Hydrocortisone is a synthetic form of cortisol, a corticosteroid hormone produced in the adrenal glands, from cholesterol. When applied directly to the skin, it can reduce inflammation and temporarily relieve minor skin irritations, itching, and rashes by reducing the secretion of histamine, a compound produced by cells of the immune system in response to the presence of pathogens or other foreign substances. Lipids are a naturally occurring group of substances that are not soluble in water but are freely soluble in organic solvents.
A triglyceride is formed by joining three glycerol molecules to a fatty acid backbone in a dehydration reaction. Skip to main content. Microbial Biochemistry. Search for:. Lipids Learning Objectives Describe the chemical composition of lipids Describe the unique characteristics and diverse structures of lipids Compare and contrast triacylglycerides triglycerides and phospholipids.
Describe how phospholipids are used to construct biological membranes. Think about It Explain why fatty acids with hydrocarbon chains that contain only single bonds are called saturated fatty acids.
Think about It How is the amphipathic nature of phospholipids significant? This video provides additional information about phospholipids and liposomes:. Think about It How are isoprenoids used in technology? Key Concepts and Summary Lipids are composed mainly of carbon and hydrogen, but they can also contain oxygen, nitrogen, sulfur, and phosphorous. They provide nutrients for organisms, store carbon and energy, play structural roles in membranes, and function as hormones, pharmaceuticals, fragrances, and pigments.
Fatty acids are long-chain hydrocarbons with a carboxylic acid functional group. Their relatively long nonpolar hydrocarbon chains make them hydrophobic. Fatty acids with no double bonds are saturated ; those with double bonds are unsaturated. Fatty acids chemically bond to glycerol to form structurally essential lipids such as triglycerides and phospholipids.
Triglycerides comprise three fatty acids bonded to glycerol, yielding a hydrophobic molecule. Phospholipids contain both hydrophobic hydrocarbon chains and polar head groups, making them amphipathic and capable of forming uniquely functional large scale structures.
Biological membranes are large-scale structures based on phospholipid bilayers that provide hydrophilic exterior and interior surfaces suitable for aqueous environments, separated by an intervening hydrophobic layer. These bilayers are the structural basis for cell membranes in most organisms, as well as subcellular components such as vesicles.
Isoprenoids are lipids derived from isoprene molecules that have many physiological roles and a variety of commercial applications. A wax is a long-chain isoprenoid that is typically water resistant; an example of a wax-containing substance is sebum, produced by sebaceous glands in the skin. Steroids are lipids with complex, ringed structures that function as structural components of cell membranes and as hormones.
Bacteria produce hopanoids, structurally similar to cholesterol, to strengthen bacterial membranes. Fungi and protozoa produce a strengthening agent called ergosterol. Multiple Choice Which of the following describes lipids?
All of the options describe lipids. Show Answer Answer b. Molecules bearing both polar and nonpolar groups are amphipathic. The principle that relates these behaviors is the tendency for nonpolar regions to clump together, seqestering these regions as much as possible from the aqueous environment.
The carboxylate groups and associated ions orient to interact with the aqueous phase. When one shakes up a soap solution, soap bubbles form - something that can be explained by the same relatively simple physiochemical principle summarized by the phrase "like interacts with like". Triacylglycerols also called triacylglycerides or triglycerides are a common, simple type of lipid consisting of three long-chain fatty acids esterified to glycerol, a three-carbon triol.
The figure at right shows a typical triacylglycerol as a structural bond-line formula. Carbon atoms are indicated at vertices and ends of lines. Hydrogen atoms are implicit. The three-carbon glycerol backbone is drawn in black.
Three fatty acids green, red, blue are shown in ester linkages with glycerol. The green fatty acid chain is from palmitate, a carbon saturated fatty acid. Triacylgycerols are mostly carbon and hydrogen, giving them a predominently nonpolar character. The ester linkages of the molecule give it a somewhat polar end. In contrast to the soaps discussed above, other lipids common in biological membranes have a larger van der Waals cross section and cannot approach one another close enough to form micelles.
Instead, they spontaneously form a lipid bilayer. At right is shown the structural formula and cartoon versions of generic membrane phospholipids. A membrane phospholipid typically consists of a glycerol backbone, two fatty acid chains in ester linkages to glycerol, a phosphate diester linkage between glycerol and a number of possible alcohols ROH.
It is the presence of two nonpolar fatty acid chains in phospholipids in contrast to the single chain of soaps that favors bilayer over micelle formation for steric reasons.
The first fatty acid is of the saturated variety, while the second is typically an unsaturated fatty acid chain. The cis configuration of the latter confers a rigid kink in the chain, thus the first phospholipid cartoon is more stereochemically accurate, but the second cartoon is more suited to cartoon versions of bilayers and biological membranes.
The existence of cells is obviously dependent on creation of a boundary that defines an inside compartment of controlled composition and character, and separates the cell from the surrounding uncontrolled environment. Cellular membranes are topologically closed surfaces based upon phospholipid bilayers that perform this bounding function. Biological membranes act as physical barriers that generally limit the passage of charged and polar species, as well as the macromolecules central to living systems.
The amphipathic nature of phospholipids is responsible for the spontaneous formation of the bilayer structure of membranes. This maximum is reached when the aLD is connected to the bilayer, which behaves as a PL reservoir. Therefore, LDs are more preset to recruit AHs than bilayers.
However, our data show that this is not the whole picture since the nature of the hydrophobic phase, to which an AH has to bind, plays a crucial role in the recruitment. This hydrophobic phase would be PL acyl chains for a bilayer and neutral lipids for LDs. AHs may preferentially bind to LD subsets containing specific neutral lipids Hsieh et al. This is would be consistent with our data indicating that AHs may have differential affinities with neutral lipids, or at least to their interface with water Fig.
These results support that the amphipathic sequence is crucial to the interactions with neutral lipids, beyond decreasing interfacial tension. The question is therefore how amino acids can discern hydrophobic regions. Hence, increasing the number of hydrophobic residues would basically decrease tension and hence annihilate specificity, as seen, for example, for NS5A Fig. This seems to be at least the case for the most abundant LD proteins, Perilipin 1—5, whose mer repeat AH sequence lacks bulky hydrophobic residues but can specifically detect LD surface Ajjaji et al.
For instance, Perilipin 3 seems to better bind to LDs in the presence of diacylglycerols Skinner et al. In cells, additional regulatory means can control the recruitment of AHs Kory et al. For example, crowding of the LD surface by strongly bound proteins, such as Perilipin 1, or hairpin-containing proteins, enables prevention of the nonspecific adsorption of soluble AH-containing proteins Ajjaji et al.
Interactions with PL headgroups may also facilitate AH recruitment specificity. This AH has a poorly developed hydrophobic face that needs the presence of charged PLs to reveal its ability to target to membrane packing defects Pranke et al. In conclusion, our data highlight an undervalued contribution of neutral lipids in controlling the binding of AHs to the surface of LDs. The PL packing density simply regulates the amount of exposed neutral lipids.
Clearly, the full picture of how AHs selectively bind to LDs in a cellular context remains not completely resolved yet, but our data bring us a major step closer to it. GUVs were prepared by electro-formation Thiam et al.
PLs and mixtures thereof in chloroform at 0. The lipid film was desiccated for 1 h. The chamber was sealed with another indium tin oxide—coated glass plate. Electro-formation is performed using Hz AC voltage at 1.
This low voltage was used to avoid hydrolysis of water and dissolution of the titanium ions on the glass plate. The mixture was sonicated. The diameter of the resulting droplets is on the order of a few hundred nanometers. Micropipettes were made from capillaries drawn out with a Sutter Instruments pipette puller. The suction was performed using a syringe.
The resulting pressure was measured with a pressure transducer, the output voltage of which was monitored with a digital voltmeter. The pressure transducer range, 55 kPa was calibrated before the experiments. Micropipettes were made from capillaries 1. The micromanipulators used were TransferMan 4r, purchased from Eppendorf. The DEVs were held by a glass micropipette and observed by confocal microscopy. Solutions were injected at similar spots from a pipette. Note that for Arfgap1-AH, due to peptide solubility issues, the buffer pH was lowered to 5.
After 5 min, the red fluorescent histidine polluting the coverslip in the observation zone was laser bleached. DEV were brought with a micromanipulator into the clean observation zone and imaged at the apex, i. All experiments were conducted at room temperature. To create oil-buffer interfaces, oil drops from 5 to After forming an oil droplet, the surface tension of the interface is measured until it stabilizes to a plateau.
This plateau of tension is the tension designated as the bare oil-buffer surface tension. After forming a droplet, the PLs relocate at the interface.
Sinusoidal area oscillations are applied to the droplet for 10 cycles as the interfacial tension is recorded. This isotherm is then fitted using the theoretical Frumkin isotherm and enables us to get an estimation of the molecular area of PLs at the interface. To simultaneously observe a micrometric-size droplet constituted of different oil under the confocal microscope, we used two micropipettes, prefilled with two different oil types, and connected to syringes.
The pipettes were introduced in a buffer medium placed under a confocal microscope. By pressing on the syringes, multiple droplets of two different compositions were formed in the observation field. The peptide was then added to the medium and peptide recruitment observed by fluorescence. DOPC PLs in chloroform were dried under argon and oil was added in such proportion to obtain a molar ratio of 0. GUVs were then imaged over time. GUV radius was measured over time.
Surface tension of oil buffer interface was measured using pendent droplet tensiometer except for silicone oil tension from Mazurek, , PFOB Astafyeva et al. To quantify the recruitment of the fluorescent peptides at the surface of LDs, we used the radial angle profile plugin of ImageJ software. This plugin measures the average signal intensity along the perimeter of concentric circles. It results in a plot of the intensity profile of a circular object for various positions relative to its center.
We chose the maximum intensity profile as a measurement of recruited peptide density. The same method was used to quantify the PL density covering oil droplet. Due to experimental limitations, the DEVs used in this study were mainly imaged along their equatorial cross-section, an easier configuration when using micropipettes for manipulation.
However, when measuring the fluorescence signal of aLDs at this equatorial level, we found a significant attenuation as compared with the signal measured at the apex of the droplet Fig. To investigate this attenuation effect, we imaged a single oil droplet homogeneously marked with histidine mCherry Fig. We noticed that the intensity of confocal fluorescence signal was varying along the z axis Fig.
S2 D , left. We quantified this signal modification with intensity-linescans at different z positions see Fig. S2 D , right. The signal was maximum at the lower apex plane, i. We reasonably assumed that this effect was due to the refraction of the laser light by the refractive index variation at the droplet—buffer interface. Indeed, when imaged at the lower apex, the light beam from the microscope objective is directly reflected back to the imaging system.
On the contrary, when the droplet is imaged at or above the equatorial cross-section, part of the light has to pass through the oil phase of the droplet and is certainly disrupted by the optical index mismatch, leading to the observed signal attenuation Fig. S2, A and D. Therefore, the signal measured at the apex not affected by the optical index mismatch is to be considered as the correct measurement of the fluorescent signal on the droplet.
On the contrary, to get the real signal on the droplet at the equatorial cross-section, the measured signal has to be corrected. To quantify the signal attenuation at the equatorial cross-section, we imaged various DEV at the equatorial cross-section and the apex. We normalized the droplet signal by the bilayer signal Fig. S2 B and compared the normalized signal measured at the apex and the equatorial cross-section.
We found an average of 2. S2 C , which was therefore equal to 2. As some of the DEVs used in this study were made with SQ oil droplets, we performed similar experiments on such systems and found comparable results Fig. The fluorescence signal attenuation between apex focal plane 1 and equatorial cross-section focal plane 2 was found at 2. S2 E , right , a value comparable to those obtained with TO droplets in Fig. The statistical comparisons were made using a nonparametric t test GraphPad Prism 7.
Micromanipulators TransferMan 4r were purchased from Eppendorf. The pendent droplet tensiometer apparatus was from Teclis Instruments. Dioleine C, 10 mg dioleyl-sn-glycerol 18—1 was from Avanti Polar Lipids.
Plin1 —, purity Penin, University of Lyon, Lyon, France. Fluorescently labeled Arf1 was generated using an Arf1 variant in which the single cysteine residue of Arf1 was replaced with serine, and the C-terminal lysine was replaced with cysteine, yielding Arf1-CS-KC. Published work has demonstrated that exchanging the C-terminal lysine of the small GTPase with a Cys residue, and subsequent fluorescent labeling using thiol-reactive dyes on Cys does not inhibit Arf1 function Beck et al.
To remove excess dye, samples were purified by gel filtration using a Superdex 75 column GE Healthcare. The figure is related to Fig. S3 displays complementary experiments for the determination of the PL density in DEV by using tagged PLs and the pending droplet experiment.
Dodecane and TO-SE oil mixtures are investigated. S4 presents the pendent droplet experimental raw data used for the determination of the area per PL as a function of the lateral pressure. S5 displays confocal micrographs corresponding to the quantification of recruitment of AHs to bare oil droplets of various oil compositions.
The recruitment as a function of tension is presented. We are thankful to all the group members for their valuable comments and critical discussions. We also thank Dr. Alain Cagna for helpful discussion and Dr. Alenka Copic for the critical read of the manuscript. Author contributions: A. Thiam designed research. Chorlay performed experiments. Thiam and A. Chorlay analyzed data and wrote the manuscript.
AHs differentially bind to aLDs emerging from a bilayer and made of different neutral lipids. Example of a DEV is shown on the right. D A solution of fluorescently tagged peptide is added to a DEV. The peptide is recruited to the DEV interfaces. See also Fig. S1, B, C, and E. SQ and TO droplet enrichment index are displayed for comparison. Related to Fig.
Related to quantification of Fig. Histidine mCherry fluorescence on the droplet and the bilayer was quantified at each cross-section. B Histidine mCherry fluorescence signal on the droplet normalized by signal on the bilayer quantified at each cross-section. Right: Fluorescence signal was quantified, and revealed significant signal attenuation at the equatorial cross-section. Right: Histidine mCherry fluorescence ratio between the droplet and the bilayer was quantified at each cross-section.
The PL packing level is identical on aLDs emerging from a bilayer, independently of neutral lipid composition tested. After min incubation with mCherry-histidine, the GUV membrane and aLD surface are visualized simultaneously at their apex same focal plane.
Control experiment without nickel lipid is shown in Fig.
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