Structure Of Biological Membranes Research Articles

Station for the investigation of the decay kinetics of the fluorescence anisotropy of biological objects

A station for the investigation of the fluorescence intensity and anisotropy decay of biological objects is described. The station has been installed on the synchrotron radiation (SR) source S-60 of P.N. Lebedev Physical Institute and can be extended to investigations of the spatial structure of lipoproteins and biological membranes.

Speculations on the origin of the genetic code.

The most primitive code is assumed to be a GC code: GG coding for glycine, CC coding for proline, GC coding for alanine, CG coding for "arginine." The genetic code is assumed to have originated with the coupling of glycine to its anticodon CC mediated by a copper-montmorillonite. The polymerization of polyproline followed when it was coupled to its anticodon GG. In this case the aminoacyl-tRNA synthetase was a copper-montmorillonite. The first membrane is considered to be a beta sheet formed from polyglycine. As the code grew more complicated, the alternative hydrophobic-hydrophilic polypeptide (alanine-"arginine") was coded for by the alternating CG copolymer. This alternating polypeptide (ala-"arg") began to function as both a primitive membrane and as an aminoacyl-tRNA synthetase. The evolution of protein structure is tightly coupled to the evolution of the membrane. The alpha helix was evolved as lipids became part of the structure of biological membranes. The membrane finally became the fluid mosaic structure that is now universal.

Effects of domain structure on in-plane reactions and interactions.

The existence of an in-plane domain structure in biological membranes raises the question of the physiological function, if any, of this structure. One important function may be to enhance or limit the equilibrium poise and rates of in-plane reactions through control by the cell of the percolation properties of the domain system. At low average domain occupancy by reactants or interactants, which must be the case for most biological membrane components, moving the domain system from connection to disconnection has marked effects on the apparent equilibrium poise and the rates of membrane-confined reactions. This conclusion is based on computer modelling of the effects of disconnection/connection of nine types of bimolecular in-plane reactions. Using the phase structure and percolation properties of two-component, two-phase phospholipid bilayers, it is possible to examine experimentally homo- and heterodimerization reactions, and enzyme-catalysed reactions in-plane as well as the effects of a transmembrane peptide on these systems. These theoretical and experimental studies suggest that percolation effects may be physiologically important in biological membranes. Whether this is in fact the case remains to be demonstrated.

Feeding S-adenosyl- l-methionine attenuates both ethanol-induced depletion of mitochondrial glutathione and mitochondrial dysfunction in periportal and perivenous rat hepatocytes

Mitochondrial glutathione plays an important role in maintaining a functionally competent organelle. Previous studies have shown that ethanol feeding selectively depletes the mitochondrial glutathione pool, more predominantly in mitochondria from perivenous hepatocytes. Because S-adenosyl- l-methionine (SAM) is a glutathione precursor and maintains the structure and function of biological membranes, the purpose of the present study was to determine the effects of SAM on glutathione and function of perivenous (PV) and peri-portal (PP) mitochondria from chronic ethanol-fed rats. SAM administration resulted in a significant increase in the basal cytosol and mitochondrial glutathione in both PP and PV cells from both pair-fed or ethanol-fed groups. When hepatocytes from ethanol-fed rats supplemented with SAM were incubated with methionine plus serine or N-acetylcysteine, mitochondrial glutathione increased in parallel with cytosol, an effect not observed in cells from ethanol-fed rats without SAM. Feeding equimolar N-acetylcysteine raised cytosol glutathione but did not prevent the mitochondrial glutathione defect. In addition, SAM feeding resulted in significant preservation of cellular adenosine triphosphate (ATP) levels (23% to 43%), mitochondrial membrane potential (17% to 25%), and the uncoupler control ratio (UCR) of respiration (from 5.1 ± 0.7 to 7.3 ± 0.6 and 2.1 ± 0.3 to 6.1 ± 0.7) for PP and PV mitochondria, respectively. Thus, these effects of SAM suggest that it may be a useful agent to preserve the disturbed mitochondrial integrity in liver disease caused by alcoholism through maintenance of mitochondrial glutathione transport.

Feeding S-adenosyl-L-methionine attenuates both ethanol-induced depletion of mitochondrial glutathione and mitochondrial dysfunction in periportal and perivenous rat hepatocytes.

Mitochondrial glutathione plays an important role in maintaining a functionally competent organelle. Previous studies have shown that ethanol feeding selectively depletes the mitochondrial glutathione pool, more predominantly in mitochondria from perivenous hepatocytes. Because S-adenosyl-L-methionine (SAM) is a glutathione precursor and maintains the structure and function of biological membranes, the purpose of the present study was to determine the effects of SAM on glutathione and function of perivenous (PV) and periportal (PP) mitochondria from chronic ethanol-fed rats. SAM administration resulted in a significant increase in the basal cytosol and mitochondrial glutathione in both PP and PV cells from both pair-fed or ethanol-fed groups. When hepatocytes from ethanol-fed rats supplemented with SAM were incubated with methionine plus serine or N-acetylcysteine, mitochondrial glutathione increased in parallel with cytosol, an effect not observed in cells from ethanol-fed rats without SAM. Feeding equimolar N-acetylcysteine raised cytosol glutathione but did not prevent the mitochondrial glutathione defect. In addition, SAM feeding resulted in significant preservation of cellular adenosine triphosphate (ATP) levels (23% to 43%), mitochondrial membrane potential (17% to 25%), and the uncoupler control ratio (UCR) of respiration (from 5.1 +/- 0.7 to 7.3 +/- 0.6 and 2.1 +/- 0.3 to 6.1 +/- 0.7) for PP and PV mitochondria, respectively. Thus, these effects of SAM suggest that it may be a useful agent to preserve the disturbed mitochondrial integrity in liver disease caused by alcoholism through maintenance of mitochondrial glutathione transport.

Structure, synthesis, and activity of dermaseptin b, a novel vertebrate defensive peptide from frog skin: relationship with adenoregulin.

A novel antimicrobial peptide, designated dermaseptin b, was isolated from the skin of the arboreal frog Phyllomedusa bicolor. This 27-residue peptide amide is basic, containing 3 lysine residues that punctuate an alternating hydrophobic and hydrophilic sequence. In helix-inducing solvent, dermaseptin b adopts an amphipathic alpha-helical conformation that most closely resembles class L amphipathic helixes, with all lysine residues on the polar face of the helix. The peptide exhibits growth inhibition activity in vitro against a broad spectrum of pathogenic microorganisms including yeast and bacteria as well as various filamentous fungi that are responsible for severe opportunistic infections accompanying acquired immunodeficiency syndrome and the use of immunosuppressive agents. Maximized pairwise sequence alignment of dermaseptin b and dermaseptin s, a 34-residue antimicrobial peptide previously isolated from Phyllomedusa sauvagii, reveals 81% amino acid identity. No other significant similarity was found between dermaseptin b and any prokaryotic or eukaryotic protein, but similarity was found with adenoregulin (38% amino acid postional identity), a 33-residue peptide that enhances binding of agonists to the A1 adenosine receptor. The synthetic replicates of dermaseptin b and adenoregulin displayed similar but nonidentical spectra of antimicrobial activity, and both peptides were devoid of lytic effect on mammalian cells. Accordingly, the observation that adenoregulin enhances binding of agonists to the adenosine receptor may in fact be a consequence of its ability to alter the structure of biological membranes and to produce signal transduction via interactions with the lipid bilayer, bypassing cell surface receptor interactions.

In Situ Modification of the Phospholipid Environment of Native Rabbit Sarcoplasmic Reticulum Membranes

A water soluble hydrogenation catalyst (palladium di(sodium alizarine monosulphonate)) in a deuterium-containing environment has been used for the in situ insertion of deuterium atoms into the fatty acyl chains of biological membranes. The thermotropic response of the stretching vibrations of the formed C-D bonds, as detected by Fourier transform IR spectroscopy, was used as a selective probe of biological membrane structure. Partial deuteration of unsaturated fatty acyl chains coupled with IR detection potentially provides a means for detecting specific biological roles of particular lipid classes. In the current study of sarcoplasmic reticulum membranes and purified phospholipid/CaATPase vesicles, it is also shown that νC-D monitors change at particular membrane locations which may remain undetected through the CH2 symmetric stretching frequency, a widely used IR spectral parameter. The latter reflects the average environment of the acyl chains. The approach described here may be suitable for wide applications to the study of biomembranes.

Polyhedral assembly of a membrane protein in its three-dimensional crystal

A novel ordered assemblage of bacteriorhodopsin, a transmembrane protein functioning as a light driven proton pump, is found in its three-dimensional crystal. Atomic force microscope images of the crystal surface reveal that spherical protein clusters with a diameter of ∼50 nm are hexagonally close-packed. Electron micrographs of mechanically disintegrated crystals show that the inside of the protein cluster is filled with the mother liquor. The crystal is made up of hollow protein clusters. When disintegrated crystals are illuminated in the presence of a lipophilic anion, a significant alkalization of the external medium occurs. This result indicates that the protein cluster contains native lipids and that the cytoplasmic side of the protein faces the external medium. X-ray diffraction patterns and the observed diameter of the spherical shell suggest that ∼200 bacteriorhodopsin trimers are aligned on a polyhedral surface lattice. Another remarkable feature of the spherical assemblies of bacteriorhodopsin is that they fuse with each other at low ionic strength and occasionally form a tubular or doughnut-like structure. The concept of membrane protein polymorphism is introduced on the basis of these observations, and it is used to describe the dynamic structure of some other biological membranes.

Electric Field-Induced Concentration Gradients in Lipid Monolayers

Externally applied electric field gradients gave rise to lateral concentration gradients in monolayers of certain binary lipid mixtures. For binary mixtures of dihydrocholesterol and dimyristoylphosphatidylcholine, the application of an electric field gradient at pressures below the critical pressure produced a liquid-liquid phase separation in a monolayer that is otherwise homogenous. At pressures slightly above the critical pressure, a field gradient produced a large concentration gradient without phase separation. The lipid concentration gradients can be described by equilibrium thermodynamic chemical potentials. The observed effects appear to be relevant to the structure and composition of biological membranes.

X-ray diffraction on biomembranes with emphasis on lipid moiety.

The theme of the fluid mosaic membrane by Singer and Nicholson (1972), in any colored variation, is known to every student of the biosciences. Yet, there exists no complete picture of the structure and dynamics of any real biological membrane, based on a sufficient set of experimentally determined atomic coordinates and trajectories. While the paradigm of static structure as a determinant of biological function has proven extremely fruitful for nucleic acids, and to a large extent also for proteins, this seems not to be the case with biological membranes. From all we know today, the static structures and mechanistic interactions are necessary to describe the protein moiety of membranes; this could be sufficiently defined in terms of three-dimensional atomic coordinates and their dislocations on conformational changes. However, a major species of membranes constituents, the lipids, appears to be physiologically in a state of dynamic disorder within certain limits given by the overall morphology—planar, tubular, or micellar—of the membrane. This is related to the situation in liquid crystals (De Gennes, 1974), and the physical description of membrane lipids has greatly benefited from the phenomenology applied to that field.KeywordsSynchrotron RadiationElectron Density ProfileLipid Phase TransitionLipid Model MembraneHydrocarbon Chain PackingThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Use of lectins as indicators for magnetic field action on erythrocyte membranes.

The effects of magnetic fields on biological membranes in general and on the red blood cell membrane in particular have been studied intensively in the last two decades. A variety of methods for evaluation of magnetic field action on the structure and function of biological membranes has been used. It has been suggested /1/ that the cell membrane can be considered as one of the primary targets affected by magnetic field exposure.

Effect of lnterdigitated Phase of DPPG Bilayer on (Ca 2+ +Mg 2+ )-ATPase From Sarcoplasmic Reticulum of Rabbit Muscle

Since 1935, cell. membranes or plasma membranes have been recognized to contain a continuously self-enclosed two-dimensional structure with a thickness equal to the dimension of two lipid molecules. With the foundation of the fluid mosaic model of membranes in 1972 and the research of lipid polymorphism, some basic structures of biological membranes have been clarified. Recently, two broad categories of bilayers have been recog-

Mechanical properties of vesicles. I. Coordinated analysis of osmotic swelling and lysis

To determine how transmembrane osmotic gradients perturb the structure and dynamics of biological membranes, we examined the effects of medium dilution on the structures of osmolyte-loaded lipid vesicles. Our preparations were characterized by dynamic light scattering (DLS) and nuclear magnetic resonance (NMR) spectroscopies. Populations of Escherichia coli phosphatidylethanolamine (PE) or dioleoylphosphatidylglycerol (DOPG) vesicles prepared by the pH jump technique were variable and polymodal in size distribution. Complex and variable structural changes occurred when PE vesicles were diluted with hypotonic buffer. Such vesicles could not be used as model systems for the analysis of membrane mechanical properties. NaCl-loaded, DOPG vesicles prepared by extrusion through 100 nm (diameter) pores were reproducible and monomodal in size distribution and unilamellar, whereas those prepared by extrusion through 200-, 400-, or 600-nm pores were variable and polymodal in size distribution and/or multilamellar. Time and pressure regimes associated with osmotic lysis of extruded vesicles were defined by monitoring release of carboxyfluorescein, a self-quenching fluorescent dye. Corresponding effects of medium dilution on vesicle structure were assessed by DLS spectroscopy. These experiments and the accompanying analysis (Hallett, F.R., J. Marsh, B.G. Nickel, and J.M. Wood. 1993. Biophys. J. 64:000-000) revealed conditions under which vesicles are expected to reside in a consistently strained state.

Mycoplasma membranes as models in membrane research.

The mycoplasmas, lacking a cell wall and intracytoplasmic membranes, have only one type of membrane, the plasma membrane. The ease with which this membrane can be isolated and the fact that controlled alterations in its composition are intraducibie have made mycoplasma membranes most effective and popular tools in biomembrane research. The aim of this chapter is to introduce the reader to the mycoplasmas and provide a historical background of the various phases in mycoplasma membrane research. Focus will be on the advantages of mycoplasma membranes as models in studying structure and function of biological membranes, and on the development of concepts that have arisen since the beginning of mycoplasma membrane research, about 30 years ago.

Biology and Materials? Part II

The October issue of the MRS Bulletin focused on two areas of research in biomolecular materials; biomineralization and protein fibers. In addition, the development of a new carbohydrate-based material with a variety of novel properties was discussed.In this issue, we turn to two other types of materials based on biological systems: (1) polymers and other materials produced through enzyme-catalyzed biosynthetic reactions and (2) protein and lipid complexes based on biological membrane structures. In each case, as in the October issue, a discussion of how Nature produces and uses these materials is followed by reports describing manipulations of these systems to enhance their properties for nonbiological applications.In the October issue, Kaplan and Cappello discussed in detail the synthesis of proteins. They described how living organisms expend a very large fraction of their energy producing these polymers to grow and repair damage to their bodies. (The other major fraction of their energy is dedicated to maintenance of brain function). Synthesis of carbohydrates, lipids, and other materials, however, is achieved through a somewhat more complex process. The organism outlines a pathway that consists of a sequence of reactions; each reaction converts a substrate-the product of the preceding reaction-to a product which is the substrate for the next reaction. The net result of the action of the pathway is the conversion of a nutrient to a useful material, often of far different chemical structure.

The resistivity and mobility functions for a model system of two equal-sized proteins in a lipid bilayer

We have calculated the mobility and resistivity functions for two equal-sized cylinders moving in a thin viscous sheet surrounded by a lower-viscosity fluid. These functions describe the hydrodynamic interactions between proteins embedded in a lipid bilayer surrounded by an aqueous solution. For a protein of radius a embedded in a biological membrane of thickness h and viscosity μ, a key parameter λ = μh/aμ′, which characterizes the viscosity ratio between the bilayer and surounding solution, is O(100). The method of solution for the hydrodynamic interactions differs depending on the separation distance, r, between the cylinders. When r = O(a), the particles are in the near-field regime, and the solution of the Stokes equations is divided between an inner and outer domain based on the asymptotically large value of λ. The inner solution neglects the flow in the lower-viscosity fluid and is solved numerically using a bipolar expansion. The outer solution is based on the fluid flows in all phases, but the cylinders are approximated by net forces. When r [Gt ] O(a), the particles are in the far-field regime, and we use the method of reflections to solve for the hydrodynamic interactions. The uniformly valid approximations, constructed from a combination of the near-field and far-field solutions, agree with the analytic solutions obtained within the lubrication and far-field regimes. Our results show that the hydrodynamic interactions between the cylinders are long range, perarting on a lengthscale of O(λ). The range of the hydrodynamic interactions is much longer than that of non-hydrodynamic interparticle forces, suggesting that hydrodynamic interactions will be significant determinants of the structure and dynamics of biological membranes.

Polyelectrolyte-sensitized phospholipid vesicles

Control of the structure and permeability of biological membranes is one of the most basic and least understood problems in contemporary biophysics. Diverse cellular processes require exquisite control of the membrane structure, including endocytosis and exocytosis, maintenance of the Golgi apparatus and the endoplasmic reticulum, and mitosis and meiosis, to mention just a few. Recently, significant progress has been made in controlling the permeability, and often the structures, of synthetic bilayer vesicles. These studies are especially important in that they help elucidate previously unimagined mechanisms for membrane control. Although no single model system is likely to utilize all of the rich capabilities exploited in nature, the synthetic systems can serve to identify concepts and mechanisms likely to be useful in a biological context. In addition, control of vesicular membrane systems may usher in novel chemistries for use in imaging, sensing, and therapeutic applications.

Molecules at interfaces: STM in materials and life sciences

The scanning tunneling microscope (STM) enables us, for the first time, to directly observe individual molecules both at surfaces under ultrahigh-vacuum conditions as well as at interfaces with fluids or soft solids. In suitable systems, structure and dynamics of single molecules or monomolecular layers can be investigated at a resolution down to the atomic length scale and the time scale of milliseconds. The structure of individual biopolymers and biological membranes on solid supports can be studied on the nanometer scale. Furthermore, the STM may be viewed as a tool to address individual molecules, e.g., for molecular electronics studies or the manipulation of individual molecular structures. The paper reviews selected STM results on organic conductors, as well as small organic molecules and large biopolymers, which have been chemi- or physisorbed to conducting substrates. Finally, some prospects for future work are discussed.

The study of lateral structure of biological and model membranes by neutron scattering

In this paper it has been shown that the difference in neutron amplitudes for isotopes (primarily hydrogen and deuterium) and the large lateral contrast (the difference in scattering amplitude density) of membrane components such as lipids, proteins, DNA, can be used for the study of membrane lateral organization. We have demonstrated the possibility to determine, with the help of elastic thermal neutron scattering, lateral pair correlation functions for membrane component packing and the possibility to study membrane inhomogeneities and aggregation processes. We have also given ratios for the dependences of neutron scattering intensities in the case of fractal distributions of membrane components.

Fröhlich and Davydov regimes in the dynamics of dipolar oscillations of biological membranes.

In this paper we present a model describing the dynamics of biological membranes. The description presented here is based both on Fr\"olich's earlier conjectures and on the actual biophysical data concerning the structure, composition, and functions of biological membranes. A model Hamiltonian is proposed that involves the oscillations of both the lipid head groups and hydrocarbon chains of the membrane. The presence of dielectrically active material, mainly water, is also accounted for. It is subsequently demonstrated that, depending on the form of the coupling constant between head- and tail-group oscillations, two limiting regimes may occur. The Fr\"ohlich regime is manifested by Bose condensation in the space of dipole oscillation frequencies. This results in self-focusing in the frequency domain. On the other hand, the Davydov regime is associated with spatial localization of the polarized state and leads to soliton formation. Thermal dissipation of the membrane's energy and damping effects are also examined.