Phase Transitions of Phospholipid Monolayers Penetrated by Apolipoproteins

Microenvironmentally controlled secondary structure motifs of apolipoprotein A-I derived peptides

The structure of apolipoprotein A-I (apoA-I), the major protein of HDL, has been extensively studied in past years. Nevertheless, its corresponding three-dimensional structure has been difficult to obtain due to the frequent conformational changes observed depending on the microenvironment. Although the function of each helical segment of this protein remains unclear, it has been observed that the apoA-I amino (N) and carboxy-end (C) domains are directly involved in receptor-recognition, processes that determine the diameter for HDL particles. In addition, it has been observed that the high structural plasticity of these segments might be related to several amyloidogenic processes. In this work, we studied a series of peptides derived from the N- and C-terminal domains representing the most hydrophobic segments of apoA-I. Measurements carried out using circular dichroism in all tested peptides evidenced that the lipid environment promotes the formation of α-helical structures, whereas an aqueous environment facilitates a strong tendency to adopt β-sheet/disordered conformations. Electron microscopy observations showed the formation of amyloid-like structures similar to those found in other well-defined amyloidogenic proteins. Interestingly, when the apoA-I peptides were incubated under conditions that promote stable globular structures, two of the peptides studied were cytotoxic to microglia and mouse macrophage cells. Our findings provide an insight into the physicochemical properties of key segments contained in apoA-I which may be implicated in disorder-to-order transitions that in turn maintain the delicate equilibrium between both, native and abnormal conformations, and therefore control its propensity to become involved in pathological processes.

Key structural arrangements at the C-terminus domain of CETP suggest a potential mechanism for lipid-transfer activity

The cholesteryl-ester transfer protein (CETP) promotes cholesteryl-ester and triglyceride transfer between lipoproteins. We evaluated the secondary structure stability of a series of small peptides derived from the C-terminus of CETP in a wide range of pH's and lipid mixtures, and studied their capability to carry out disorder-to-order secondary structure transitions dependent of lipids. We report that while a mixture of phosphatidylcholine/cholesteryl-esters forms large aggregated particles, the inclusion of a series of CETP carboxy-terminal peptides in a stable α-helix conformation, allows the formation of small homogeneous micelle-like structures. This phenomenon of lipid ordering was directly connected to secondary structural transitions at the C-terminus domain when lysophosphatidic acid and lysophosphatidylcholine lipids were employed. Circular dichroism, cosedimentation experiments, electron microscopy, as well as molecular dynamics simulations confirm this phenomenon. When purified CETP is studied, the same type of phenomenon occurs by promoting the reorganization of lipid from large to smaller particles. Our findings extend the emerging view for a novel mechanism of lipid transfer carried out by CETP, assigning its C-terminus domain the property to accomplish lipid ordering through secondary structure disorder-to-order transitions.

Insertion of apoLp-III into a lipid monolayer is more favorable for saturated, more ordered, acyl-chains

Neutral lipid transport in mammals is complicated involving many types of apolipoprotein. The exchangeable apolipoproteins mediate the transfer of hydrophobic lipids between tissues and particles, and bind to cell surface receptors. Amphipathic α-helices form a common structural motif that facilitates their lipid binding and exchangeability. ApoLp-III, the only exchangeable apolipoprotein found in insects, is a model amphipathic α-helix bundle protein and its three dimensional structure and function mimics that of the mammalian proteins apoE and apoAI. Even the intracellular exchangeable lipid droplet protein TIP47/perilipin 3 contains an α-helix bundle domain with high structural similarity to that of apoE and apoLp-III. Here, we investigated the interaction of apoLp-III from Locusta migratoria with lipid monolayers. Consistent with earlier work we find that insertion of apoLp-III into fluid lipid monolayers is highest for diacylglycerol. We observe a preference for saturated and more highly ordered lipids, suggesting a new mode of interaction for amphipathic α-helix bundles. X-ray reflectivity shows that apoLp-III unfolds at a hydrophobic interface and flexible loops connecting the amphipathic α-helices stay in solution. X-ray diffraction indicates that apoLp-III insertion into diacylglycerol monolayers induces additional ordering of saturated acyl-chains. These results thus shed important new insight into the protein-lipid interactions of a model exchangeable apolipoprotein with significant implications for its mammalian counterparts.

Apolipoprotein C-I binds more strongly to phospholipid/triolein/water than triolein/water interfaces: a possible model for inhibiting cholesterol ester transfer protein activity and triacylglycerol-rich lipoprotein uptake

Apolipoprotein C-I (apoC-I) is an important constituent of high-density lipoprotein (HDL) and is involved in the accumulation of cholesterol ester in nascent HDL via inhibition of cholesterol ester transfer protein and potential activation of lecithin:cholesterol acyltransferase (LCAT). As the smallest exchangeable apolipoprotein (57 residues), apoC-I transfers between lipoproteins via a lipid-binding motif of two amphipathic α-helices (AαHs), spanning residues 7-29 and 38-52. To understand apoC-I's behavior at hydrophobic lipoprotein surfaces, oil drop tensiometry was used to compare the binding to triolein/water (TO/W) and palmitoyloleoylphosphatidylcholine/triolein/water (POPC/TO/W) interfaces. When apoC-I binds to either interface, the surface tension (γ) decreases by ~16-18 mN/m. ApoC-I can be exchanged at both interfaces, desorbing upon compression and readsorbing on expansion. The maximal surface pressures at which apoC-I begins to desorb (Π(max)) were 16.8 and 20.7 mN/m at TO/W and POPC/TO/W interfaces, respectively. This suggests that apoC-I interacts with POPC to increase its affinity for the interface. ApoC-I is more elastic on POPC/TO/W than TO/W interfaces, marked by higher values of the elasticity modulus (ε) on oscillations. At POPC/TO/W interfaces containing an increasing POPC:TO ratio, the pressure at which apoC-I begins to be ejected increases as the phospholipid surface concentration increases. The observed increase in apoC-I interface affinity due to higher degrees of apoC-I-POPC interactions may explain how apoC-I can displace larger apolipoproteins, such as apoE, from lipoproteins. These interactions allow apoC-I to remain bound to the interface at higher Π values, offering insight into apoC-I's rearrangement on triacylglycerol-rich lipoproteins as they undergo Π changes during lipoprotein maturation by plasma factors such as lipoprotein lipase.

The formation of pepsin monomolecular layer by the Langmuir-Blodgett film deposition technique

We report herein the formation of pepsin monomolecular layer by the Langmuir-Blodgett film deposition technique. An effort was made to find an optimal subphase by adjusting the concentration of salt (KCl) and pH by monitoring the growth kinetics of pepsin for the formation of Langmuir monolayer by using as little as possible pepsin molecules to build up ultra thin film and to measure the extent of denaturation. Significant changes of area/molecule, compressibility, rigidity and unfolding of pepsin are observed at optimized subphase than pure water subphase. Observations at optimal subphase are explained in context of the modified DLVO theory and the site dissociation model. FTIR analysis of amide band together with the observed surface morphology of pepsin film in FE-SEM images indicate that at optimal subphase the pepsin molecules modify their structures by incrementing the beta-structure, resulting into larger unfolding and inter-molecular aggregates.

Disorder-to-order conformational transitions in protein structure and its relationship to disease

Function in proteins largely depends on the acquisition of specific structures through folding at physiological time scales. Under both equilibrium and non-equilibrium states, proteins develop partially structured molecules that being intermediates in the process, usually resemble the structure of the fully folded protein. These intermediates, known as molten globules, present the faculty of adopting a large variety of conformations mainly supported by changes in their side chains. Taking into account that the mechanism to obtain a fully packed structure is considered more difficult energetically than forming partially "disordered" folding intermediates, evolution might have conferred upon an important number of proteins the capability to first partially fold and-depending on the presence of specific partner ligands-switch on disorder-to-order transitions to adopt a highly ordered well-folded state and reach the lowest energy conformation possible. Disorder in this context can represent segments of proteins or complete proteins that might exist in the native state. Moreover, because this type of disorder-to-order transition in proteins has been found to be reversible, it has been frequently associated with important signaling events in the cell. Due to the central role of this phenomenon in cell biology, protein misfolding and aberrant disorder-to-order transitions have been at present associated with an important number of diseases.

Forces between hydrophilic surfaces adsorbed with apolipoprotein AII alpha helices

To provide better understanding of how a protein secondary structure affects protein-protein and protein-surface interactions, forces between amphiphilic alpha-helical proteins (human apolipoprotein AII) adsorbed on a hydrophilic surface (mica) were measured using an interferometric surface force apparatus (SFA). Forces between surfaces with adsorbed layers of this protein are mainly composed of electrostatic double layer forces at large surface distances and of steric repulsive forces at small distances. We suggest that the amphiphilicity of the alpha-helix structure facilitates the formation of protein multilayers next to the mica surfaces. We found that protein-surface interaction is stronger than protein-protein interaction, probably due to the high negative charge density of the mica surface and the high positive charge of the protein at our experimental conditions. Ellipsometry was used to follow the adsorption kinetics of this protein on hydrophilic silica, and we observed that the adsorption rate is not only controlled by diffusion, but rather by the protein-surface interaction. Our results for dimeric apolipoprotein AII are similar to those we have reported for the monomeric apolipoprotein CI, which has a similar secondary structure but a different peptide sequence and net charge. Therefore, the observed force curves seem to be a consequence of the particular features of the amphiphilic alpha-helices.

Interaction of N-nitrosodiethylamine/bovine serum albumin complexes with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine monolayers at the air-water interface

We report the effect of N-nitrosodiethylamine (NDA) on the interaction between bovine serum albumin (BSA) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine monolayers (DPPC) at the air-water interface. We prepared aqueous solutions of NDA/BSA complexes maintaining a constant concentration of BSA of 1.49 x 10(-9) M and using NDA concentrations to obtain 2000, 4000, 6000, 12,500, and 25,000 NDA/BSA molar ratios. The hysteresis area and the compressional modulus of the compression-expansion cycles performed at different times were dependent on the NDA concentration. The cycles performed demonstrate the stability of the new phase of DPPC/BSA and DPPC/NDA/BSA monolayers. This was achieved probably because the BSA concentration used was lower than the one needed for BSA to inhibit the return of DPPC molecules to the interface. Results of the compressional modulus at the onset of the new phase, obtained around 17 mN/m, 15 min and 1, 3, 5, and 12 h after DPPC deposition, indicated that the 3.0 x 10(-6) M NDA concentration produced a more rigid film, probably due to the higher alpha-helix content of BSA. AFM images were obtained for DPPC/BSA and two DPPC/NDA/BSA complexes. Our images show that 12,500 NDA/BSA molecules were mostly adsorbed in the liquid condensed phase. However, BSA molecules were distributed more homogeneously.

Incorporation of ovalbumin within cationic octadecylamine monolayer and a comparative study with zwitterionic DPPC and anionic stearic acid monolayer

In this communication we demonstrated the incorporation of water-soluble surface-active protein OVA within an insoluble cationic ODA monolayer and compared with zwitterionic (DPPC) and anionic (SA) monolayer. The incorporation of OVA is found to be more in ODA as compared to that of DPPC and SA. The kinetics of protein adsorption in lipid monolayer gives the idea that unfolding of OVA is less in case of DPPC than SA and ODA. The pi-A isotherm and compressibility study gives the information about the different states of the protein-lipid mixed monolayer. At higher pressure, OVA tend to squeeze out from the lipids monolayer. High-resolution field emission scanning electron microscope (FE-SEM) images confirm this observation. The surface morphology of DPPC-OVA LB film is far better than ODA-OVA and SA-OVA LB film. OVA forms large irregular aggregates on SA and ODA monolayer. Fluorescence study reveals that protein structure is perturbed more in SA and ODA system compared to that of DPPC. The overall results indicate that DPPC monolayer is better to get protein lipid mixed film than SA and ODA monolayer.

Lipid dependant disorder-to-order conformational transitions in apolipoprotein CI derived peptides

In contrast to the notion established for many years that protein function depends on rigid 3D structures, nowadays there is important evidence suggesting that non-structured segments of proteins play important roles in protein function. Therefore, disorder-to-order dynamic conformational transitions have been proposed as an attractive mechanism involved in protein-protein recognition. Our laboratory using Langmuir monolayers of apolipoproteins has previously shown that upon lateral compression at the air/water and phospholipid/water interfaces, there is an important movement of the C-terminal segment of apolipoprotein CI towards the air, considered the hydrophobic region of the monolayer and the acyl-chain region of the interface when phospholipids are used. Here, in an attempt to define secondary structure changes that might occur within this C-terminal segment of apoCI while moving from the monolayer interface back and forth its hydrophobic region, employing three peptides derived from apoCI we studied by circular dichroism and dynamic light scattering their conformational properties when associated to a series of amphipathic lipids and lipid-like molecules. Our results show that a series of lysophospholipids present the ability to modulate the formation of an alpha helix at the C-terminal peptide of apoCI through a disorder-to-order transition while forming small lipid/peptide aggregates below 10nm in diameter.

Interfacial Behavior of N-Nitrosodiethylamine/Bovine Serum Albumin Complexes at the Air−Water and the Chloroform−Water Interfaces by Axisymmetric Drop Tensiometry

Interfacial properties of N-nitrosodiethylamine/bovine serum albumin (NDA/BSA) complexes were investigated at the air-water interface. The interfacial behavior at the chloroform-water interface of the interaction product of phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), dissolved in the chloroform phase, and NDA/BSA complex, in the aqueous phase, were also analyzed by using a drop tensiometer. The secondary structure changes of BSA with different NDA concentrations were monitored by circular dichroism spectroscopy at different pH and the NDA/BSA interaction was probed by fluorescence spectroscopy. Different NDA/BSA mixtures were prepared from 0, 7.5 x 10(-5), 2.2 x 10(-4), 3.7 x 10(-4), 5 x 10(-4), 1.6 x 10(-3), and 3.1 x 10(-3) M NDA solutions in order to afford 0, 300/1, 900/1, 1 500/1, 2 000/1, 6 000/1, and 12 500/1 NDA/BSA molar ratios, respectively, in the aqueous solutions. Increments of BSA alpha-helix contents were obtained up to the 2 000/1 NDA/BSA molar ratio, but at ratios beyond this value, the alpha-helix content practically disappeared. These BSA structure changes produced an increment of the surface pressure at the air-water interface, as the alpha-helix content increased with the concentration of NDA. On the contrary, when alpha-helix content decreased, the surface pressure also appeared lower than the one obtained with pure BSA solutions. The interaction of DPPC with NDA/BSA molecules at the chloroform-water interface produced also a small, but measurable, pressure increment with the addition of NDA molecules. Dynamic light scattering measurements of the molecular sizes of NDA/BSA complex at pH 4.6, 7.1, and 8.4 indicated that the size of extended BSA molecules at pH 4.6 increased in a greater proportion with the increment in NDA concentration than at the other studied pH values. Diffusion coefficients calculated from dynamic surface tension values, using a short-term solution of the general adsorption model of Ward and Tordai, also showed differences with pH and the NDA concentration. Both, the storage and loss dilatational elastic modulus were obtained at the air-water and at the chloroform-water interfaces. The interaction of NDA/BSA with DPPC at the chloroform-water produced a less rigid monolayer than the one obtained with pure DPPC (1 x 10(-5) M), indicating a significant penetration of NDA/BSA molecules at the interface. At short times and pH 4.6, the values of the storage elastic modulus were larger and more sensible to the NDA addition than the ones at pH 7.1 and 8.4, probably due to a gel-like network formation at the air-water interface.

Interaction of Ovalbumin with Phospholipids Langmuir−Blodgett Film

Interaction of native ovalbumin (OVA) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) Langmuir-Blodgett monolayer has been studied at the air-water interface. A compressibility study shows the positive association with DPPC. Adsorption kinetics shows that the protein adsorption is a one-step process and the amount of protein adsorbed depends on the concentration of protein at the water subphase. Incorporation of protein into the DPPC layer is surface-pressure dependent. The compressibility study indicates that the DPPC-OVA interaction is hydrophobic in nature and structural reorganization is eminent to adjust the hydrophobic mismatch between DPPC acyl chains and OVA hydrophobic moieties. At higher pressure, OVA tends to squeeze out from the DPPC monolayer. A nanometer scale FE-SEM image confirms this observation. Globular aggregates of protein of dimension 60-80 nm were observed in DPPC-OVA supported film. Steady-state fluorescence spectroscopy suggests that the tryptophan residues of OVA are main emitting species. The blue shift of tryptophan fluorescence in supported film may be due to the tryptophan molecule of protein exposed to the hydrophobic air phase.

Penetration of surfactin into phospholipid monolayers: nanoscale interfacial organization

Atomic force microscopy (AFM) combined with surface pressure-area isotherms were used to probe the interfacial behavior of phospholipid monolayers following penetration of surfactin, a cyclic lipopeptide produced by Bacillus subtilis strains. Prior to penetration experiments, interfacial behavior of different surfactin molecules (cyclic surfactins with three different aliphatic chain lengths--S13, S14, and S15--and a linear surfactin obtained by chemical cleavage of the cycle of the surfactin S15) has been investigated. A more hydrophobic aliphatic chain induces greater surface-active properties of the lipopeptide. The opening of the peptide ring reduces the surface activity. The effect of phospholipid acyl chain length (dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine- (DPPC), and distearoylphosphatidylcholine) and phospholipid polar head (DPPC, dipalmitoylphosphatidylethanolamine and dipalmitoylphosphatidylserine) on monolayer penetration properties of the surfactin S15 has been explored. Results showed that while the lipid monolayer thickness and the presence of electrostatic repulsions from the interfacial film do not significantly influence surfactin insertion, these parameters strongly modulate the ability of the surfactin to alter the nanoscale organization of the lipid films. We also probed the effect of surfactin structure (influence of the aliphatic chain length and of the cyclic structure of the peptide ring) on the behavior of DPPC monolayers. AFM images and isotherms showed that surfactin penetration is promoted by longer lipopeptide chain length and a cyclic polar head. This indicates that hydrophobic interactions are of main importance for the penetration power of surfactin molecules.

Fabrication of phytic acid sensor based on mixed phytase-lipid Langmuir-Blodgett films

This paper reports the surface activity of phytase at the air-water interface, its interaction with lipid monolayers, and the construction of a new phytic acid biosensor on the basis of the Langmuir-Blodgett (LB) technique. Phytase was inserted in the subphase solution of dipalmitoylphosphatidylglycerol (DPPG) Langmuir monolayers, and its incorporation to the air-water interface was monitored with surface pressure measurements. Phytase was able to incorporate into DPPG monolayers even at high surface pressures, ca. 30 mN/m, under controlled ionic strength, pH, and temperature. Mixed Langmuir monolayers of phytase and DPPG were characterized by surface pressure-area and surface potential-area isotherms, and the presence of the enzyme provided an expansion in the monolayers (when compared to the pure lipid at the interface). The enzyme incorporation also led to significant changes in the equilibrium surface compressibility (in-plane elasticity), especially in liquid-expanded and liquid-condensed regions. The dynamic surface elasticity for phytase-containing interfaces was investigated using harmonic oscillation and axisymmetric drop shape analysis. The insertion of the enzyme at DPPG monolayers caused an increase in the dynamic surface elasticity at 30 mN m(-)(1), indicating a strong interaction between the enzyme and lipid molecules at a high-surface packing. Langmuir-Blodgett (LB) films containing 35 layers of mixed phytase-DPPG were characterized by ultraviolet-visible and fluorescence spectroscopy and crystal quartz microbalance nanogravimetry. The ability in detecting phytic acid was studied with voltammetric measurements.

Miscibility of binary monolayers at the air-water interface and interaction of protein with immobilized monolayers by surface plasmon resonance technique

The miscibility and stability of the binary monolayers of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and cationic dioctadecyldimethylammonium bromide (DOMA) at the air-water interface and the interaction of ferritin with the immobilized monolayers have been studied in detail using surface pressure-area isotherms and surface plasmon resonance technique, respectively. The surface pressure-area isotherms indicated that the binary monolayers of DPPC and DOMA at the air-water interface were miscible and more stable than the monolayers of the two individual components. The surface plasmon resonance studies indicated that ferritin binding to the immobilized monolayers was primarily driven by the electrostatic interaction and that the amount of adsorbed protein at saturation was closely related not only to the number of positive charges in the monolayers but also to the pattern of positive charges at a given mole fraction of DOMA. The protein adsorption kinetics was determined by the properties of the monolayers (i.e., the protein-monolayer interaction) and the structure of preadsorbed protein molecules (i.e., the protein-protein interaction).

Chitosan−Cholesterol and Chitosan−Stearic Acid Interactions at the Air−Water Interface

We report in this work the isotherms of cholesterol and stearic acid at the air-water interface modified by different chitosans (chitosan chloride, hydrophobic modified chitosan, and medium and high molecular weight chitosans) in the aqueous subphase. The Langmuir-Blodgett films of the complexes cholesterol-chitosan and stearic acid-chitosan are analyzed by atomic force microscopy (AFM), and a molecular simulation was performed to visualize the chitosan-lipid interactions. Strong modifications are obtained in the isotherms as a result of the chitosan interactions with cholesterol and stearic acid at the air-water interface. These modifications were dependent on the type and concentration of chitosan. Severe modifications of all phases were noticed with larger molecular areas, and the observed changes in the compressional modulus were dependent on the type of chitosan used. The complexes of chitosan-stearic acid were more flexible than the ones of chitosan-cholesterol. The AFM images demonstrated that chitosan was disaggregated by the cholesterol and stearic acid interactions producing more homogeneous surfaces in some cases. The hydrophobic chitosan showed more affinity with stearic acid, while both medium and high molecular weight chitosans produced homogeneous surfaces with cholesterol. The simulated chitosan chains interacting with cholesterol and stearic acid demonstrated the possibility of specific sites of electrostatic bonds between these molecules. Adsorption of cholesterol on the different powdered chitosans, performed by HPLC, showed that the medium and high molecular weight chitosans could retain higher proportions of cholesterol compared with the other analyzed samples.

In search of new structural states of exchangeable apolipoproteins

Based upon state of the art biophysical experimentation, this article focuses on the different structural arrangements exchangeable apolipoproteins achieve when placed on Langmuir monolayers and subjected to changes in lateral pressure. We have studied the monolayers of apolipoproteins CI, CIII, AI, AII, and E that show as secondary structure a high percentage of amphipathic alpha-helix. This has been achieved employing techniques such as Brewster angle microscopy, synchrotron X-ray diffraction, and surface pressure measurements. In addition, the lateral order of protein arrays has been also studied by atomic force microscopy. These monolayers show that a phase transition from a two-dimensional disorder fluid to an ordered state is detected at relatively high lateral pressure, where unusual one-dimensional solid phases are discovered. While several helices that conform the apolipoprotein are confined to the interface, others are uniformly tilted toward the hydrophobic air or the phospholipid fatty acid chains. Our results suggest that a similar ordering might also occur when these apolipoproteins are attached to a lipoprotein particle such as a high density lipoprotein (HDL) particle. Therefore, changes from a nascent or discoidal HDL to a mature spherical HDL might in parallel involve structural changes as those described in our Langmuir interfaces. Current experimentation is being carried out in order to elucidate if the structural states already found are related to the efficiency of lipid transfer between lipoprotein particles or lipoproteins and the plasma membrane of cells, as well as receptor ligand recognition.