Characterization of cytochrome bo(3) activity in a native-like surface-tethered membrane

We have developed a simple native-like surface-tethered membrane system to investigate the activity of cbo3 (cytochrome bo3), a terminal oxidase in Escherichia coli. The tethered membranes consist of E. coli inner-membrane extracts mixed with additional E. coli lipids containing various amounts of the cbo3 substrate UQ-10 (ubiquinol-10). Tethered membranes are formed by self-assembly from vesicles on to gold electrodes functionalized with cholesterol derivatives. cbo3 activity was monitored using CV (cyclic voltammetry) with electron transfer to cbo3 mediated by UQ-10. The apparent Km for oxygen with this system is 1.1±0.4 μM, in good agreement with values reported in the literature for whole-cell experiments and for purified cbo3. Increasing the concentration of lipophilic UQ-10 in the membrane leads to an increase in cbo3 activity. The activity of cbo3 with long-chain ubiquinones appears to be different from previous reports using short-chain substrate analogues such as UQ-1 in that typical Michaelis–Menten kinetics are not observed using UQ-10. This native-like membrane model thus provides new insights into the interaction of transmembrane enzymes with hydrophobic substrates which contrasts with studies using hydrophilic UQ analogues.