenoxyl radical active sites, make the preparation and investigation of stable phenoxyl radical metal model complexes an attractive goal. 

In this review, current understanding of the structure and function of galactose oxidase and amine oxidases will be described together with comparisons between them and future directions in this field. 

The major purpose of this review is to describe studies on the structure and function of galactose oxidase carried out over the last decade. As with all science, these advances are based on many earlier studies and it will not be possible to document with due respect how these studies advanced the field. Earlier work (up to 1984) on GOase has been reviewed (Malm-strom et al., 1975 1981 Kosman, 1984, 13nl5) and more recent work is covered in reviews by Whittaker (1994) and by Knowles and Ito (1993). 

FIGURE 7. A functional model for galactose oxidase (Wang et cd., 1998). 

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), ... 

The simple coordination chemistry characteristic of the majority of protein-metal interactions is replaced in certain cases by irreversible covalent modifications of the protein mediated by the metal ion. These modifications are essential for the function and are templated by the structure of the protein, as no other proteins are required for the reaction to occur. These self-processing reactions result in the biogenesis of redox cofactors in some enzymes (amine oxidases, galactose oxidase, cytochrome c oxidase) and activation of hydrolytic sites in others (nitrile hydratase). The active sites of all of these enzymes are bifunctional, directing not only the catalytic turnover reaction of the mature enzyme but the modification steps required for maturation. 

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