Ron Wever
Van 't Hoff Institute for Molecular Sciences
University of Amsterdam
Nieuwe Achtergracht 129
1018 WS Amsterdam
The Netherlands, e-mail rwever@science.uva.nl
Oxidative biocatalysts form a group of oxidoreductases that have found relatively little application in contrast to the dehydrogenases which catalyze hydrogen removal but do not involve oxygen intermediates. Oxidases catalyze the oxidation of substrates and they use oxygen as the terminal acceptor and water or hydrogen peroxide is the final product. Reduction of oxygen to water requires 4 electrons and 4 protons and multiple metal sites are in general present in the oxidases to shuttle the electrons towards oxygen. An example is the copper-containing laccase that sometimes require mediators that act as intermediate substrates of the enzyme in the overall reaction. The oxidases that produce hydrogen peroxide are in general more simple enzymes containing a single prosthetic group.
Peroxidases also oxidize substrates, however, they reduce hydrogen peroxide to water which is two-electron process. There is a large variety of peroxidases and of reactions that are catalyzed by these enzymes. The peroxidases are able
(i) to oxidize an organic molecule in which a single electron from the substrate is abstracted and as a result free radicals are produced. These radicals may recombine to form a variety of polymerized products.
(ii) Some of these enzymes better known as haloperoxidases catalyze the oxidation of chloride and bromide to hypochlorous or hypobromous acid.
(iii) Some peroxidases catalyze oxygen transfer from hydrogen peroxide to an organic molecule in which for example an alkene is epoxidized and sulfides are oxidized to sulfoxides. Two main classes of peroxidase can be distinguished, the heme proteins in which in general a protoheme group is present and the vanadium haloperoxidases which have vanadate (HVO42-) as a cofactor in their active site.
Vanadium haloperoxidases show a much higher chemical and operational stability as compared to the well-known heme-containing haloperoxidases partially due to their robust vanadate-cofactor (HVO42-). The cofactor is very stable under highly oxidative conditions, whereas the heme-group in heme peroxidases is susceptible to oxidative attack by substrates and products (H2O2, HOX, and 1O2). The vanadium enzymes are much less promiscuous than their heme counterparts. They essentially only oxidize halides at a substantial rate (Eqn. 1). A secondary reaction occurs when these enzymes oxidize halides producing singlet oxygen (Eqn. 2).
H2O2 + X- + H
HOX + H2O2 ——> 1O2 + X- + H+ + H2O (Eqn. 2)
X= Cl, Br
In my lecture I will first address the properties of the oxidases and peroxidases and then discuss applications of these enzymes in detergents e.g. the use in laundry detergents for bleaching of dyes in solution and thus preventing surplus dye from one garment to deposit on and discolourize other garments. Also the toolboxes such as directed evolution that are used these days to create enzymes with desired properties will be discussed.
