Monolayer and calorimetric studies of phosphatidylcholines containing branched-chain fatty acids and of their interactions with cholesterol and with a bacterial hopanoid in model membranes

Hopanoids Confer Robustness to Physicochemical Variability in the Niche of the Plant Symbiont Bradyrhizobium diazoefficiens

Rhizobia are a group of bacteria that increase soil nitrogen content through symbiosis with legume plants. The soil and symbiotic host are potentially stressful environments, and the soil will likely become even more stressful as the climate changes. Many rhizobia within the Bradyrhizobium clade, like Bradyrhizobium diazoefficiens, possess the genetic capacity to synthesize hopanoids, steroid-like lipids similar in structure and function to cholesterol. Hopanoids are known to protect against stresses relevant to the niche of B. diazoefficiens. Paradoxically, mutants unable to synthesize the extended class of hopanoids participate in symbioses with success similar to that of the wild type, despite being delayed in root nodule initiation. Here, we show that in B. diazoefficiens, the growth defects of extended-hopanoid-deficient mutants can be at least partially compensated for by the physicochemical environment, specifically, by optimal osmotic and divalent cation concentrations. Through biophysical measurements of lipid packing and membrane permeability, we show that extended hopanoids confer robustness to environmental variability. These results help explain the discrepancy between previous in-culture and in planta results and indicate that hopanoids may provide a greater fitness advantage to rhizobia in the variable soil environment than the more controlled environments within root nodules. To improve the legume-rhizobium symbiosis through either bioengineering or strain selection, it will be important to consider the full life cycle of rhizobia, from soil to symbiosis. IMPORTANCE Rhizobia, such as B. diazoefficiens, play an important role in the nitrogen cycle by making nitrogen gas bioavailable through symbiosis with legume plants. As climate change threatens soil health, this symbiosis has received increased attention as a more sustainable source of soil nitrogen than the energy-intensive Haber-Bosch process. Efforts to use rhizobia as biofertilizers have been effective; however, long-term integration of rhizobia into the soil community has been less successful. This work represents a small step toward improving the legume-rhizobium symbiosis by identifying a cellular component-hopanoid lipids-that confers robustness to environmental stresses rhizobia are likely to encounter in soil microenvironments as sporadic desiccation and flooding events become more common.

Hopanoid Hopene Locates in the Interior of Membranes and Affects Their Properties

Hopanoids are proposed as sterol surrogates in some bacteria, and it has been proved that some hopanoids are able to induce a liquid-order phase state in lipid membranes. The members of this group of molecules have diverse structures, and not all of them have been studied in detail yet. Here, we study membranes with the hopanoid hopene (hop-22 (29)-ene or diploptene), which is the product of the cycling of squalene by squalene-hopene cyclase, and thus is present in the first step of hopanoid biosynthesis. Hopene is particularly interesting because it lacks a polar head group, which opens the question of how does this molecule accommodate in a lipid membrane, and what are the effects promoted by its presence. In order to get an insight into this, we prepared monolayers and bilayers of a phospholipid with hopene and studied their properties in comparison with pure phospholipid membranes, and with the sterol cholesterol or the hopanoid diplopterol. Film stiffness, shear viscosity, and bending dynamics were very affected by the presence of hopene, while zeta-potential, generalized polarization of Laurdan, and conductivity were affected moderately by this molecule. The results suggest that at very low percentages, hopene locates parallel to the phospholipid molecules, while the excess of the hopene molecules stays between leaflets, as previously proposed using molecular dynamics simulations.

The role of hopanoids in fortifying rhizobia against a changing climate

Bacteria are a globally sustainable source of fixed nitrogen, which is essential for life and crucial for modern agriculture. Many nitrogen-fixing bacteria are agriculturally important, including bacteria known as rhizobia that participate in growth-promoting symbioses with legume plants throughout the world. To be effective symbionts, rhizobia must overcome multiple environmental challenges: from surviving in the soil, to transitioning to the plant environment, to maintaining high metabolic activity within root nodules. Climate change threatens to exacerbate these challenges, especially through fluctuations in soil water potential. Understanding how rhizobia cope with environmental stress is crucial for maintaining agricultural yields in the coming century. The bacterial outer membrane is the first line of defence against physical and chemical environmental stresses, and lipids play a crucial role in determining the robustness of the outer membrane. In particular, structural remodelling of lipid A and sterol-analogues known as hopanoids are instrumental in stress acclimation. Here, we discuss how the unique outer membrane lipid composition of rhizobia may underpin their resilience in the face of increasing osmotic stress expected due to climate change, illustrating the importance of studying microbial membranes and highlighting potential avenues towards more sustainable soil additives.

Bacterial Analogs to Cholesterol Affect Dimerization of Proteorhodopsin and Modulates Preferred Dimer Interface

Hopanoids, the bacterial analogues of sterols, are ubiquitous in bacteria and play a significant role in organismal survival under stressful environments. Unlike sterols, hopanoids have a high degree of variation in the size and chemical nature of the substituent attached to the ring moiety, leading to different effects on the structure and dynamics of biological membranes. While it is understood that hopanoids can indirectly tune membrane physical properties, little is known on the role that hopanoids may play in affecting the organization and behavior of bacterial membrane proteins. In this work we used coarse-grained molecular dynamics simulations to characterize the effects of two hopanoids, diploptene (DPT) and bacteriohopanetetrol (BHT), on the oligomerization of proteorhodopsin (PR) in a model membrane composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phophoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-3-phosphoglycerol (POPG). PR is a bacterial membrane protein that functions as a light-activated proton pump. We chose PR based on its ability to adopt a distribution of oligomeric states in different membrane environments. Furthermore, the efficiency of proton pumping in PR is intimately linked to its organization into oligomers. Our results reveal that both BHT and DPT indirectly affect dimerization by tuning membrane properties in a fashion that is concentration-dependent. Variation in their interaction with PR in the membrane-embedded and the cytoplasmic regions leads to distinctly different effects on the plasticity of the dimer interface. BHT has the ability to intercalate between monomers in the dimeric interface, whereas DPT shifts dimerization interactions via packing of the interleaflet region of the membrane. Our results show a direct relationship between hopanoid structure and lateral organization of PR, providing a first glimpse at how these bacterial analogues to eukaryotic sterols produce very similar biophysical effects within the cell membrane.

The antimicrobial peptide Polybia-MP1 differentiates membranes with the hopanoid, diplopterol from those with cholesterol

Polybia-MP1 is an antimicrobial peptide that shows a decreased activity in membranes with cholesterol (CHO). Since it is now accepted that hopanoids act as sterol-surrogates in some sterol-lacking bacteria, we here inquire about the impact of Polybia-MP1 on membranes containing the hopanoid diplopterol (DP) in comparison to membranes with CHO. We found that, despite the properties induced on lipid membranes by DP are similar to those induced by CHO, the effect of Polybia-MP1 on membranes with CHO or DP was significantly different. DP did not prevent dye release from LUVs, nor the insertion of Polybia-MP1 into monolayers, and peptide-membrane affinity was higher for those with DP than with CHO. Zeta potentials ( ζ ) for DP-containing LUVs showed a complex behavior at increasing peptide concentration. The effect of the peptide on membrane elasticity, investigated by nanotube retraction experiments, showed that peptide addition softened all membrane compositions, but membranes with DP got stiffer at long times. Considering this, and the ζ results, we propose that peptides accumulate at the interface adopting different arrangements, leading to a non-monotonic behavior. Possible correlations with cell membranes were inquired testing the antimicrobial activity of Polybia-MP1 against hopanoid-lacking bacteria pre-incubated with DP or CHO. The fraction of surviving cells was lower in cultures incubated with DP compared to those incubated with CHO. We propose that the higher activity of Polybia-MP1 against some bacteria compared to mammalian cells is not only related to membrane electrostatics, but also the composition of neutral lipids, particularly the hopanoids, could be important.

Distinct functional roles for hopanoid composition in the chemical tolerance of Zymomonas mobilis

Hopanoids are a class of membrane lipids found in diverse bacterial lineages, but their physiological roles are not well understood. The ethanol fermenter Zymomonas mobilis features the highest measured concentration of hopanoids, leading to the hypothesis that these lipids can protect against the solvent toxicity. However, the lack of genetic tools for manipulating hopanoid composition in this bacterium has limited their further functional analysis. Due to the polyploidy (>50 genome copies per cell) of Z. mobilis, we found that disruptions of essential hopanoid biosynthesis (hpn) genes act as genetic knockdowns, reliably modulating the abundance of different hopanoid species. Using a set of hpn transposon mutants, we demonstrate that both reduced hopanoid content and modified hopanoid polar head group composition mediate growth and survival in ethanol. In contrast, the amount of hopanoids, but not their head group composition, contributes to fitness at low pH. Spectroscopic analysis of bacterial-derived liposomes showed that hopanoids protect against several ethanol-driven phase transitions in membrane structure, including lipid interdigitation and bilayer dissolution. We propose that hopanoids act through a combination of hydrophobic and inter-lipid hydrogen bonding interactions to stabilize bacterial membranes during solvent stress.

Effect of Triclosan and Chloroxylenol on Bacterial Membranes

Triclosan and chloroxylenol are broad-spectrum biocides used extensively in healthcare and consumer products. They have been suggested to perturb the structure of bacterial membranes, but studies so far have not considered that most bacterial membranes contain large amounts of branched-chain lipids. Here, molecular dynamics simulation is used to examine the effect of the two biocides on membranes consisting of lipids with methyl-branched chains, cyclopropanated chains, and nonbranched chains. It is shown that triclosan and chloroxylenol induced a phase transition in membranes from a liquid-crystalline to a liquid-ordered phase irrespective of the presence and nature of branching groups. At high concentration, chloroxylenol promoted chain interdigitation. Our results suggest that triclosan and chloroxylenol decrease the degree of fluidity of membranes and that this effect is more pronounced in bacterial membranes. As a result, their biocidal activity could be associated with a change in the function of membrane proteins.

Variations in carotenoid content and acyl chain composition in exponential, stationary and biofilm states of Staphylococcus aureus, and their influence on membrane biophysical properties

Bacteria are often found in close association with surfaces, resulting in the formation of biofilms. In Staphylococcus aureus (S. aureus), biofilms are implicated in the resilience of chronic infections, presenting a serious clinical problem world-wide. Here, S. aureus biofilms are grown under flow within clinical catheters at 37 °C. The lipid composition and biophysical properties of lipid extracts from these biofilms are compared with those from exponential growth and stationary phase cells. Biofilms show a reduction in iso and anteiso branching compensated by an increase in saturated fatty acids compared to stationary phase. A drastic reduction in carotenoid levels is also observed during biofilm formation. Thermotropic measurements of Laurdan GP and DPH polarization, show a reduction of lipid packing at 37 °C for biofilms compared to stationary phase. We studied the effects of carotenoid content on DMPG and DPPG model membranes showing trends in thermotropic behavior consistent with those observed in bacterial isolates, indicating that carotenoids participate in modulating lipid packing. Additionally, bending elastic constant (k_c) measurements using vesicle fluctuation analysis (VFA) show that the presence of carotenoids can increase membrane bending rigidity. The antimicrobial peptide Magainin H2 was less activity on liposomes composed of stationary phase compared to biofilms or exponential growth isolates. This study contributes to an understanding of how Staphylococcus aureus modulates the composition of its membrane lipids, and how those changes affect the biophysical properties of membranes, which in turn may play a role in its virulence and its resistance to different membrane-active antimicrobial agents.

Human meibum chain branching variability with age, gender and meibomian gland dysfunction

NMR spectroscopy was used to measure hydrocarbon chain branching variability in meibum with age, gender and Meibomian gland dysfuction (MGD). A cohort of 65 meibum donors without dry eye and 31 donors with MGD was studied. Heteronuclear 2D NMR was used to confirm CH₃ resonance assignments allowing us to positively identify 23 of the 27 proton resonances of cholesteryl ester and ¹H resonances due to straight chain, iso-branched and anteiso-branched hydrocarbons. Meibum from donors without dry eye contained 57 ± 1% straight-chains, 23 ± 1% iso-branched chains and 20 ± 1% anteiso-branched hydrocarbon chains. Compared with meibum from donors without dry eye, meibum from donors with MGD contained less, 50 ± 2% straight-chains, more, 32 ± 2% iso-branched chains and the same amount, 18.0 ± 0.07%, of anteiso-branched hydrocarbon chains. Meibum hydrocarbon chain branching did not change with age between 22 and 68 years of age, nor was it influenced by gender. Based on previous studies, one would expect anteiso-branched chains would contribute to lowering the phase transition temperature of meibum, decrease the elasticity of the tear film lipid layer and increase the molecular area and spreading of meibum on the surface of the eye. Although we speculate that the observed differences in iso-chain branching do not influence tear film stability or rheology, the speculation has yet to be tested.

Hopanoid lipids: from membranes to plant-bacteria interactions

Lipid research represents a frontier for microbiology, as showcased by hopanoid lipids. Hopanoids, which resemble sterols and are found in the membranes of diverse bacteria, have left an extensive molecular fossil record. They were first discovered by petroleum geologists. Today, hopanoid-producing bacteria remain abundant in various ecosystems, such as the rhizosphere. Recently, great progress has been made in our understanding of hopanoid biosynthesis, facilitated in part by technical advances in lipid identification and quantification. A variety of genetically tractable, hopanoid-producing bacteria have been cultured, and tools to manipulate hopanoid biosynthesis and detect hopanoids are improving. However, we still have much to learn regarding how hopanoid production is regulated, how hopanoids act biophysically and biochemically, and how their production affects bacterial interactions with other organisms, such as plants. The study of hopanoids thus offers rich opportunities for discovery.

Hopanoids as functional analogues of cholesterol in bacterial membranes

The functionality of cellular membranes relies on the molecular order imparted by lipids. In eukaryotes, sterols such as cholesterol modulate membrane order, yet they are not typically found in prokaryotes. The structurally similar bacterial hopanoids exhibit similar ordering properties as sterols in vitro, but their exact physiological role in living bacteria is relatively uncharted. We present evidence that hopanoids interact with glycolipids in bacterial outer membranes to form a highly ordered bilayer in a manner analogous to the interaction of sterols with sphingolipids in eukaryotic plasma membranes. Furthermore, multidrug transport is impaired in a hopanoid-deficient mutant of the gram-negative Methylobacterium extorquens, which introduces a link between membrane order and an energy-dependent, membrane-associated function in prokaryotes. Thus, we reveal a convergence in the architecture of bacterial and eukaryotic membranes and implicate the biosynthetic pathways of hopanoids and other order-modulating lipids as potential targets to fight pathogenic multidrug resistance.

Methylation at the C-2 position of hopanoids increases rigidity in native bacterial membranes

Sedimentary rocks host a vast reservoir of organic carbon, such as 2-methylhopane biomarkers, whose evolutionary significance we poorly understand. Our ability to interpret this molecular fossil record is constrained by ignorance of the function of their molecular antecedents. To gain insight into the meaning of 2-methylhopanes, we quantified the dominant (des)methylated hopanoid species in the membranes of the model hopanoid-producing bacterium Rhodopseudomonas palustris TIE-1. Fluorescence polarization studies of small unilamellar vesicles revealed that hopanoid 2-methylation specifically renders native bacterial membranes more rigid at concentrations that are relevant in vivo. That hopanoids differentially modify native membrane rigidity as a function of their methylation state indicates that methylation itself promotes fitness under stress. Moreover, knowing the in vivo (2Me)-hopanoid concentration range in different cell membranes, and appreciating that (2Me)-hopanoids' biophysical effects are tuned by the lipid environment, permits the design of more relevant in vitro experiments to study their physiological functions.

DOI:http://dx.doi.org/10.7554/eLife.05663.001

The cell membrane that separates the inside of a cell from its outside environment is not a fixed structure. A cell can change the amount and type of different molecules in its membrane, which can alter the rigidity and permeability of the membrane and allow the cell to adapt to changing conditions.

The cell membranes of many bacteria contain molecules called hopanoids. Hopanes are the fossilized forms of these molecules and many hopanes are found extensively in sedimentary rocks. For example, 2-methylated hopanes—the fossilized forms of hopanoids that have a methyl group added to a particular carbon atom—have been found in ancient rocks that formed up to 1.6 billion years ago.

Many researchers have suggested that 2-methylated hopanes (and other molecular fossils) in sedimentary rocks could act as ‘biomarkers’ and be used to deduce what primitive life and ancient living conditions were like. Millions of years ago, several periods occurred where the Earth's oceans lost almost all of their oxygen; this likely placed all life on Earth under great stress. A greater proportion of the hopanes found in rocks formed during those periods are methylated than those seen in rocks from other time periods. However, it was difficult to interpret this observation about the fossil record, as the role of 2-methylated hopanoids in living bacterial cells was unknown.

Wu et al. have now investigated the role of 2-methylated hopanoids by performing experiments on bacterial membranes and found that 2-methylated hopanoids help the other molecules that make up the membrane to pack more tightly together. This makes the membrane more rigid, and the extent of this stiffening depends on the length of the 2-methylated hopanoid and on the other molecules that are present in the membrane. A more rigid membrane would protect the bacteria more in times of stress; therefore, rock layers containing an increased amount of 2-methylhopane are likely to indicate times when the bacteria living at that time were under a great deal of stress.

DOI:http://dx.doi.org/10.7554/eLife.05663.002

Search for the most 'primitive' membranes and their reinforcers: a review of the polyprenyl phosphates theory

Terpenoids have an essential function in present-day cellular membranes, either as membrane reinforcers in Eucarya and Bacteria or as principal membrane constituents in Archaea. We have shown that some terpenoids, such as cholesterol and α, ω-dipolar carotenoids reinforce lipid membranes by measuring the water permeability of unilamellar vesicles. It was possible to arrange the known membrane terpenoids in a ‘phylogenetic’ sequence, and a retrograde analysis led us to conceive that single-chain polyprenyl phosphates might have been ‘primitive’ membrane constituents. By using an optical microscopy, we have observed that polyprenyl phosphates containing 15 to 30 C-atoms form giant vesicles in water in a wide pH range. The addition of 10 % molar of some polyprenols to polyprenyl phosphate vesicles have been shown to reduce the water permeability of membranes even more efficiently than the equimolecular addition of cholesterol. A ‘prebiotic’ synthesis of C10 and C15 prenols from C5 monoprenols was achieved in the presence of a montmorillonite clay. Hypothetical pathway from C1 or C2 units to ‘primitive’ membranes and that from ‘primitive’ membranes to archaeal lipids are presented.

A polycyclic terpenoid that alleviates oxidative stress

Polycyclic terpenoid lipids such as hopanes and steranes have been widely used to understand ancient biology, Earth history, and the oxygenation of the ocean-atmosphere system. Some of these lipids are believed to be produced only by aerobic organisms, whereas others actually require molecular oxygen for their biosynthesis. A persistent question remains: Did some polycyclic lipids initially evolve in response to certain environmental or metabolic stresses, including the presence of oxygen? Here, we identify tetracyclic isoprenoids in spores of the bacterium Bacillus subtilis. We call them sporulenes. They are produced by cyclization of regular polyprenes, a reaction that is more favorable chemically than the formation of terpenoids such as hopanoids and steroids from squalene. The simplicity of the reaction suggests that the B. subtilis cyclase may be analogous to evolutionarily ancient cyclases. We show that these molecules increase the resistance of spores to a reactive oxygen species, demonstrating a specific physiological role for a nonpigment bacterial lipid biomarker. Geostable derivatives of these compounds in sediments could thus be used as direct indicators of oxidative stress and aerobic environments.

Steroids, triterpenoids and molecular oxygen

There is a close connection between modern-day biosynthesis of particular triterpenoid biomarkers and presence of molecular oxygen in the environment. Thus, the detection of steroid and triterpenoid hydrocarbons far back in Earth history has been used to infer the antiquity of oxygenic photosynthesis. This prompts the question: were these compounds produced similarly in the past? In this paper, we address this question with a review of the current state of knowledge surrounding the oxygen requirement for steroid biosynthesis and phylogenetic patterns in the distribution of steroid and triterpenoid biosynthetic pathways. The hopanoid and steroid biosynthetic pathways are very highly conserved within the bacterial and eukaryotic domains, respectively. Bacteriohopanepolyols are produced by a wide range of bacteria, and are methylated in significant abundance at the C2 position by oxygen-producing cyanobacteria. On the other hand, sterol biosynthesis is sparsely distributed in distantly related bacterial taxa and the pathways do not produce the wide range of products that characterize eukaryotes. In particular, evidence for sterol biosynthesis by cyanobacteria appears flawed. Our experiments show that cyanobacterial cultures are easily contaminated by sterol-producing rust fungi, which can be eliminated by treatment with cycloheximide affording sterol-free samples. Sterols are ubiquitous features of eukaryotic membranes, and it appears likely that the initial steps in sterol biosynthesis were present in their modern form in the last common ancestor of eukaryotes. Eleven molecules of O2 are required by four enzymes to produce one molecule of cholesterol. Thermodynamic arguments, optimization of function and parsimony all indicate that an ancestral anaerobic pathway is highly unlikely. The known geological record of molecular fossils, especially steranes and triterpanes, is notable for the limited number of structural motifs that have been observed. With a few exceptions, the carbon skeletons are the same as those found in the lipids of extant organisms and no demonstrably extinct structures have been reported. Furthermore, their patterns of occurrence over billion year time-scales correlate strongly with environments of deposition. Accordingly, biomarkers are excellent indicators of environmental conditions even though the taxonomic affinities of all biomarkers cannot be precisely specified. Biomarkers are ultimately tied to biochemicals with very specific functional properties, and interpretations of the biomarker record will benefit from increased understanding of the biological roles of geologically durable molecules.

Impact on lipid membrane organization by free branched-chain fatty acids

Here, we exploit the non-invasive techniques of solid-state NMR (nuclear magnetic resonance) and differential scanning calorimetry (DSC) to study the effect of free iso and ante-iso branched chain fatty acids (BCFAs) on the physicochemical properties of lipid membranes. Free fatty acids are present in biological membranes at low abundance, but can influence the cellular function by modulating the membrane organization. Solid state NMR spectra of dimyristoylphosphatidylcholine (DMPC) lipid membranes containing either free 12-methyltetradecanoic acid (a15:0) or free 13-methyltetradecanoic acid (i15:0), show significant differences in their impact on the lipid bilayer. Chain order profiles obtained by deuterium NMR on fully deuterated DMPC-d(67) bilayers revealed an ordering effect induced by both fatty acids on the hydrophobic membrane core. This behavior was also visible in the corresponding DSC thermograms where the main phase transition of DMPC bilayers-indicative of the hydrophobic membrane region-was shifted to higher temperatures, with the iso isomer triggering more pronounced changes as compared to the ante-iso isomer. This is probably due to a higher packing density in the core of the lipid bilayer, which causes reduced diffusion across membranes. By utilizing the naturally occurring spin reporters nitrogen-14 and phosphorus-31 present in the hydrophilic DMPC headgroup region, even fatty acid induced changes at the membrane interface could be detected, an observation reflecting changes in the lipid headgroup dynamics.

Synthesis of new fluidity-enhanced amphiphilic compounds for soluble protein two-dimensional crystallization purpose

The synthesis of new amphiphilic compounds is described. The structures are rationally designed for soluble protein two-dimensional (2D) crystallization purpose. Special attention is devoted to fluidity properties expected of resulting monolayers. A series of 13 compounds was prepared containing unsaturated, branched or fluorinated alkyl chains. Structures are either symmetrical or dissymmetrical and present a hydroxyl group as polar head, eventually complemented with two other 'secondary' hydrophilic functions.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Evidence that nonbilayer phase propensity of the membrane is important for the side chain cleavage activity of cytochrome P450SCC

To analyze whether specific protein-lipid interactions or physical features of the membrane contribute to cytochrome P450SCC (CYP11A1) activation by lipids, dimyristoylphosphatidylcholine/cardiolipin and dimyristoylphosphatidylcholine/branched phosphatidylcholine vesicles of defined acyl chain structure were studied for their ability to stimulate the side chain cleavage activity of the enzyme. Activation was found to increase with the mole percent of nonbilayer lipids in the system and the chain lengths of both the branched and main fatty acyl chains of the activator lipid. Unsaturation provided by dioleoylphosphatidylcholine as host lipid leads to a further increase in the potency of the branched phosphatidylcholines to activate the enzyme. The observed activation can be qualitatively interpreted in terms of the effect of these lipids on the hydrophobic volume of the membrane. Using differential scanning calorimetry, we showed that the branched phosphatidylcholines perturb the bilayer membrane structure of dimyristoylphosphatidylcholine and lower the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine, i.e., promote hexagonal phase formation. We also examined the effect of eicosane on both the cytochrome P450SCC activity and the lipid polymorphism and found that eicosane increases both the activity and the hexagonal phase propensity of the vesicle membrane. Because of these correlations, we conclude that the nonbilayer phase propensity of the membrane rather than specific binding of activator lipids to the enzyme explains best the observed activation of enzymatic activity by the lipids.

Properties of unusual phospholipids, II: Synthesis, NMR studies and monolayer investigations of diacylglycerophosphocholines containing alpha-branched acyl chains

Diacylglycerophosphocholines, containing various alpha-branched fatty acids, were synthesized and purified to homogeneity. The influence of the alpha-branched acyl chains on the monolayer packing properties of these phospholipids was studied by the Langmuir-Blodgett film balance technique. NMR investigations were used for conformational studies, including the determination of rotamer populations about the dihedral angles (theta 2, theta 4 and alpha 1) in the glycerol region of the diacylglycerophosphocholines.

Regulation and phase equilibria of membrane lipids from Bacillus megaterium and Acholeplasma laidlawii strain A containing methyl-branched acyl chains

Phosphatidylethanolamine (PE) was isolated from Bacillus megaterium grown at 20 and 55 degrees C (PE-20 and PE-55). Iso and anteiso methyl-branched, saturated acyl chains are predominant in B. megaterium, and the value of the molar ratio of iso/anteiso acyl chains is more than 20-fold higher in PE-55 than in PE-20. Moreover, about 21 mol% of the acyl chains of PE-20 are monounsaturated. The phase equilibria differ between the two PE preparations: (1) PE-20 is more prone to form reversed nonlamellar phases than PE-55; (2) PE-20 forms both reversed cubic (I2) and reversed hexagonal (H(II)) phases while PE-55 forms only an HII phase; and (3) the lamellar liquid-crystalline (L alpha) phase of PE-20 takes up about 70% more water than the L alpha phase of PE-55. These differences can be explained by the differences in the acyl chain composition. When the growth temperature is raised, PE molecules with a reduced tendency to form nonlamellar phases are probably synthesized by B. megaterium in order to counteract the bilayer destabilizing effect of the temperature. The regulation of the acyl chain composition is not needed in order to regulate the temperature for the transition between gel/crystalline and L alpha phases of the membrane lipids. Acholeplasma laidlawii strain A-EF22 was grown at 37 degrees C on 15-(1,1,1(-2) H3)methylhexadecanoic acid, 14-(1,1,1(-2)H3)methylhexadecanoic acid or 13-(1,1,1(-2)H3)methylhexadecanoic acid, and these acids constituted 84-89 mol% of the acyl chains in the membrane lipids. The molar ratio between the two dominating lipids, monoglucosyldiacylglycerol (MGLcDAG) and diglucosyldiacylglycerol (DGlcDAG), decreased, and the molar fraction of the anionic lipids increased, when the methyl branch was moved from position 15 to position 13. Concomitantly, the order of the methyl branch increased in cells as well as in total lipid extracts. The phase equilibria of total lipid extracts (neutral lipids removed) were studied with 20 wt % of water, and HII and I2 phases were formed above 63-67 degrees C. These results indicate that the regulation of the polar head-group composition compensates for the difference in acyl chain packing introduced into the bilayer by the three branched-chain fatty acids. The regulation of the polar head-group composition of the A. laidlawii lipids cannot regulate the temperature for the transition between gel/crystalline and L alpha phases of the lipids, i.e. the transition to fluid acyl chains.(ABSTRACT TRUNCATED AT 400 WORDS)

The interaction of various cholesterol 'ancestors' with lipid membranes: a 2H-NMR study on oriented bilayers

The effect of putative cholesterol 'precursors' on model membranes has been studied by deuterium nuclear, magnetic resonance (2H-NMR) spectroscopy. Oriented bilayers were prepared from 1-myristoyl-2-[2H27 myristoyl-sn-glycero-3-phosphocholine (DMPC-d27) and tricyclohexaprenols or octaprenediols. Order parameter profiles were determined and showed that tricyclohexaprenols and octaprenediols increase the acyl chain order in DMPC bilayers, but to a smaller extent than cholesterol. The order parameter increases, depending on the chain position, from 5% to 7% in the presence of ditertiary octaprenediol, and from 16% to 21% in the presence of tricyclohexaprenol-Z,Z. Aqueous multilamellar dispersions of DMPC-d27 and of DMPC-d27 containing 30 mol% tricyclohexaprenol-E,E were prepared, and the first moments calculated from 2H-NMR spectra over the temperature range 5-55 degrees C. Tricyclohexaprenol-E,E almost abolishes the phase transition of DMPC. Thus, as predicted, tricyclohexaprenols and octaprenediols have a cholesterol-like behaviour in lipid membranes; however their effect on the model DMPC system is weak. On the contrary, isoarborinol has no effect on the lipid chain order in the liquid-crystalline phase of DMPC bilayers. 2H-NMR spectra of aqueous dispersions of DMPC-d27 and 30 mol% isoarborinol between 25 and 60 degrees C showed the coexistence of two lamellar phases over a wide temperature range, which was confirmed by differential scanning calorimetry (DSC) and 31P-NMR spectroscopy. This absence of ordering effect of isoarborinol might be related to some inherent structural features.

Purification and some properties of the squalene-tetrahymanol cyclase from Tetrahymena thermophila

The membrane-bound enzyme from Tetrahymena thermophila responsible for the conversion of squalene into the quasi-hopanoid tetrahymanol was purified 297-fold to near homogeneity. Purification involved solubilization by octylthioglucoside, chromatography on DEAE-trisacryl, hydroxyapatite and FPLC ion-exchange on Mono Q. The apparent KM was found to be 18 microM. 2,3-Iminosqualene and N,N-dimethyldodecylamine-N-oxide are effective inhibitors of the cyclase with I50 values of 50 and 30 nM, respectively. The cyclase has a molecular mass of 72 kDa as judged by electrophoresis in polyacrylamide gels under denaturating conditions. The optimal enzymatic activity was obtained at pH 7.0 and 30 degrees C. The solubilized enzyme needs the presence of detergent for maintaining activity. The influence of different detergents on cyclase activity was studied. Triton X-100 proved to be a strong inactivator of the enzyme. Solubilization of the cyclase in Tween 80 and digitonin inactivates the enzyme. However, its activity can be recovered by complementation of the assay buffer with octylthioglucoside above its critical micellar concentration. We suggest that this approach might be applicable to other membrane-bound proteins.

Phase transitions of polymerizable phospholipids

Polymerizable lipids have received considerable attention in the last ten years as polymerization of lipids in vesicle systems is a possibility to increase the stability of lipid bilayers. Lipids with various polymerizable groups have been synthesized in the last years. This paper is focussed on those lipids which are closely related to natural phospholipids, i.e. molecules which have two hydrophobic chains and a head group containing a phosphate moiety. The phase behaviour of polymerizable phospholipids as lipid monomers and in the polymerized state is reviewed and discussed.

Intracellular sterol trafficking

Sterols are acquired by cells either biosynthetically by the interaction of cytoplasmic and endoplasmic reticulum elements, or by endocytosis. The subcellular distribution of sterols, however, argues that sterols are trafficked quickly from sites of acquisition to target membranes, particularly the plasma membrane. The mechanisms mediating this movement might include aqueous diffusion, vesicles of either a unique pathway or of the protein secretory pathway, or carrier proteins. These mechanisms are discussed and the limited data concerning each are presented. Finally, a theory is proposed which describes how sterols and other membrane reinforcing molecules might have driven the evolution of intracellular membranes, thus establishing the dynamic membrane system of modern eukaryotes.

Permeability studies of lipid vesicles from alkalophilic Bacillus firmus showing opposing effects of membrane isoprenoid and diacylglycerol fractions and suggesting a possible basis for obligate alkalophily

Previous studies of the membrane lipids of extremely alkalophilic bacilli had indicated that both facultative and obligate alkalophiles contained a substantial fraction of isoprenoid lipid as well as high concentrations of cardiolipin. Facultative alkalophiles differed from obligate strains in having a phospholipid fatty acid composition that would be expected to result in a more ordered membrane structure. Current studies of ion permeability in vesicles prepared from lipids from obligately alkalophilic Bacillus firmus RAB and its facultatively alkalophilic strain, OF4, support the suggestion that membranes of the latter strain form a tighter barrier structure, with the difference especially pronounced at near neutral pH values. The water permeability of whole cells and the reflection coefficients for acetamide in vesicles were also consistent with a tighter membrane in the facultatively alkalophilic strain than in the obligately alkalophilic strain. The permeability properties of vesicles prepared from phospholipids from these organisms were studied as a function of the addition of either homologous membrane isoprenoid or diacylglycerol. For each permeability parameter that was assayed, in lipids from both strains, the isoprenoid fraction decreased the permeability, whereas the diacylglycerol fraction increased the permeability of the vesicles to solute.

The bioenergetics of alkalophilic bacilli

A summary, cum speculation, of the major bioenergetic characteristics of alkalophilic bacilli is presented in Figure 5. Further progress will depend heavily on the purification and characterization of the relevant proteins that catalyze the ion fluxes and on the development of much more potent genetic approaches to the outstanding issues of this interesting group of bacteria.

Physical properties of membrane lipids isolated from a thermophilic eubacterium (Thermus sp.)

Membranes from a thermophilic eubacterium, Thermus sp. strain SPS 11, isolated from thermal springs of São Pedro do Sul spa (Portugal), are characterized for having two main polar lipids, a glycolipid (GL) with four monosaccharide residues, which at 73 degrees C accounts for 95% of the carbohydrate in the total lipid extracts, and a glycophospholipid (PL) which at 73 degrees C accounts for about 90% of the lipid phosphorous. A complex mixture of carotenoids (CA) makes up 11% by weight of the total membrane lipids. The branched fatty acyl chains (iso C15 and iso C17) comprise about 90% of the alifatic moieties of the polar lipids of this bacterium. Moreover, when the growth temperature increases from 50 to 73 degrees C there is an increase of the iso C17/ iso C15 ratio and of the GL/PL ratio. We have studied the biophysical properties of bilayers (as multilamellar liposomes) prepared from GL, PL and the mixtures of PL, GL and CA in proportions found in the membranes of bacteria growing at their optimal growth temperature, using polarization of DPH fluorescence, low and wide-angle X-ray diffraction and differential scanning calorimetry. The three techniques agree in showing the presence of a broad phase transition from a gel (L beta) phase to a liquid-crystal (L alpha) phase between 8 and 30 degrees C, for all the lipid dispersions studied except for the GL. Although all the dispersions studied form a bilayer structure at all the temperatures studied, only the mixture of the three components (PL, GL + CA) avoids the phase separation present in the mixtures of PL + CA at temperatures lower than 30 degrees C and PL + GL at temperatures lower than 55 degrees C. Our results are compared with those of Pinheiro et al. (1978) obtained with the 31p-NMR technique and applied to the study of the same samples.

Lipid shape as a determinant of lipid composition in Clostridium butyricum. The effects of incorporation of various fatty acids on the ratios of the major ether lipids

The lipid composition of Clostridium butyricum is strongly influenced by the aliphatic chain compositions of the membrane lipids. Growth on cis-monounsaturated fatty acids in the absence of biotin was shown to affect the relative proportions of phosphatidylethanolamine, plasmenylethanolamine, and the glycerol acetal of plasmenylethanolamine most strongly, with smaller effects on the acidic lipids, phosphatidylglycerol and cardiolipin. The ratio of the glycerol acetal of plasmenylethanolamine to total phosphatidylethanolamine in cells grown on a series of fatty acids is shown to decrease in the following order; cis-vaccenic acid greater than or equal to oleic acid = C19-cyclopropane fatty acid greater than linoleic acid greater than petroselinic acid greater than elaidic acid greater than 14-methylhexadecanoic acid (anteiso-C17) greater than 12-methyltridecanoic acid (iso-C14). All fatty acids were extensively incorporated into the lipid acyl, alkenyl, and alkyl chains. There was considerable chain-elongation of the iso-C14 to iso-C16. The results are consistent with the hypothesis that the membrane lipid composition is strongly influenced by lipid shape and that the observed changes in lipid composition serve to stabilize the bilayer arrangement of the cell membrane.

Dilatometric studies of isobranched phosphatidylcholines

Absolute apparent specific volumes have been obtained for phosphatidylcholine lipids with saturated, isobranched hydrocarbon chains with ni = 15 to 20 carbons, with an emphasis upon phase transition behavior, both equilibrium and kinetic. The temperature of the chain-melting transition extrapolates with increasing chain length to the melting temperature of polyethylene with a small odd/even alternation. There are also odd/even alternations in the volume of transition and in the hysteresis of the chain-melting transition, but with the odd and even reversed when compared with the larger odd/even alternation in the lower solid-solid transition that occurs in the longer chain ni lipids. A phenomenological picture is given for the coalescence of the two transitions for shorter ni lipids and this picture is used to sharpen the discussion of the kinetic mechanism of melting. A temperature-reversal experiment shows that the melting from the lowest temperature crystal or C phase to the fluid F phase does not proceed via the metastable gel G phase for 16i. The dilatometric results are combined with recent X-ray structural results for the C and G phases of 17i and 20i to deduce various structural information, including the hydration numbers and the volume of the headgroup, VH = 341 A3, which agrees very well with VH for straight-chain phosphatidylcholines. For the chain-melted F phase the assumption that the methylene volumes of the different ni lipids should be the same at the same temperature is used to obtain the volumes of the methylene and the methyl groups.

Effects of lipid fatty acyl chain structure on the activity of the (Ca2+ + Mg2+)-ATPase

The (Ca2+ + Mg2+)-ATPase purified from rabbit muscle sarcoplasmic reticulum has been reconstituted into a series of phosphatidylcholines in the liquid crystalline phase. For phosphatidylcholines containing monounsaturated fatty acyl chains, optimal activity is observed for a chain length of C18, with longer or shorter chains supporting lower activities. Phospholipids with methyl-branched chain saturated fatty acids support somewhat lower activities than the corresponding phospholipids with mono-unsaturated fatty acids. Mixed chain phospholipids support ATPase activities comparable to those shown by an unmixed chain phospholipid with the same average chain length. However, the response of the ATPase reconstituted with mixed chain phospholipids to the addition of oleyl alcohol is dominated by the longest fatty acyl chain. Based on their ability to displace brominated phospholipids, relative binding constants to the ATPase of a series of phosphatidylcholines have been determined. Binding to the ATPase is virtually unaffected by fatty acyl chain length or the presence of methyl branches.

Properties of lecithin-dodecaprenol macrovesicular bilayer membranes

The ionic transport properties, capacitance and breakdown voltage of bilayer macrovesicles made from lecithin, dodecaprenol and their mixtures have been studied. The electrical measurements showed that polyprenol in lipid bilayers increases membrane permeability and elasticity, and decreases membrane thickness. Some physiological implications of these findings are indicated.