Direct electrochemistry of spinach plastocyanin at a lipid bilayer-modified electrode: cyclic voltammetry as a probe of membrane-protein interactions

Electroactive hemoglobin-surfactant-polymer biomembrane-like films

Polyions complex 2C12N+ PSS- was prepared by reacting poly(sodium styrenesulfonate) (Na+ PSS-) with didodecyldimethylammonium bromide (2Cl2N+ Br-). Stable thin films made from 2C12N+ PSS- with incorporated redox protein hemoglobin (Hb) on pyrolytic graphite (PG) electrodes were then characterized by electrochemistry and other techniques. Cyclic voltammetry (CV) of Hb-2C12N+ PSS- films showed a pair of well-defined and nearly reversible peaks for HbFe(III)/Fe(II) couple at about -0.17 V vs. saturated calomel electrode (SCE) in pH 5.5 buffers. The electron transfer rate between Hb and PG electrode was greatly facilitated in microenvironment of 2C12N+ PSS- films. Positions of Soret absorption band suggest that Hb keeps its secondary structure similar to its native state in 2C12N+ PSS- films at the medium pH. The results of X-ray diffraction and differential scanning calorimetry (DSC) suggest synthesized lipid 2C12N+ PSS- films have an ordered bilayer structure intercalated between PSS- polyion layers, and the incorporated Hb expands the layer spacing of the films. HbFe(I), a highly reduced form of Hb, might also be produced in these films at about -1.09 V, and could be used to catalytically reduce organohalide pollutants.

Coupled plasmon-waveguide resonance spectroscopy studies of the cytochrome b6f/plastocyanin system in supported lipid bilayer membranes

The incorporation of cytochrome (cyt) b6f into a solid-supported planar egg phosphatidylcholine (PC) bilayer membrane and complex formation with plastocyanin have been studied by a variant of surface plasmon resonance called coupled plasmon-waveguide resonance (CPWR) spectroscopy, developed in our laboratory. CPWR combines greatly enhanced sensitivity and spectral resolution with direct measurement of anisotropies in refractive index and optical extinction coefficient, and can therefore probe structural properties of lipid-protein and protein-protein interactions. Cyt b6f incorporation into the membrane proceeds in two stages. The first occurs at low protein concentration and is characterized by an increase in total proteolipid mass without significant changes in the molecular order of the system, as demonstrated by shifts of the resonance position to larger incident angles without changing the refractive index anisotropy. The second stage, occurring at higher protein concentrations, results in a decrease in both the mass density and the molecular order of the system, evidenced by shifts of the resonance position to smaller incident angles and a large decrease in the membrane refractive index anisotropy. Plastocyanin can bind to such a proteolipid system in three different ways. First, the addition of plastocyanin before the second stage of b6f incorporation begins results in complex formation between the two proteins with a KD of approximately 10 microM and induces structural changes in the membrane that are similar to those occurring during the second stage of complex incorporation. The addition of larger amounts of plastocyanin under these conditions leads to nonspecific binding to the lipid phase with a KD of approximately 180 microM. Finally, the addition of plastocyanin after the completion of the second phase of b6f incorporation results in tighter binding between the two proteins (KD approximately 1 microM). Quantitation of the binding stoichiometry indicates that two plastocyanin molecules bind tightly to the dimeric form of the cyt b6f complex, assuming random insertion of the cytochrome into the bilayer. The structural basis for these results and formation of the proteolipid membrane are discussed.

Voltammetry of Plastocyanin at a Graphite Electrode: Effects of Structure, Charge, and Electrolyte

Comparative voltammetric studies on Anabaena variabilis plastocyanin (positively charged at neutral pH) and spinach and poplar plastocyanins (negatively charged at neutral pH) have been undertaken at an edge-plane graphite electrode as a function of ionic strength, pH, and Mg(2+) concentration at 3 degrees C. The aim was to provide a more detailed understanding of the influence of the electrode-protein (solution) interfacial characteristics, as well as the variation of the formal potential with both the nature of the plastocyanin species and the pH. As might be expected, some of the interfacial properties associated with the positive charge on A. variabilis plastocyanin are the opposite of those observed with the negatively charged plastocyanins. For example, the linear diffusion component of the mass transport process for A. variabilis plastocyanin under the conditions of cyclic voltammetry is decreased and the radial diffusion component is increased by the addition of Mg(2+), whereas the reverse occurs with poplar and spinach plastocyanins. The voltammetrically determined reversible potentials for A. variabilis plastocyanin are considerably less positive than those for spinach and poplar plastocyanins, in agreement with values calculated from chemically based redox studies. Ionic strength effects, as determined by addition of NaClO(4) over the concentration range 0.005-0.20 M, are negligible for all three proteins. The addition of Mg(2+) causes a significant shift in the reversible potential toward more positive values for spinach and poplar plastocyanin but only a small positive shift for A. variabilis plastocyanin. The difference is attributed to a specific binding effect. The addition of Mg(2+) also dramatically alters the pH dependence of the reversible potential, indicating that the equilibrium between the protonated and unprotonated forms of reduced plastocyanin is modified by binding of Mg(2+) to the protein. It is concluded that the effects of biologically relevant redox-inactive cations such as Mg(2+) or Ca(2+) have to be considered carefully in studies of the redox chemistry of metalloproteins.

Photobleaching of the photoactive yellow protein from Ectothiorhodospira halophila promotes binding to lipid bilayers: evidence from surface plasmon resonance spectroscopy

The photoactive yellow protein (PYP) from the phototrophic bacterium Ectothiorhodospira halophila is a small, soluble protein that undergoes reversible photobleaching upon blue light irradiation and may function to mediate the negative phototactic response. Based on previous studies of the effects of solvent viscosity and of aliphatic alcohols on PYP photokinetics, we proposed that photobleaching is concomitant with a protein conformational change that exposes a hydrophobic region on the protein surface. In the present investigation, we have used surface plasmon resonance (SPR) spectroscopy to characterize the binding of PYP to lipid bilayers deposited on a thin silver film. SPR spectra demonstrate that the net negatively charged PYP molecule can bind in a saturable manner to electrically neutral, net positively, and net negatively charged bilayers. Illumination with either blue or white light of a PYP solution, which is in contact with the bilayer, at concentrations below saturation results in an increase in the extent of binding, consistent with exposure of a high affinity hydrophobic surface in the photobleached state, a property that may contribute to its biological function. A value for the thickness of the bound PYP layer (23 A), obtained from theoretical fits to the SPR spectra, is consistent with the structure of the protein determined by x-ray crystallography and indicates that the molecule binds with its long axis parallel to the membrane surface.

Assembly and molecular organization of self-assembled lipid bilayers on solid substrates monitored by surface plasmon resonance spectroscopy

The structural properties of lipid films, made from a squalene/butanol solution containing varying amounts (0-15 mg/ml) of egg phosphatidylcholine and deposited on a thin metallic silver layer, were investigated using surface plasmon resonance (SPR) spectroscopy. Optical parameters (thickness, refractive index and extinction coefficient) of such supported self-assembled lipid membranes were obtained from a theoretical analysis of the experimental SPR curves. The mass of the lipid membrane and the area and volume occupied by one lipid molecule were also calculated. The results were consistent with the formation of durable and homogeneous lipid bilayers on the solid substrate, and indicated similarities in structural properties between the present lipid bilayers and freely suspended and Langmuir-Blodgett bilayer membranes. Such bilayers represent a simple model for biological membranes, as well as providing a means of immobilizing proteins for various practical applications, including receptor-based sensors and molecular devices. The results confirm the value of the SPR technique for investigating the properties of thin biomolecular dielectric films deposited on a metal surface.

Interaction of phospholipids with proteins and peptides. New advances IV

1. The review deals with the newest achievements in the field of the various interactions between phospholipids and proteins and peptides. 2. Interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecular assemblies is also discussed.

Direct measurement of cyclic current-voltage responses of integral membrane proteins at a self-assembled lipid-bilayer-modified electrode: cytochrome f and cytochrome c oxidase

Direct cyclic voltage-current responses, produced in the absence of redox mediators, for two detergent-solubilized integral membrane proteins, spinach cytochrome f and beef heart cytochrome c oxidase, have been obtained at an optically transparent indium oxide electrode modified with a self-assembled lipid-bilayer membrane. The results indicate that both proteins interact with the lipid membrane so as to support quasi-reversible electron transfer redox reactions at the semiconductor electrode. The redox potentials that were obtained from analysis of the cyclic "voltammograms," 365 mV for cytochrome f and 250 and 380 mV for cytochrome c oxidase (vs. normal hydrogen electrode), compare quite well with the values reported by using conventional titration methods. The ability to obtain direct electrochemical measurements opens up another approach to the investigation of the properties of integral membrane redox proteins.

Direct electrochemistry of thioredoxins and glutathione at a lipid bilayer-modified electrode

By using direct electrochemical analysis we have established that the reduction of Escherichia coli thioredoxin (EcT), T4 thioredoxin (T4T), and glutathione (GSSG) occurs at a self-assembled lipid bilayer-modified gold electrode via two separate one-electron processes. The first electron transfer has half-wave potentials of -0.05 +/- 0.01, -0.07 +/- 0.01, and -0.06 +/- 0.01 V, whereas the second one has values of -0.48 +/- 0.01, -0.39 +/- 0.01, and -0.45 +/- 0.01 V, for EcT, T4T, and GSSG, respectively. The scan-rate dependence of the cyclic voltammetry indicates, for both waves, that the process of electron transfer is dominated by a bulk diffusion of free species to and from the electrode, and that strongly adsorbed species do not significantly contribute at the scan rates used. The voltage separation of the peak currents indicates a quasi-reversible electron transfer process with an electrochemical rate constant which is larger for the second (lower potential) electron than for the first one. Using the above half-wave potentials of the one-electron steps, one can calculate a thermodynamic half-wave potential for the two-electron reduction processes. The values of these potentials are -0.265, -0.23, and -0.25 V for EcT, T4T, and GSSG, respectively. These are in excellent agreement with literature values obtained from equilibrium measurements of enzyme-catalyzed reactions involving these species. It is quite clear from these results that lipid bilayer-modified electrodes provide a biocompatible and direct means of efficiently carrying out electrochemical reactions with sulfur-based redox systems, as we have previously shown to be the case with metalloproteins.