Electrodes modified with lipid membranes to study quinone oxidoreductases

Abstract

Quinone oxidoreductases are a class of membrane enzymes that catalyse the oxidation or reduction of membrane-bound quinols/quinones. The conversion of quinone/quinol by these enzymes is difficult to study because of the hydrophobic nature of the enzymes and their substrates. We describe some biochemical properties of quinones and quinone oxidoreductases and then look in more detail at two model membranes that can be used to study quinone oxidoreductases in a native-like membrane environment with their native lipophilic quinone substrates. The results obtained with these model membranes are compared with classical enzyme assays that use water-soluble quinone analogues.

Figure 1

Model membrane systems used to study quinone oxidoreductases. (A) A hybrid bilayer on alumin oxide modified with trichloro(octadecyl)silane (OTS). This system has been used to study a peripheral membrane enzyme, pyruvate-ubiquinone oxidoreductase[32]. (B) A tethered bilayer lipid membrane (tBLM) formed on a mixed self-assembled monolayer of EO3-Cholesterol and 6-mercaptohexanol. This system has been used to study a ubiquinol oxidase, cytochrome bo₃[33, 34].

Figure 2

The enzyme activity of cytochrome bo₃ from E. coli as a function of ubqiuinol-10 concentration in the membrane. Data determined using a tethered bilayer lipid membrane system as explained in the text. The dotted line is guide for the eye to highlight possible cooperative behaviour as described in the text. Details of the experiment are described in [34].