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Introduction

Bilirubin oxidase is an enzyme known to catalyze the oxidation of bilirubin to biliverdin by reducting O2 in water. It’s a multicopper oxidase which can exist under two different structures in the ascomycete Myrothecium verrucaria (see the 3abg one ^[3]). The 2xII form is a 4 chain structure with 8 ligands NAG-NAG belonging to the family of oxidoreductases.

It was first isolated in 1981 by the scientists Sawao Murao and Noriaki Tanaka as they tried to find a microorganism able to decolore raw sewage and to use it as an analytical tool in clinical fields. Now, bilirubin oxidase may be used to determine free hemoglobin in icteric specimens and can be use as a treatment for neonatal jaundice.

Moreover, the bilirubin oxidase is an enzyme that is active in porphyrin and chlorophyll metabolism.

Structure

Multicopper oxidases are enzymes which oxidise their substrate by accepting electrons at a mononuclear copper centre and transferring them to a trinuclear copper centre.^[4]

Bilirubin oxidases are multicopper oxidases containing type 1, type 2, and type 3 coppers. Indeed, there is strong sequence homology between bilirubin oxidase and multicopper oxidases like laccase, ascorbate oxidase and even ceruloplasmin. Moreover, the His-Cys-His sequence, characteristic of multicopper oxidase, is present in bilirubin oxidase. Copper is essential for the enzyme activity.^[5]

Copper is classified into three types according to their optical and magnetic properties. Type 1 copper (or blue copper) shows many charge-transfer bands around 450 nm, 600 nm and 750 nm. The most peculiar band appears around 600 nm and represents the Cys to Cu(II) charge transfer. Type 2 copper (or nonblue copper) does not show any strong charge-transfer bands in the visible region. Type 3 coppers are not detectable by ESR because some are antiferromagnetically coupled. However, a hydroxide ion links them and so gives a strong absorption at 330 nm.^[5]

While type 2 and 3 coppers reduce dioxygen to two water molecules by forming a trinuclear center, type 1 copper transfers electrons from substrate to the trinuclear center. This is the peculiar sequence His456-Cys457-His458 that forms an intramolecular electron-transfer pathway between the type 1 copper site and the trinuclear center composed of the type 2 and type 3 copper sites.^[5]

Function

Applications

Biofuel cell

In the field of power generation, biofuel cells provide an alternative to fossil energy. So far biofuell cells that were developed required specific enzymes that degrade and transform substrates present in physiologic fluids such as glucose and O2. These biofuel cells were used for medical devices power supply.^[6] Now a new generation of biofuel cells use hydrogenase. Indeed, H2/O2 biofuel cells are designed based on two thermostable enzymes: an hyperthermophilic O2-tolerant hydrogenase for H2 oxidation and bilirubin oxidase for O2 reduction. ^[7] Both enzymes are immobilized on carbon nanofibers. This biofuel cell can deliver electricity over a wide range of températures and pH. Indeed, the thermostability of the enzymes allows the biofuel cell to work under extreme conditions, from 30 to 80°C.^[6] H2/O2 biofuel cells can be used as alternative power supply for small electronic devices in a sustainable manner. However, research is ongoing; one issue is the enzyme instability on long term.^[7]

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