Analysis of Ion Transport through a Single Channel of Gramicidin A in Bilayer Lipid Membranes

Model Lipid Raft Membranes for Embedding Integral Membrane Proteins: Reconstitution of HMG-CoA Reductase and Its Inhibition by Statins

For the first time, HMG-CoA reductase, the membrane protein responsible for cholesterol synthesis, was incorporated into a lipid membrane consisting of DOPC:Chol:SM at a 1:1:1 molar ratio, which mimics the lipid rafts of cell membranes. The membrane containing the protein was generated in the form of either a proteoliposomes or a film obtained by spreading the proteoliposomes at the air-water interface to prepare a protein-rich and stable lipid layer over time. The lipid vesicle parameters were characterized using dynamic light scattering (DLS) and fluorescence microscopy. The incorporation of HMG-CoA reductase was reflected in the increased size of the proteoliposomes compared to that of the empty liposomes of model rafts. Enzyme reconstitution was confirmed by measuring the activity of NADPH, which participates in the catalytic process. The thin lipid raft films formed by spreading liposomes and proteoliposomes at the air-water interface were investigated using the Langmuir technique. The activities of the HMG-CoA reductase films were preserved over time, and the two lipid raft systems, nanoparticles and films, were exposed to solutions of fluvastatin, a HMG-CoA reductase inhibitor commonly used in the treatment of hypercholesterolemia. Both lipid raft systems constructed were useful membrane models for the determination of reductase activity and for monitoring the statin inhibitory effects and may be used for investigating other integral membrane proteins during exposure to inhibitors/activators considered to be potential drugs.

Properties of electrode-supported lipid cubic mesophase films with embedded gramicidin A: structure and ion-transport studies

The lipid cubic phase (LCP) is a nanomaterial composed of water channels surrounded by lipid bilayers. LCPs are stable at room temperature and are biocompatible. These features make the lipid cubic phases similar to biological membranes, and hence, are favorable for embedding membrane proteins. We show that the monoolein cubic phase deposited on the electrode forms a 3D lipid bilayer film convenient for electrochemical investigations of membrane proteins. In this research, we studied the effect of embedding an ionophoric peptide, gramicidin A (gA), on the structure and properties of the LCP film. The phase identity and structural parameters of the gramicidin-doped phase were characterized by small-angle X-ray scattering (SAXS). The potassium ion transport through the film were studied by electroanalytical methods: alternating current voltammetry (ACV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). Increased values for the current of the gramicidin-doped cubic phase compared to the empty cubic phase and changes of the EIS parameters confirmed that the peptide remained in the film in its active dimeric form. Our results show that the LCP can be considered a suitable 3D biomimetic film for the investigation of ion channels and other transporting membrane proteins, and for their application in electrochemical sensors.