Redox enzymes in tethered membranes

An electrode surface is presented that enables the characterization of redox-active membrane enzymes in a native-like environment. An ubiquinol oxidase from Escherichia coli, cytochrome bo₃ (cbo₃), has been co-immobilized into tethered bilayer lipid membranes (tBLMs). The tBLM is formed on gold surfaces functionalized with cholesterol tethers which insert into the lower leaflet of the membrane. The planar membrane architecture is formed by self-assembly of proteoliposomes, and its structure is characterized by surface plasmon resonance (SPR), electrochemical impedance spectroscopy (EIS), and tapping-mode atomic force microscopy (TM-AFM). The functionality of cbo ₃ is investigated by cyclic voltammetry (CV) and is confirmed by the catalytic reduction of oxygen. Interfacial electron transfer to cbo₃ is mediated by the membrane-localized ubiquinol-8, the physiological electron donor of cbo₃. Enzyme coverages observed with TM-AFM and CV coincide (2-8.5 fmol·cm^-2), indicating that most-if not all-cbo ₃ on the surface is catalytically active and thus retains its integrity during immobilization.