Effect of vesicle composition and curvature on the dissociation of phosphatidic acid in small unilamellar vesicles--a 31P-NMR study

Phospholipids in Motion: High-Resolution 31P NMR Field Cycling Studies

Diverse phospholipid motions are key to membrane function but can be quite difficult to untangle and quantify. High-resolution field cycling ³¹P NMR spin-lattice relaxometry, where the sample is excited at high field, shuttled in the magnet bore for low-field relaxation, then shuttled back to high field for readout of the residual magnetization, provides data on phospholipid dynamics and structure. This information is encoded in the field dependence of the ³¹P spin-lattice relaxation rate (R₁). In the field range from 11.74 down to 0.003 T, three dipolar nuclear magnetic relaxation dispersions (NMRDs) and one due to ³¹P chemical shift anisotropy contribute to R₁ of phospholipids. Extraction of correlation times and maximum relaxation amplitudes for these NMRDs provides (1) lateral diffusion constants for different phospholipids in the same bilayer, (2) estimates of how additives alter the motion of the phospholipid about its long axis, and (3) an average ³¹P-¹H angle with respect to the bilayer normal, which reveals that polar headgroup motion is not restricted on a microsecond timescale. Relative motions within a phospholipid are also provided by comparing ³¹P NMRD profiles for specifically deuterated molecules as well as ¹³C and ¹H field dependence profiles to that of ³¹P. Although this work has dealt exclusively with phospholipids in small unilamellar vesicles, these same NMRDs can be measured for phospholipids in micelles and nanodisks, making this technique useful for monitoring lipid behavior in a variety of structures and assessing how additives alter specific lipid motions.

Cholesterol moieties as building blocks for assembling nanoparticles to achieve effective oral delivery of insulin

The amphiphilic cholesterol-phosphate conjugate can fabricate into cholesterol-coated nanoparticles by reverse emulsion method. The nanoparticles generated a rapid-onset and long-lasting hypoglycemic effect following gavage in T1DM rats.

Complex Behavior of Phosphatidylcholine-Phosphatidic Acid Bilayers and Monolayers: Effect of Acyl Chain Unsaturation

Phosphatidic acids (PAs) have many biological functions in biomembranes, e.g., they are involved in the proliferation, differentiation, and transformation of cells. Despite decades of research, the molecular understanding of how PAs affect the properties of biomembranes remains elusive. In this study, we explored the properties of lipid bilayers and monolayers composed of PAs and phosphatidylcholines (PCs) with various acyl chains. For this purpose, the Langmuir monolayer technique and atomistic molecular dynamics (MD) simulations were used to study the miscibility of PA and PC lipids and the molecular organization of mixed bilayers. The monolayer experiments demonstrated that the miscibility of membrane components strongly depends on the structure of the hydrocarbon chains and thus on the overall lipid shape. Interactions between PA and PC molecules vary from repulsive, for systems containing lipids with saturated and unsaturated acyl tails (strongly positive values of the excess free energy of mixing), to attractive, for systems in which all lipid tails are saturated (negative values of the excess free energy of mixing). The MD simulations provided atomistic insight into polar interactions (formation of hydrogen bonds and charge pairs) in PC-PA systems. H-bonding between PA monoanions and PCs in mixed bilayers is infrequent, and the lipid molecules interact mainly via electrostatic interactions. However, the number of charge pairs significantly decreases with the number of unsaturated lipid chains in the PA-PC system. The PA dianions weakly interact with the zwitterionic lipids, but their headgroups are more hydrated as compared to the monoanionic form. The acyl chains in all PC-PA bilayers are more ordered compared to single-component PC systems. In addition, depending on the combination of lipids, we observed a deeper location of the PA phosphate groups compared to the PC phosphate groups, which can alter the presentation of PAs for the peripheral membrane proteins, affecting their accessibility for binding.

Divide & Conquer: Surfactant Protein SP-C and Cholesterol Modulate Phase Segregation in Lung Surfactant

Lung surfactant (LS) is an essential system supporting the respiratory function. Cholesterol can be deleterious for LS function, a condition that is reversed by the presence of the lipopeptide SP-C. In this work, the structure of LS-mimicking membranes has been analyzed under the combined effect of SP-C and cholesterol by deuterium NMR and phosphorus NMR and by electron spin resonance. Our results show that SP-C induces phase segregation at 37°C, resulting in an ordered phase with spectral features resembling an interdigitated state enriched in dipalmitoylphosphatidylcholine, a liquid-crystalline bilayer phase, and an extremely mobile phase consistent with small vesicles or micelles. In the presence of cholesterol, POPC and POPG motion seem to be more hindered by SP-C than dipalmitoylphosphatidylcholine. The use of deuterated cholesterol did not show signs of specific interactions that could be attributed to SP-C or to the other hydrophobic surfactant protein SP-B. Palmitoylation of SP-C had an indirect effect on the extent of protein-lipid perturbations by stabilizing SP-C structure, and seemed to be important to maximize differences among the lipids participating in each phase. These results shed some light on how SP-C-induced lipid perturbations can alter membrane structure to sustain LS functionality at the air-liquid interface.

Monitoring phosphatidic acid formation in intact phosphatidylcholine bilayers upon phospholipase D catalysis

We have monitored the production of the negatively charged lipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid acid (POPA), in supported lipid bilayers via the enzymatic hydrolysis of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC), a zwitterionic lipid. Experiments were performed with phospholipase D (PLD) in a Ca(2+) dependent fashion. The strategy for doing this involved using membrane-bound streptavidin as a biomarker for the charge on the membrane. The focusing position of streptavidin in electrophoretic-electroosmotic focusing (EEF) experiments was monitored via a fluorescent tag on this protein. The negative charge increased during these experiments due to the formation of POPA lipids. This caused the focusing position of streptavidin to migrate toward the negatively charged electrode. With the use of a calibration curve, the amount of POPA generated during this assay could be read out from the intact membrane, an objective that has been otherwise difficult to achieve because of the lack of unique chromophores on PA lipids. On the basis of these results, other enzymatic reactions involving the change in membrane charge could also be monitored in a similar way. This would include phosphorylation, dephosphorylation, lipid biosynthesis, and additional phospholipase reactions.

Phase changes in mixed lipid/polymer membranes by multivalent nanoparticle recognition

Selective addressing of membrane components in complex membrane mixtures is important for many biological processes. The present paper investigates the recognition between multivalent surface functionalized nanoparticles (NPs) and amphiphilic block copolymers (BCPs), which are successfully incorporated into lipid membranes. The concept involves the supramolecular recognition between hybrid membranes (composed of a mixture of a lipid (DPPC or DOPC), an amphiphilic triazine-functionalized block copolymer TRI-PEO13-b-PIB83 (BCP 2), and nonfunctionalized BCPs (PEO17-b-PIB87 BCP 1)) with multivalent (water-soluble) nanoparticles able to recognize the triazine end group of the BCP 2 at the membrane surface via supramolecular hydrogen bonds. CdSe-NPs bearing long PEO47-thymine (THY) polymer chains on their surface specifically interacted with the 2,4-diaminotriazine (TRI) moiety of BCP 2 embedded within hybrid lipid/BCP mono- or bilayers. Experiments with GUVs from a mixture of DPPC/BCP 2 confirm selective supramolecular recognition between the THY-functionalized NPs and the TRI-functionalized polymers, finally resulting in the selective removal of BCP 2 from the hybrid vesicle membrane as proven via facetation of the originally round and smooth vesicles. GUVs (composed of DOPC/BCP 2) show that a selective removal of the polymer component from the fluid hybrid membrane results in destruction of hybrid vesicles via membrane rupture. Adsorption experiments with mixed monolayers from lipids with either BCP 2 or BCP 1 (nonfunctionalized) reveal that the THY-functionalized NPs specifically recognize BCP 2 at the air/water interface by inducing significantly higher changes in the surface pressure when compared to monolayers from nonspecifically interacting lipid/BCP 1 mixtures. Thus, recognition of multivalent NPs with specific membrane components of hybrid lipid/BCP mono- and bilayers proves the selective removal of BCPs from mixed membranes, in turn inducing membrane rupture. Such recognition events display high potential in controlling permeability and fluidity of membranes (e.g., in pharmaceutics).

Ionization behavior of polyphosphoinositides determined via the preparation of pH titration curves using solid-state 31P NMR

Detailed knowledge of the degree of ionization of lipid titratable groups is important for the evaluation of protein-lipid and lipid-lipid interactions. The degree of ionization is commonly evaluated by acid-base titration, but for lipids localized in a multicomponent membrane interface this is not a suitable technique. For phosphomonoester-containing lipids such as the polyphosphoinositides, phosphatidic acid, and ceramide-1-phosphate, this is more conveniently accomplished by (31)P NMR. Here, we describe a solid-state (31)P NMR procedure to construct pH titration curves to determine the degree of ionization of phosphomonoester groups in polyphosphoinositides. This procedure can also be used, with suitable sample preparation conditions, for other important signaling lipids. Access to a solid-state, i.e., magic angle spinning, capable NMR spectrometer is assumed. The procedures described here are valid for a Bruker instrument, but can be adapted for other spectrometers as needed.

Molecular recognition: from solution science to nano/materials technology

In the 25 years since its Nobel Prize in chemistry, supramolecular chemistry based on molecular recognition has been paid much attention in scientific and technological fields. Nanotechnology and the related areas seek breakthrough methods of nanofabrication based on rational organization through assembly of constituent molecules. Advanced biochemistry, medical applications, and environmental and energy technologies also depend on the importance of specific interactions between molecules. In those current fields, molecular recognition is now being re-evaluated. In this review, we re-examine current trends in molecular recognition from the viewpoint of the surrounding media, that is (i) the solution phase for development of basic science and molecular design advances; (ii) at nano/materials interfaces for emerging technologies and applications. The first section of this review includes molecular recognition frontiers, receptor design based on combinatorial approaches, organic capsule receptors, metallo-capsule receptors, helical receptors, dendrimer receptors, and the future design of receptor architectures. The following section summarizes topics related to molecular recognition at interfaces including fundamentals of molecular recognition, sensing and detection, structure formation, molecular machines, molecular recognition involving polymers and related materials, and molecular recognition processes in nanostructured materials.

The physical chemistry of the enigmatic phospholipid diacylglycerol pyrophosphate

Phosphatidic acid (PA) is a lipid second messenger that is formed transiently in plants in response to different stress conditions, and plays a role in recruiting protein targets, ultimately enabling an adequate response. Intriguingly, this increase in PA concentration in plants is generally followed by an increase in the phospholipid diacylglycerolpyrophosphate (DGPP), via turnover of PA. Although DGPP has been shown to induce stress-related responses in plants, it is unclear to date what its molecular function is and how it exerts its effect. Here, we describe the physicochemical properties, i.e., effective molecular shape and charge, of DGPP. We find that unlike PA, which imparts a negative curvature stress to a (phospho)lipid bilayer, DGPP stabilizes the bilayer phase of phosphatidylethanolamine (PE), similar to the effect of phosphatidylcholine (PC). DGPP thus has zero curvature. The pKa(2) of the phosphomonoester of DGPP is 7.44 ± 0.02 in a PC bilayer, compared to a pKa(2) of 7.9 for PA. Replacement of half of the PC with PE decreases the pKa(2) of DGPP to 6.71 ± 0.02, similar to the behavior previously described for PA and summarized in the electrostatic-hydrogen bond switch model. Implications for the potential function of DGPP in biomembranes are discussed.

Flip-flop-induced relaxation of bending energy: implications for membrane remodeling

Cellular and organellar membranes are dynamic materials that underlie many aspects of cell biology. Biological membranes have long been thought of as elastic materials with respect to bending deformations. A wealth of theory and experimentation on pure phospholipid membranes provides abundant support for this idea. However, biological membranes are not composed solely of phospholipids--they also incorporate a variety of amphiphilic molecules that undergo rapid transbilayer flip-flop. Here we describe several experimental systems that demonstrate deformation-induced molecular flip-flop. First we use a fluorescence assay to track osmotically controlled membrane deformation in single component fatty acid vesicles, and show that the relaxation of the induced bending stress is mediated by fatty acid flip-flop. We then look at two-component phospholipid/cholesterol composite vesicles. We use NMR to show that the steady-state rate of interleaflet diffusion of cholesterol is fast relative to biological membrane remodeling. We then use a Förster resonance energy transfer assay to detect the transbilayer movement of cholesterol upon deformation. We suggest that our results can be interpreted by modifying the area difference elasticity model to account for the time-dependent relaxation of bending energy. Our findings suggest that rapid interleaflet diffusion of cholesterol may play a role in membrane remodeling in vivo. We suggest that the molecular characteristics of sterols make them evolutionarily preferred mediators of stress relaxation, and that the universal presence of sterols in the membranes of eukaryotes, even at low concentrations, reflects the importance of membrane remodeling in eukaryotic cells.

Phospholipid reorientation at the lipid/water interface measured by high resolution 31P field cycling NMR spectroscopy

The magnetic field dependence of the 31P spin-lattice relaxation rate, R1, of phospholipids can be used to differentiate motions for these molecules in a variety of unilamellar vesicles. In particular, internal motion with a 5- to 10-ns correlation time has been attributed to diffusion-in-a-cone of the phosphodiester region, analogous to motion of a cylinder in a liquid hydrocarbon. We use the temperature dependence of 31P R1 at low field (0.03-0.08 T), which reflects this correlation time, to explore the energy barriers associated with this motion. Most phospholipids exhibit a similar energy barrier of 13.2 +/- 1.9 kJ/mol at temperatures above that associated with their gel-to-liquid-crystalline transition (Tm); at temperatures below Tm, this barrier increases dramatically to 68.5 +/- 7.3 kJ/mol. This temperature dependence is broadly interpreted as arising from diffusive motion of the lipid axis in a spatially rough potential energy landscape. The inclusion of cholesterol in these vesicles has only moderate effects for phospholipids at temperatures above their Tm, but significantly reduces the energy barrier (to 17 +/- 4 kJ/mol) at temperatures below the Tm of the pure lipid. Very-low-field R1 data indicate that cholesterol inclusion alters the averaged disposition of the phosphorus-to-glycerol-proton vector (both its average length and its average angle with respect to the membrane normal) that determines the 31P relaxation.

Thioesterase superfamily member 2 (Them2)/acyl-CoA thioesterase 13 (Acot13): a homotetrameric hotdog fold thioesterase with selectivity for long-chain fatty acyl-CoAs

Them2 (thioesterase superfamily member 2) is a 140-amino-acid protein of unknown biological function that comprises a single hotdog fold thioesterase domain. On the basis of its putative association with mitochondria, accentuated expression in oxidative tissues and interaction with StarD2 (also known as phosphatidylcholine-transfer protein, PC-TP), a regulator of fatty acid metabolism, we explored whether Them2 functions as a physiologically relevant fatty acyl-CoA thioesterase. In solution, Them2 formed a stable homotetramer, which denatured in a single transition at 59.3 degrees C. Them2 exhibited thioesterase activity for medium- and long-chain acyl-CoAs, with Km values that decreased exponentially as a function of increasing acyl chain length. Steady-state kinetic parameters for Them2 were characteristic of long-chain mammalian acyl-CoA thioesterases, with minimal values of Km and maximal values of kcat/Km observed for myristoyl-CoA and palmitoyl-CoA. For these acyl-CoAs, substrate inhibition was observed when concentrations approached their critical micellar concentrations. The acyl-CoA thioesterase activity of Them2 was optimized at physiological temperature, ionic strength and pH. For both myristoyl-CoA and palmitoyl-CoA, the addition of StarD2 increased the kcat of Them2. Enzymatic activity was decreased by the addition of phosphatidic acid/phosphatidylcholine small unilamellar vesicles. Them2 expression, which was most pronounced in mouse heart, was associated with mitochondria and was induced by activation of PPARalpha (peroxisome-proliferator-activated receptor alpha). We conclude that, under biological conditions, Them2 probably functions as a homotetrameric long-chain acyl-CoA thioesterase. Accordingly, Them2 has been designated as the 13th member of the mammalian acyl-CoA thioesterase family, Acot13.

Magic-angle phosphorus NMR of functional mitochondria: in situ monitoring of lipid response under apoptotic-like stress

Using a noninvasive, solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) approach, we track ex vivo the behavior of individual membrane components in isolated, active mitochondria (model system: potato tubers) during physiological processes. The individual phosphatidylcholine (PC), phosphatidylethanolamine (PE), and cardiolipin (CL) membrane constituents were identified as distinct lines by applying MAS (31)P NMR on extracted lipid membranes. However, the CL NMR signal appeared to be very broad in functional mitochondria, indicating a tight complex formation with membrane protein. Calcium stress induced severe membrane degradation without recovery of a single CL NMR resonance. This suggests that calcium overload destroys the outer mitochondrial membrane and does not modify strongly the CL protein complexes in the inner membrane; a conclusion confirmed by respiratory controls. Conversely, mitochondrial membrane disruption on time degradation or mechanical stress generates clearly visible identical CL NMR signals, similar to those observed in rehydrated lipid extracts. Similarly, noninvasive based NMR tracking of lipids in response to diverse physiological stimuli can easily be used for other organelles and whole living cells.

Biophysics and function of phosphatidic acid: a molecular perspective

Phosphatidic acid is the simplest (diacyl)glycerophospholipid present in cells and is now a well established second messenger with direct biological functions. It is specifically recognized by diverse proteins and plays an important role in cellular signaling and membrane dynamics in all eukaryotes. An important determinant of the biological functions of phosphatidic acid is its anionic headgroup. In this review we will focus on the peculiar ionization properties of phosphatidic acid and their crucial role in lipid-protein interactions. We will take a molecular approach focusing entirely on the physical chemistry of the lipid and develop a model explaining the ionization properties of phosphatidic acid, termed the electrostatic-hydrogen bond switch model. Diverse examples from recent literature in support of this model will be presented and the broader implications of our findings will be discussed.

Correlation of vesicle binding and phospholipid dynamics with phospholipase C activity: insights into phosphatidylcholine activation and surface dilution inhibition

The enzymatic activity of the peripheral membrane protein, phosphatidylinositol-specific phospholipase C (PI-PLC), is increased by nonsubstrate phospholipids with the extent of enhancement tuned by the membrane lipid composition. For Bacillus thuringiensis PI-PLC, a small amount of phosphatidylcholine (PC) activates the enzyme toward its substrate PI; above 0.5 mol fraction PC (XPC), enzyme activity decreases substantially. To provide a molecular basis for this PC-dependent behavior, we used fluorescence correlation spectroscopy to explore enzyme binding to multicomponent lipid vesicles composed of PC and anionic phospholipids (that bind to the active site as substrate analogues) and high resolution field cycling 31P NMR methods to estimate internal correlation times (tauc) of phospholipid headgroup motions. PI-PLC binds poorly to pure anionic phospholipid vesicles, but 0.1 XPC significantly enhances binding, increases PI-PLC activity, and slows nanosecond rotational/wobbling motions of both phospholipid headgroups, as indicated by increased tauc. PI-PLC activity and phospholipid tauc are constant between 0.1 and 0.5 XPC. Above this PC content, PI-PLC has little additional effect on the substrate analogue but further slows the PC tauc, a motional change that correlates with the onset of reduced enzyme activity. For PC-rich bilayers, these changes, together with the reduced order parameter and enhanced lateral diffusion of the substrate analogue in the presence of PI-PLC, imply that at high XPC, kinetic inhibition of PI-PLC results from intravesicle sequestration of the enzyme from the bulk of the substrate. Both methodologies provide a detailed view of protein-lipid interactions and can be readily adapted for other peripheral membrane proteins.

Membrane organization and ionization behavior of the minor but crucial lipid ceramide-1-phosphate

Ceramide-1-phosphate (Cer-1-P), one of the simplest of all sphingophospholipids, occurs in minor amounts in biological membranes. Yet recent evidence suggests important roles of this lipid as a novel second messenger with crucial tasks in cell survival and inflammatory responses. We present a detailed description of the physical chemistry of this hitherto little explored membrane lipid. At full hydration Cer-1-P forms a highly organized subgel (crystalline) bilayer phase (L(c)) at low temperature, which transforms into a regular gel phase (L(beta)) at approximately 45 degrees C, with the gel to fluid phase transition (L(beta)-L(alpha)) occurring at approximately 65 degrees C. When incorporated at 5 mol % in a phosphatidylcholine bilayer, the pK(a2) of Cer-1-P, 7.39 +/- 0.03, lies within the physiological pH range. Inclusion of phosphatidylethanolamine in the phosphatidylcholine bilayer, at equimolar ratio, dramatically reduces the pK(a2) to 6.64 +/- 0.03. We explain these results in light of the novel electrostatic/hydrogen bond switch model described recently for phosphatidic acid. In mixtures with dielaidoylphosphatidylethanolamine, small concentrations of Cer-1-P cause a large reduction of the lamellar-to-inverted hexagonal phase transition temperature, suggesting that Cer-1-P induces, like phosphatidic acid, negative membrane curvature in these types of lipid mixtures. These properties place Cer-1-P in a class more akin to certain glycerophospholipids (phosphatidylethanolamine, phosphatidic acid) than to any other sphingolipid. In particular, the similarities and differences between ceramide and Cer-1-P may be relevant in explaining some of their physiological roles.

Diazeniumdiolate reactivity in model membrane systems

The effect of small unilamellar phospholipid vesicles on the acid-catalyzed dissociation of nitric oxide from diazeniumdiolate ions, R(1)R(2)N[N(O)NO](-), [1: R(1)=H(2)N(CH(2))(3)-, R(2)=H(2)N(CH(2))(3)NH(CH(2))(4)-; 2: R(1)=R(2)=H(2)N(CH(2))(3)-; 3: R(1)=n-butyl-, R(2)=n-butyl-NH2+(CH(2))(6)-; 4: R(1)=R(2)=nPr-] has been examined at pH 7.4 and 37 degrees C. NO release was catalyzed by anionic liposomes (DPPG, DOPG, DMPS, POPS and DOPA) and by mixed phosphatidylglycerol/phosphatidylcholine (DPPG/DPPC and DOPG/DPPC) covesicles, while cationic liposomes derived from 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the zwitterionic liposome DMPC did not significantly affect the dissociation rates of the substrates examined. Enhancement of the dissociation rate constant in DPPG liposome media (0.010M phosphate buffer, pH 7.4, 37 degrees C) at 10mM phosphoglycerol levels, ranged from 37 for 1 to 1.2 for the anionic diazeniumdiolate 4, while DOPA effected the greatest rate enhancement, achieving 49-fold rate increases with 1 under similar conditions. The observed catalysis decreases with increase in the bulk concentration of electrolytes in the reaction media. Quantitative analysis of catalytic effects has been obtained through the application of pseudo-phase kinetic models and equilibrium binding constants at different liposome interfaces are compared. The stoichiometry of nitric oxide release from 1 and 2 in DPPG/DPPC liposome media has been obtained through oxyhemoglobin assay. DPPG=1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DOPG=1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DMPS=1,2-dimyristoyl-sn-glycero-3-[phospho-l-serine], POPS=1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine], DOPA=1,2-dioleoyl-sn-glycero-3-phosphate; DPPC=1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DMPC=1,2-dimyristoyl-sn-glycero-3-phosphocholine, DOTAP=1,2-dioleoyl-3-trimethylammonium-propane.

An improved NMR study of liposomes using 1-palmitoyl-2-oleoyl-sn-glycero-3-phospatidylcholine as model

In this paper we report a comparative characterization of Small Unilamellar Vesicles (SUVs), Large Unilamellar Vesicles (LUVs) and Multilamellar Vesicles (MLVs) prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phospatidylcholine (POPC), carried out using two NMR techniques, namely High Resolution NMR in solution and High Resolution-Magic Angle Spinning (HR-MAS). The size and size distributions of these vesicles were investigated using the dynamic light scattering technique. An improved assignment of the (1)H-NMR spectrum of MLVs is also reported.

Conformational states and thermodynamics of alpha-lactalbumin bound to membranes: a case study of the effects of pH, calcium, lipid membrane curvature and charge

The study of the conformational changes of bovine alpha-lactalbumin, switching from soluble states to membrane-bound states, deepens our knowledge of the behaviour of amphitropic proteins. The binding and the membrane-bound conformations of alpha-lactalbumin are highly sensitive to environmental factors, like calcium and proton concentrations, curvature and charge of the lipid membrane. The interactions between the protein and the membrane result from a combination of hydrophobic and electrostatic interactions and the respective weights of these interactions depend on the physicochemical conditions. As inferred by macroscopic as well as residue-level methods, the conformations of the membrane-bound protein range from native-like to molten globule-like states. However, the regions anchoring the protein to the membrane are similar and restricted to amphiphilic alpha-helices. H/(2)H-exchange experiments also yield residue-level data that constitute comprehensive information providing a new point of view on the thermodynamics of the interactions between the protein and the membrane.

Monolayer studies of single-chain polyprenyl phosphates

The monolayer properties of some single-chain polyprenyl phosphates (phytanyl, phytyl, and geranylgeranyl phosphates), which we regard as hypothetical primitive membrane lipids, were investigated at the air-water interface by surface pressure-area (pi-A) isotherm measurements. The molecular area/ pressure at various pH conditions dependence revealed the acid dissociation constants (pKa values) of the phosphate. The pKa values thus obtained at the air-water interface (pKa1 = 7.1 and pKa2 = 9.4 for phytanyl phosphate) were significantly shifted to higher pH than those observed in the bilayer state in water (pKa1 = 2.9 and pKa2 = 7.8). The difference in pKa values leads to a stability of the phosphate as both monolayer and bilayer states in a pH range of 2-6. In addition, the presence of ions such as sodium, magnesium, calcium, and lanthanum in the subphase significantly altered the stability of the polyprenyl phosphate monolayers, as shown by the determination of monolayer collapse and compression/expansion hysteresis. Although sodium ions in the subphase showed only a weak effect on the stabilization of the monolayer, addition of magnesium ions or of a small amount of calcium ions significantly suppressed the dissolution of the monolayer into the subphase and increased its mechanical stability against collapse. In contrast, the presence of larger amounts of calcium or of lanthanum ions induced collapse of the monolayers. Based on these experimental facts, a plausible scenario for the formation of primitive cell membrane by transformation of a monolayer to vesicle structures is proposed.

Phosphohydrolase and transphosphatidylation reactions of two Streptomyces phospholipase D enzymes: covalent versus noncovalent catalysis

A kinetic comparison of the hydrolase and transferase activities of two bacterial phospholipase D (PLD) enzymes with little sequence homology provides insights into mechanistic differences and also the more general role of Ca(2+) in modulating PLD reactions. Although the two PLDs exhibit similar substrate specificity (phosphatidylcholine preferred), sensitivity to substrate aggregation or Ca(2+), and pH optima are quite distinct. Streptomyces sp. PMF PLD, a member of the PLD superfamily, generates both hydrolase and transferase products in parallel, consistent with a mechanism that proceeds through a covalent phosphatidylhistidyl intermediate where the rate-limiting step is formation of the covalent intermediate. For Streptomyces chromofuscus PLD, the two reactions exhibit different pH profiles, a result consistent with a mechanism likely to involve direct attack of water or an alcohol on the phosphorus. Ca(2+), not required for monomer or micelle hydrolysis, can activate both PLDs for hydrolysis of PC unilamellar vesicles. In the case of Streptomyces sp. PMF PLD, Ca(2+) relieves product inhibition by interactions with the phosphatidic acid (PA). A similar rate enhancement could occur with other HxKx(4)D-motif PLDs as well. For S. chromofuscus PLD, Ca(2+) is absolutely critical for binding of the enzyme to PC vesicles and for PA activation. That the Ca(2+)-PA activation involves a discreet site on the protein is suggested by the observation that the identity of the C-terminal residue in S. chromofuscus PLD can modulate the extent of product activation.

Spectroscopic evidence for a preferential location of lidocaine inside phospholipid bilayers

We examined the effect of uncharged lidocaine on the structure and dynamics of egg phosphatidylcholine (EPC) membranes at pH 10.5 in order to assess the location of this local anesthetic in the bilayer. Changes in the organization of small unilamellar vesicles were monitored either by electron paramagnetic resonance (EPR)-in the spectra of doxyl derivatives of stearic acid methyl esters labeled at different positions in the acyl chain (5-, 7-, 12- and 16-MeSL)-or by fluorescence, with pyrene fatty-acid (4-, 6-, 10- and 16-Py) probes. The largest effects were observed with labels located at the upper positions of the fatty-acid acyl-chain. Dynamic information was obtained by 1H-NMR. Lidocaine protons presented shorter longitudinal relaxation times (T(1)) values due to their binding, and consequent immobilization to the membrane. In the presence of lidocaine the mobility of all glycerol protons of EPC decreased, while the choline protons revealed a higher degree of mobility, indicating a reduced participation in lipid-lipid interactions. Two-dimensional Nuclear Overhauser Effect experiments detected contacts between aromatic lidocaine protons and the phospholipid-choline methyl group. Fourier-transform infrared spectroscopy spectra revealed that lidocaine changes the access of water to the glycerol region of the bilayer. A "transient site" model for lidocaine preferential location in EPC bilayers is proposed. The model is based on the consideration that insertion of the bulky aromatic ring of the anesthetic into the glycerol backbone region causes a decrease in the mobility of that EPC region (T(1) data) and an increased mobility of the acyl chains (EPR and fluorescence data).

Asymmetrical membranes and surface tension

The (31)P-nuclear magnetic resonance chemical shift of phosphatidic acid in a membrane is sensitive to the lipid head group packing and can report qualitatively on membrane lateral compression near the aqueous interface. We have used high-resolution (31)P-nuclear magnetic resonance to evaluate the lateral compression on each side of asymmetrical lipid vesicles. When monooleoylphosphatidylcholine was added to the external monolayer of sonicated vesicles containing dioleoylphosphatidylcholine and dioleoylphosphatidic acid, the variation of (31)P chemical shift of phosphatidic acid indicated a lateral compression in the external monolayer. Simultaneously, a slight dilation was observed in the inner monolayer. In large unilamellar vesicles on the other hand the lateral pressure increased in both monolayers after asymmetrical insertion of monooleoylphosphatidylcholine. This can be explained by assuming that when monooleoylphosphatidylcholine is added to large unilamellar vesicles, the membrane bends until the strain is the same in both monolayers. In the case of sonicated vesicles, a change of curvature is not possible, and therefore differential packing in the two layers remains. We infer that a variation of lipid asymmetry by generating a lateral strain in the membrane can be a physiological way of modulating the conformation of membrane proteins.

The role of interfacial binding in the activation of Streptomyces chromofuscus phospholipase D by phosphatidic acid

The Streptomyces chromofuscus phospholipase D (PLD) cleavage of phosphatidylcholine in bilayers can be enhanced by the addition of the product phosphatidic acid (PA). Other anionic lipids such as phosphatidylinositol, oleic acid, or phosphatidylmethanol do not activate this PLD. This allosteric activation by PA could involve a conformational change in the enzyme that alters PLD binding to phospholipid surfaces. To test this, the binding of intact PLD and proteolytically cleaved isoforms to styrene divinylbenzene beads coated with a phospholipid monolayer and to unilamellar vesicles was examined. The results indicate that intact PLD has a very high affinity for PA bilayers at pH >/= 7 in the presence of EGTA that is weakened as Ca(2+) or Ba(2+) are added to the system. Proteolytically clipped PLD also binds tightly to PA in the absence of metal ions. However, the isolated catalytic fragment has a considerably weaker affinity for PA surfaces. In contrast to PA surfaces, all PLD forms exhibited very low affinity for PC interfaces with an increased binding when Ba(2+) was added. All PLD forms also bound tightly to other anionic phospholipid surfaces (e.g. phosphatidylserine, phosphatidylinositol, and phosphatidylmethanol). However, this binding was not modulated in the same way by divalent cations. Chemical cross-linking studies suggested that a major effect of PLD binding to PA.Ca(2+) surfaces is aggregation of the enzyme. These results indicate that PLD partitioning to phospholipid surfaces and kinetic activation are two separate events and suggest that the Ca(2+) modulation of PA.PLD binding involves protein aggregation that may be the critical interaction for activation.

Activation of phospholipase D by phosphatidic acid. Enhanced vesicle binding, phosphatidic acid-Ca2+ interaction, or an allosteric effect?

The activity of bacterial phospholipase D (PLD), a Ca2+-dependent enzyme, toward phosphatidylcholine bilayers was enhanced 7-fold by incorporation of 10 mol % phosphatidic acid (PA) in the vesicle bilayer. Addition of other negatively charged lipids such as phosphatidylinositol, phosphatidylmethanol, and oleic acid either inhibited or had no effect on enzyme activity. Only negatively charged lipids with a free phosphate group, phosphatidylinositol 4-phosphate and lyso-PA, had the same effect as PA on enzyme activity. Changes in vesicle curvature and fusion were not the reason for PA activation; rather, a metal ion-induced lateral segregation of PA in the vesicle bilayer correlated with PLD activation. Significant PA activation was also observed with monomer phosphatidylcholine substrate upon the addition of PA vesicles. The PA activation was caused by Ca2+.PA interacting with PLD at an allosteric site other than active site.

Solid-state NMR for the study of membrane systems: the use of anisotropic interactions

The use of solid-state nuclear magnetic resonance (NMR) as a tool to determine the structure of membrane molecules is reviewed with a particular emphasis on techniques that provide information on orientation or order. Experiments reported here have been performed in membranes, rather than in micelles or organic solvents. Several ways to prepare and handle the samples are discussed, like sample orientation and magic-angle spinning (MAS). Results concerning lipids, membrane peptides and proteins are included, as well as a discussion regarding the potential of such methods and their pitfalls.

Annexin V binding to the outer leaflet of small unilamellar vesicles leads to altered inner-leaflet properties: 31P- and 1H-NMR studies

Calcium-dependent binding to phospholipid membranes is closely associated with annexin functional properties. In these studies, 31P- and 1H-nuclear magnetic resonance (NMR) experiments have been performed to study the effects of binding of recombinant rat annexin V to sonicated small unilamellar vesicles (SUVs). High-resolution 31P-NMR spectra of SUVs containing mixtures of synthetic phosphatidic acid (PA) and phosphatidylcholine (PC) show resolvable resonances corresponding to the inner-leaflet PA, outer-leaflet PA, and PC phosphoryl groups. When annexin binding occurs, the outer-leaflet PA 31P resonance shifts while that of PC is unaffected, consistent with selective binding of the protein to the phosphoryl moiety of the PA component. Further, annexin V binding to membrane outer-leaflet phospholipids has a measurable effect on inner-leaflet phospholipids of intact vesicles. 1H-NMR T1 relaxation measurements of SUVs containing acyl-chain-perdeuterated PC show no effects on the PA hydrocarbon-chain segmental motions upon annexin binding. Circular dichroism measurements indicate that the protein does not undergo a significant conformational change upon binding to the vesicles. The observed NMR changes do not correspond to proton or calcium gradients, nor to lateral segregation of extended patches of homogeneous phospholipids. The combined evidence suggests that selective, peripheral annexin-membrane interactions influence the environment of the inner vesicular surface. The mechanism proposed is a protein-induced change in vesicle morphology that corresponds to reduced curvature.