Lipids And Cell Membranes
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Lipids and cell membranes Topics
1.
Lipids: Introduction
2.
Classification of Lipids
3.
Organization of Biomembranes
4.
Fluid Mosaic Model of Biological membrane
1. Lipids: Introduction Lipids: Lipids can be defined on the basis of a physical property of “solubility”. Lipids are biological molecules extracted from living organisms which are soluble in organic solvents, such as methanol, and are sparingly soluble in water. Propionate, Limonene, Squalene, Chrysin and Vitamin E are some the popular examples of lipids which are insoluble in water and soluble in organic solvents. The insolubility in water is due to the high proportion of carbon and hydrogen present in these molecules. In short, Lipids are hydrophobic compounds, soluble in organic solvents. Most of the lipids present in the membrane are amphipathic, which means having both the non-polar end and polar end. Fatty Acids: Fatty acids are made up of a hydrocarbon chain having a carboxylic acid at one end. Some of the common examples of fatty acids are myristic acid, palmitic acid, stearic acid oleic acid, linoleic acid and arachidonic acid. Structure: Fatty acids can be saturated or unsaturated. Saturated fatty acids contain no double bonds in the acyl chain whereas unsaturated ones contain either one double bond (monounsaturated) or multiple double bonds (polyunsaturated).
Common Saturated Fatty Acids: Acetic Acid
CH3COOH
Propionic Acid
C2H5COOH
Butyric Acid
C3H7COOH
Lauric Acid
C11H23COOH
Myristic Acid
C13H27COOH
Palmitic Acid
C15H31COOH
Stearic Acid
C17H35COOH
Archidonic Acid
C19H39COOH
Lignoceric Acid
C23H47COOH
Common Unsaturated Fatty Acids: Oleic Acid is a common monounsaturated fatty acid containing one double bond. Linoleic acid and arachidonic acid are polyunsaturated fatty acids having more than one double bond. Common name
Structure
Palmitoleic acid
CH3(CH2)5CH=CH-(CH2)7COOH
Oleic acid
CH3(CH2)7CH=CH-(CH2)7COOH
Linoleic acid
CH3(CH2)4(CH=CHCH2)2(CH2)6COOH
α-Linolenic acid
CH3CH2(CH=CHCH2)3(CH2)6COOH
Arachidonic acid
CH3(CH2)4(CH=CHCH2)4(CH2)2COOH
Fatty acid Nomenclature: Fatty acids can be named in a number of ways; mostly used are common name (denoting their origin and place of occurrence etc), symbolic name (based upon the structure and arrangement of carbon atoms), and systematic name (based on IUPAC nomenclature).
Common name: Oleic acid got its common name as it is present abundant in olive oil.
Symbolic name: Symbolic name can be written as x:y (Δ a,b,c) where x is the number of Carbons in the chain, y is the number of double bonds. a, b, and c represent the positions of the start of the double bonds from C1,the carboxyl C. • • • • •
Saturated fatty acids contain no C-C double bonds. Monounsaturated fatty acids contain one C=C bond. Polyunsaturated fatty acids contain more than 1 C=C bond. cis configuration is present usually in the double bonds in fatty acids. Almost all the fatty acids occurring in nature have an even number of carbon atoms.
Systematic Name: Systematic name is given using IUPAC nomenclature. It gives the number of carbons present in a particular fatty acid. If the fatty acid is unsaturated, the base name reflects the number of double bonds. Examples:
Common name: α-linolenic acid Systematic name: octadecatrienoic acid Symbolic Representation: 18:3Δ 9,12,15
Common name: palmitic acid Systematic name: hexadecanoic acid Symbolic Representation: 16:0 (here the number of double bonds is 0 since it is a saturated fatty acid.
2. Classification of Lipids Depending on their reactivity with strong bases, lipids are classified into two major classes, saponifiable lipids and nonsaponifiable lipids. Saponification: Saponification is the process in which soap is produced from the reaction between lipids and a strong base.
Saponifiable Lipids: The saponifiable lipids consist of long chain fatty acids, esterified to a “backbone” molecule. This back bone molecule can be either glycerol or sphingosine.
The three major saponifiable lipids are triacylglycerides, glycerophospholipids, and the sphingolipids. •
Triacylglycerides contain three fatty acids esterified to the three OHs on glycerol.
•
Glycerophospholipids have two fatty acids esterified at first and second and a phospho-X group esterifed at then third carbon.
For both Triacylglycerides and Glycerophospholipids, glycerol serves as the backbone molecule.
•
Spingolipids possess ‘Spingosine’ as the backbone, which has a long alkyl group connected at C1 and a free amine at C2. In this type of saponifiable
lipids, at C2, a fatty acid is attached through an amide link, and a H or esterified phospho-X group is present at C3. Non Saponifiable Lipids: Non Saponifiable Lipids do not undergo saponification and the most common examples of cholesterol, vitamin D2, and α – tocopherol. Among these, cholesterol is an important constituent of cell membranes. It consists of a stiff ring system and a short branched tail of hydrocarbon.
+ Glycerophospholipids: Glycerophospholipids (phosphoglycerides) are common constituents of cellular membranes. They possess a glycerol backbone and the hydroxyls at C1 & C2 of glycerol are esterified into fatty acids.
Formation of ester linkage: An ester is formed when a hydroxyl reacts with a carboxylic acid, with the reduction of a water molecule.
In phosphatidate, fatty acids are esterified to the hydroxyls on C1 and C2, while the C3 hydroxyl is esterified to phosphate. In most phosphoglycerides, the phosphate is in turn esterified to an alcohol of one of the polar head groups like ethanolamine, serine, glycerol, choline or inositol. Phosphatidylinositol: Phosphatidylinositol is a glycerophospholipid with inositol as polar head group.
Phosphatidylinositol is a significant membrane lipid. It also serves an important role in cell signaling.
Phosphatidylcholine: Phosphatidylcholine is another glycerophospholipid with choline as its polar head group. It is a common membrane lipid.
The normal constituents of a glycerophospholipid are (i) a polar region consists of glycerol, carbonyl oxygen atoms of fatty acids, phosphate, and the polar head group.
(ii) non-polar region which includes the hydrocarbon tails of fatty acids. Sphingolipids: Sphingolipids are derivatives of the lipid sphingosine which has a long hydrocarbon tail, and a polar domain that comprises an amino group. Normally the other derivatives of sphingosine are components of biological membranes. Sphingosine-1-phosphate: Sphingosine-1-phosphate is an important signal molecule and it is produced by the reversible phosphorylation.
Sphingomyelin: Sphingomyelins are common constituents of plasma membranes and have a phosphocholine or phosphoethanolamine head group. •
A glycerophospholipid phosphatidyl choline can be comparable in size and shape to a Sphingomyelin, possessing a phosphocholine head group.
Glycosphingolipids:
A ganglioside is a ceramide (sphingolipid) with a polar head group that is a complex oligosaccharide, including the acidic sugar derivative sialic acid. Gangliosides are very common in the brain.
A cerebroside is a sphingolipid with a monosaccharide such as glucose or galactose as polar head group.
Sulfatides are subtypes of cerebrosides which are sulfate derivatives of the galactose residues.
Cerebrosides and gangliosides, are collectively called glycosphingolipids. They are present in the outer leaflet of the plasma membrane bilayer, having their sugar chains extending out from the cell surface. Amphipathic lipids associates with water to form complexes in such a way that their polar regions are in contact with water and their hydrophobic regions are away from water.
3. Organization of Biomembranes The phospholipid bilayer forms the fundamental structure of all cellular membranes. It consists of two layers of phospholipid molecules. The fatty acyl tails of the phospholipid molecules form the hydrophobic interior of the bilayer and their hydrophilic head line the peripheral surface of the bilayer on both sides. Hence it can be stated that the phospholipid bilayer is the most stable configuration for amphipathic lipids with a cylindrical shape (phospholipids). The lipid bilayer interior is normally in high fluid state. Liquid crystal state: In the liquid crystal state, hydrocarbon chains of phospholipids are disordered and in constant motion. Crystalline state: At lower temperatures, a membrane containing a single phospholipid type undergoes transition into a crystalline state. In this state, the following phenomenon can be noticed.
Completely extended fatty acid tails Highly ordered packing Maximal van der Waals interaction between adjacent chains.
Cholesterol: Cholesterol, a highly hydrophobic molecule is an important constituent of cell membranes. It is present abundant in the plasma membrane of the mammalian cells and it is absent from most of the prokaryotic cells. It is characterized by a rigid ring system and a short branched hydrocarbon tail. The presence of one polar group and a hydroxyl makes it amphipathic. Cholesterol inserts itself into bilayer membranes in such a way that its hydroxyl group is oriented toward the aqueous phase and its hydrophobic ring system is contiguous to fatty acid tails of phospholipids. Formation of hydrogen bonds takes place between the hydroxyl group of cholesterol and the polar phospholipid head groups.
The major portion of cholesterol molecule is hydrophobic but only the hydroxyl (-OH) group is hydrophilic. Interaction with the comparatively rigid cholesterol decreases the mobility of hydrocarbon tails of phospholipids. Also the transition to the crystalline state is avoided by the interference of cholesterol in a phospholipid membrane with the close packing of fatty acid tails in the crystal state. Phospholipid membranes with a high concentration of cholesterol have a fluidity intermediate between the liquid crystal and crystal states. Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-chain saturated fatty acids. If cholesterol is not present, such membranes would crystallize at physiological temperatures.
Membrane proteins: Membrane Proteins may be classified as (i)
Integral membrane proteins and
(ii)
Peripheral membrane proteins
Peripheral Membrane Proteins: Peripheral proteins occur on the surface of the membrane. They are water-soluble, with mostly hydrophilic surfaces. They are commonly referred as extrinsic proteins. Peripheral membrane proteins bound to the membrane in two different ways (i) Directly by interaction with the lipid polar head groups. (ii) Indirectly by interaction with the integral membrane proteins. Peripheral proteins can be dislodged from membranes frequently by conditions that disturb ionic and H -bond interactions. These activities include the extraction with solutions containing high concentrations of salts, change of pH etc. Modular design of the proteins allows them to form domains with the different segments of their primary structure folded together. The so formed domains perform various functions depending upon the number of folds. There are some cytosolic proteins that have domains binding to polar head groups of lipids which momentarily exist in a membrane.
The enzymes that create or degrade these lipids are subject to signal-mediated regulation. This provides them a mechanism for modulating affinity of a protein for a membrane surface. The best example is the pleckstrin homology (PH) domains bind to phosphorylated derivatives of phosphatidylinositol (PI).
Integral Membrane Proteins: Integral membrane proteins are also called as intrinsic proteins. They possess one or more domains that extend into the hydrocarbon core of the membrane. Most of the integral proteins span the phospholipid bilayer. Intramembrane domains have largely hydrophobic surfaces, which interact with membrane lipids. Integral proteins are complicated and cannot be crystallized for X-ray analysis. Due to the presence of their hydrophobic transmembrane domains, detergents must be present during crystallization. Atomic-resolution structures have been determined for few integral membrane proteins. In all those transmembrane proteins examined, the membrane-spanning domains are found to be α-helix or β-strands. On the other hand, some integral proteins are anchored into the membrane by covalently attached lipid anchors to one of the leaflets of the phospholipid bilayer.
α Helices: In α -helix, amino acid R-groups projected out from the helically coiled polypeptide backbone. They are embedded in the membranes by hydrophobic interactions with the lipid interior of the bilayer and also by the ionic interactions with the polar head groups of phospholipids. The hydrophobic helix forms Van der Waals interactions with the fatty acyl chains. This hydrophobic helix is prevented from sliding across the membrane by particular amino acids (positively charged) that occur at different positions relative to the surface or interior of the bilayer in transmembrane segments of integral proteins. This positively charged amino acids interact with the negatively charged phospholipid head groups and thus help to stabilize the α –helices. Leucine, isoleucine, alanine, valine are those amino acid residues with aliphatic side-chains that predominate in the middle of the bilayer.
Tyrosine and tryptophan are common near the membrane surface while Lysine and arginine are often at the lipid/water interface, with the positively charged groups at the ends of their aliphatic side chains that extend towards the polar membrane surface.
•
Glycophorin, a homodimeris protein present in the erythrocyte plasma membrane is an example of an integral protein whose intramembrane domains consist mainly of transmembrane α -helices.
•
Bacteriorhodopsin, a protein present in the photosynthetic bacterium is characterized by seven membrane-spanning α –helices.
Transmembrane α-helices lining a water-filled channel may possibly have polar amino acid R-groups (side-chains) facing the lumen, and non-polar amino acid R-groups facing lipids or other hydrophobic α-helices. β-strands: Transmembrane α-helices are the most common structural design for integral proteins. In contrast, a family of bacterial outer envelope channel proteins called porins consists of β barrel structures. Various types of porins are present in the outer membrane of the bacteria like E.coli. A β barrel is a β -sheet rolled up to form a cylindrical pore. Almost all the porins are trimeric transmembrane proteins. Each subunit (monomer) is barrel-shaped containing the β strands as the wall and a transmembrane pore at the centre. In a β -sheet, amino acid R-groups alternately point above and below the sheet. The primary structure of a porin mainly consists of alternating polar and non-polar amino acids. Polar residues face the aqueous lumen and the non-polar ones are in contact with membrane lipids.
Protease enzymes and monoclonal antibodies are used to test the transmembrane topology. These are all impermeant probes, added on one side of a membrane.
Protease enzymes degrade the protein segment with their addition and this enables exposure to the aqueous phase on the side of the membrane to which the protease was added.
Monoclonal antibodies are raised to peptides which are on par with the individual segments of the protein. Binding of these antibodies shows surface exposure of that protein segment on the side to which they were added.
From these observations, it has been identified that all copies of a given type of integral protein have the same orientation relative to the bilayer membrane and flip-flopping of integral proteins does not occur. However in some natural membranes, occasionally flip-flopping of integral proteins from one leaflet of the membrane to the other happens. This is catalyzed by some enzymes called as flippases. But when considering energetic barriers, such reaction are highly unfavourable.
Solubilization of integral proteins: Amphipathic detergents are necessary for solubilization of integral proteins. Hydrophobic domains of detergents serve as substitute for lipids and polar domains of detergents interact with water. If detergents are removed, purified integral proteins tend to aggregate and come out of solution.
Proteins with Lipid anchor: Some proteins bind to membranes through a covalently attached lipid anchor that inserts into the bilayer. A very common example of this type of protein is Glycosylphosphatidylinositol. It contains two fatty acyl groups, N-acetylglucoasamine, mannose and inositol. Glycosylphosphatidylinositol can be abbreviated GPI. It is a complex glycolipid and attaches some proteins to the outer surface of the plasma membrane. Alkaline phosphatase is a common enzyme belonging to this type of proteins. GPI-linked proteins may be released from the outer cell surface by phospholipases which have an important role in the degradation of aged and damaged cell membranes. In another group of lipid-anchored cytosolic proteins, a fatty acyl group like palmitate or myristate is linked through an amide bond to the N-terminal glycine residue. Palmitate is usually attached via an ester linkage to the thiol of a cysteine residue.
4. Fluid Mosaic Model of Biological Membrane The cell membrane forms a boundary around the cell and it allows the regulation of molecules moving in or out from the cell. This cell membrane is composed of a lipid bilayer with protein molecules implanted throughout in an arbitrary manner. Several experiments have shown that many integral membrane proteins are floating freely in the two-dimensional space of the natural membrane. Based on this concept fluid mosaic model is raised in which the membrane is observed as a two-dimensional mosaic consisting of laterally mobile proteins and phospholipids. Fluid: The lipid bilayer is fluid in nature and lipid molecules are free to move. Mosaic: The embedded proteins found in the bilayer are scattered in various places and protruding in varying amounts to produce a mosaic pattern. The fluid-mosaic model helps the researchers to explain the behaviour and function of cell membranes as it is reliable with the restrictions forced by thermodynamics.
An overview of the biological molecules associated with cell membranes are as follows: Proteins: Membrane proteins have many different functions. Protein molecules characterized by their tertiary structure serve as channels, pumps, proteins and receptors for hormones or other signal molecules, and enzymes.
Phospholipids: Phospholipids form the most abundant lipid components of the membrane. The hydrophilic heads of the molecule face the outside and inside of the cell (aqueous region) and the hydrophobic tails face each other in the middle of the membrane. Phosphatidylcholine is a common phospholipid.
Sphingolipids: Sphingolipids lack a glycerol backbone and contain sphingosine which is an amino alcohol with a long unsaturated hydrocarbon chain. Sphingomyelin is an example of a glycolipid.
Cholesterol: This is a significant lipid with the four fused rings structure acts to strengthen the membrane and control its fluidity. Cholesterol and its derivatives represent an important group of membrane lipids called the “steroids”. Carbohydrates: Carbohydrate chains are found attached to membrane lipids and proteins on outer surface of the cells. They covalently bound to the proteins as components of glycoproteins or to the lipids as glycolipids. FRAP: Using Fluorescence Recovery After Photobleaching (FRAP) technique, the lateral movement of proteins and lipids on the surface of the membrane can be quantified. In this method, diffusion coefficient (the rate at which the surface lipid or protein move) can be determined.
Several similar experiments have shown that nearly 30-90 percent of all integral proteins present in the plasma membrane are free to move. The percent varies depending upon the type of the cells.
Points to Remember: •
Lipids are hydrophobic compounds, soluble in organic solvents.
•
Most of the lipids present in the membrane are amphipathic, which means having both the non-polar end and polar end.
•
Fatty acids are made up of a hydrocarbon chain having a carboxylic acid at one end.
•
Fatty acids can be saturated (no double bonds in the acyl chain) or unsaturated (having double bonds). Monounsaturated fatty acids contain one double bond and Polyunsaturated contains multiple double bonds.
•
Saponification is the process in which soap is produced from the reaction between lipids and a strong base.
•
Triacylglycerides, glycerophospholipids, and sphingolipids are the three major saponifiable lipids.
•
Non Saponifiable Lipids do not undergo saponification and the most common examples of cholesterol, vitamin D2, and α– tocopherol.
•
Phosphatidylinositol is a glycerophospholipid with inositol as polar head group. It is a significant membrane lipid and serves an important role in cell signaling.
•
Phosphatidylcholine is another glycerophospholipid with choline as its polar head group. It is a common membrane lipid.
•
Sphingolipids are derivatives of the lipid sphingosine which has a long hydrocarbon tail.
•
Cerebrosides and gangliosides, are collectively called glycosphingolipids.
•
The phospholipid bilayer forms the fundamental structure of all cellular membranes. It consists of two layers of phospholipid molecules.
•
The fatty acyl tails of the phospholipid molecules form the hydrophobic interior of the bilayer and their hydrophilic head line the peripheral surface of the bilayer on both sides.
•
The major portion of cholesterol molecule is hydrophobic but only the hydroxyl (OH) group is hydrophilic.
•
Membrane Proteins may be classified as Integral membrane proteins and Peripheral membrane proteins
•
Peripheral proteins occur on the surface of the membrane. They are watersoluble, with mostly hydrophilic surfaces and are commonly referred as extrinsic proteins.
•
Integral membrane proteins are also called as intrinsic proteins. They possess one or more domains that extend into the hydrocarbon core of the membrane.
•
In all transmembrane proteins, the membrane-spanning domains are found to be α -helix or β-strands.
•
Some integral proteins are anchored into the membrane by covalently attached lipid anchors to one of the leaflets of the phospholipid bilayer.
•
Fluid mosaic model is viewed as the membrane is a two-dimensional mosaic consisting of laterally mobile proteins and phospholipids.
•
Fluorescence Recovery after Photobleaching (FRAP) is a technique through which the lateral movement of proteins and lipids on the surface of the membrane can be quantified.
1.
Lipids: Introduction
2.
Classification of Lipids
3.
Organization of Biomembranes
4.
Fluid Mosaic Model of Biological membrane
1. Lipids: Introduction Lipids: Lipids can be defined on the basis of a physical property of “solubility”. Lipids are biological molecules extracted from living organisms which are soluble in organic solvents, such as methanol, and are sparingly soluble in water. Propionate, Limonene, Squalene, Chrysin and Vitamin E are some the popular examples of lipids which are insoluble in water and soluble in organic solvents. The insolubility in water is due to the high proportion of carbon and hydrogen present in these molecules. In short, Lipids are hydrophobic compounds, soluble in organic solvents. Most of the lipids present in the membrane are amphipathic, which means having both the non-polar end and polar end. Fatty Acids: Fatty acids are made up of a hydrocarbon chain having a carboxylic acid at one end. Some of the common examples of fatty acids are myristic acid, palmitic acid, stearic acid oleic acid, linoleic acid and arachidonic acid. Structure: Fatty acids can be saturated or unsaturated. Saturated fatty acids contain no double bonds in the acyl chain whereas unsaturated ones contain either one double bond (monounsaturated) or multiple double bonds (polyunsaturated).
Common Saturated Fatty Acids: Acetic Acid
CH3COOH
Propionic Acid
C2H5COOH
Butyric Acid
C3H7COOH
Lauric Acid
C11H23COOH
Myristic Acid
C13H27COOH
Palmitic Acid
C15H31COOH
Stearic Acid
C17H35COOH
Archidonic Acid
C19H39COOH
Lignoceric Acid
C23H47COOH
Common Unsaturated Fatty Acids: Oleic Acid is a common monounsaturated fatty acid containing one double bond. Linoleic acid and arachidonic acid are polyunsaturated fatty acids having more than one double bond. Common name
Structure
Palmitoleic acid
CH3(CH2)5CH=CH-(CH2)7COOH
Oleic acid
CH3(CH2)7CH=CH-(CH2)7COOH
Linoleic acid
CH3(CH2)4(CH=CHCH2)2(CH2)6COOH
α-Linolenic acid
CH3CH2(CH=CHCH2)3(CH2)6COOH
Arachidonic acid
CH3(CH2)4(CH=CHCH2)4(CH2)2COOH
Fatty acid Nomenclature: Fatty acids can be named in a number of ways; mostly used are common name (denoting their origin and place of occurrence etc), symbolic name (based upon the structure and arrangement of carbon atoms), and systematic name (based on IUPAC nomenclature).
Common name: Oleic acid got its common name as it is present abundant in olive oil.
Symbolic name: Symbolic name can be written as x:y (Δ a,b,c) where x is the number of Carbons in the chain, y is the number of double bonds. a, b, and c represent the positions of the start of the double bonds from C1,the carboxyl C. • • • • •
Saturated fatty acids contain no C-C double bonds. Monounsaturated fatty acids contain one C=C bond. Polyunsaturated fatty acids contain more than 1 C=C bond. cis configuration is present usually in the double bonds in fatty acids. Almost all the fatty acids occurring in nature have an even number of carbon atoms.
Systematic Name: Systematic name is given using IUPAC nomenclature. It gives the number of carbons present in a particular fatty acid. If the fatty acid is unsaturated, the base name reflects the number of double bonds. Examples:
Common name: α-linolenic acid Systematic name: octadecatrienoic acid Symbolic Representation: 18:3Δ 9,12,15
Common name: palmitic acid Systematic name: hexadecanoic acid Symbolic Representation: 16:0 (here the number of double bonds is 0 since it is a saturated fatty acid.
2. Classification of Lipids Depending on their reactivity with strong bases, lipids are classified into two major classes, saponifiable lipids and nonsaponifiable lipids. Saponification: Saponification is the process in which soap is produced from the reaction between lipids and a strong base.
Saponifiable Lipids: The saponifiable lipids consist of long chain fatty acids, esterified to a “backbone” molecule. This back bone molecule can be either glycerol or sphingosine.
The three major saponifiable lipids are triacylglycerides, glycerophospholipids, and the sphingolipids. •
Triacylglycerides contain three fatty acids esterified to the three OHs on glycerol.
•
Glycerophospholipids have two fatty acids esterified at first and second and a phospho-X group esterifed at then third carbon.
For both Triacylglycerides and Glycerophospholipids, glycerol serves as the backbone molecule.
•
Spingolipids possess ‘Spingosine’ as the backbone, which has a long alkyl group connected at C1 and a free amine at C2. In this type of saponifiable
lipids, at C2, a fatty acid is attached through an amide link, and a H or esterified phospho-X group is present at C3. Non Saponifiable Lipids: Non Saponifiable Lipids do not undergo saponification and the most common examples of cholesterol, vitamin D2, and α – tocopherol. Among these, cholesterol is an important constituent of cell membranes. It consists of a stiff ring system and a short branched tail of hydrocarbon.
+ Glycerophospholipids: Glycerophospholipids (phosphoglycerides) are common constituents of cellular membranes. They possess a glycerol backbone and the hydroxyls at C1 & C2 of glycerol are esterified into fatty acids.
Formation of ester linkage: An ester is formed when a hydroxyl reacts with a carboxylic acid, with the reduction of a water molecule.
In phosphatidate, fatty acids are esterified to the hydroxyls on C1 and C2, while the C3 hydroxyl is esterified to phosphate. In most phosphoglycerides, the phosphate is in turn esterified to an alcohol of one of the polar head groups like ethanolamine, serine, glycerol, choline or inositol. Phosphatidylinositol: Phosphatidylinositol is a glycerophospholipid with inositol as polar head group.
Phosphatidylinositol is a significant membrane lipid. It also serves an important role in cell signaling.
Phosphatidylcholine: Phosphatidylcholine is another glycerophospholipid with choline as its polar head group. It is a common membrane lipid.
The normal constituents of a glycerophospholipid are (i) a polar region consists of glycerol, carbonyl oxygen atoms of fatty acids, phosphate, and the polar head group.
(ii) non-polar region which includes the hydrocarbon tails of fatty acids. Sphingolipids: Sphingolipids are derivatives of the lipid sphingosine which has a long hydrocarbon tail, and a polar domain that comprises an amino group. Normally the other derivatives of sphingosine are components of biological membranes. Sphingosine-1-phosphate: Sphingosine-1-phosphate is an important signal molecule and it is produced by the reversible phosphorylation.
Sphingomyelin: Sphingomyelins are common constituents of plasma membranes and have a phosphocholine or phosphoethanolamine head group. •
A glycerophospholipid phosphatidyl choline can be comparable in size and shape to a Sphingomyelin, possessing a phosphocholine head group.
Glycosphingolipids:
A ganglioside is a ceramide (sphingolipid) with a polar head group that is a complex oligosaccharide, including the acidic sugar derivative sialic acid. Gangliosides are very common in the brain.
A cerebroside is a sphingolipid with a monosaccharide such as glucose or galactose as polar head group.
Sulfatides are subtypes of cerebrosides which are sulfate derivatives of the galactose residues.
Cerebrosides and gangliosides, are collectively called glycosphingolipids. They are present in the outer leaflet of the plasma membrane bilayer, having their sugar chains extending out from the cell surface. Amphipathic lipids associates with water to form complexes in such a way that their polar regions are in contact with water and their hydrophobic regions are away from water.
3. Organization of Biomembranes The phospholipid bilayer forms the fundamental structure of all cellular membranes. It consists of two layers of phospholipid molecules. The fatty acyl tails of the phospholipid molecules form the hydrophobic interior of the bilayer and their hydrophilic head line the peripheral surface of the bilayer on both sides. Hence it can be stated that the phospholipid bilayer is the most stable configuration for amphipathic lipids with a cylindrical shape (phospholipids). The lipid bilayer interior is normally in high fluid state. Liquid crystal state: In the liquid crystal state, hydrocarbon chains of phospholipids are disordered and in constant motion. Crystalline state: At lower temperatures, a membrane containing a single phospholipid type undergoes transition into a crystalline state. In this state, the following phenomenon can be noticed.
Completely extended fatty acid tails Highly ordered packing Maximal van der Waals interaction between adjacent chains.
Cholesterol: Cholesterol, a highly hydrophobic molecule is an important constituent of cell membranes. It is present abundant in the plasma membrane of the mammalian cells and it is absent from most of the prokaryotic cells. It is characterized by a rigid ring system and a short branched hydrocarbon tail. The presence of one polar group and a hydroxyl makes it amphipathic. Cholesterol inserts itself into bilayer membranes in such a way that its hydroxyl group is oriented toward the aqueous phase and its hydrophobic ring system is contiguous to fatty acid tails of phospholipids. Formation of hydrogen bonds takes place between the hydroxyl group of cholesterol and the polar phospholipid head groups.
The major portion of cholesterol molecule is hydrophobic but only the hydroxyl (-OH) group is hydrophilic. Interaction with the comparatively rigid cholesterol decreases the mobility of hydrocarbon tails of phospholipids. Also the transition to the crystalline state is avoided by the interference of cholesterol in a phospholipid membrane with the close packing of fatty acid tails in the crystal state. Phospholipid membranes with a high concentration of cholesterol have a fluidity intermediate between the liquid crystal and crystal states. Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-chain saturated fatty acids. If cholesterol is not present, such membranes would crystallize at physiological temperatures.
Membrane proteins: Membrane Proteins may be classified as (i)
Integral membrane proteins and
(ii)
Peripheral membrane proteins
Peripheral Membrane Proteins: Peripheral proteins occur on the surface of the membrane. They are water-soluble, with mostly hydrophilic surfaces. They are commonly referred as extrinsic proteins. Peripheral membrane proteins bound to the membrane in two different ways (i) Directly by interaction with the lipid polar head groups. (ii) Indirectly by interaction with the integral membrane proteins. Peripheral proteins can be dislodged from membranes frequently by conditions that disturb ionic and H -bond interactions. These activities include the extraction with solutions containing high concentrations of salts, change of pH etc. Modular design of the proteins allows them to form domains with the different segments of their primary structure folded together. The so formed domains perform various functions depending upon the number of folds. There are some cytosolic proteins that have domains binding to polar head groups of lipids which momentarily exist in a membrane.
The enzymes that create or degrade these lipids are subject to signal-mediated regulation. This provides them a mechanism for modulating affinity of a protein for a membrane surface. The best example is the pleckstrin homology (PH) domains bind to phosphorylated derivatives of phosphatidylinositol (PI).
Integral Membrane Proteins: Integral membrane proteins are also called as intrinsic proteins. They possess one or more domains that extend into the hydrocarbon core of the membrane. Most of the integral proteins span the phospholipid bilayer. Intramembrane domains have largely hydrophobic surfaces, which interact with membrane lipids. Integral proteins are complicated and cannot be crystallized for X-ray analysis. Due to the presence of their hydrophobic transmembrane domains, detergents must be present during crystallization. Atomic-resolution structures have been determined for few integral membrane proteins. In all those transmembrane proteins examined, the membrane-spanning domains are found to be α-helix or β-strands. On the other hand, some integral proteins are anchored into the membrane by covalently attached lipid anchors to one of the leaflets of the phospholipid bilayer.
α Helices: In α -helix, amino acid R-groups projected out from the helically coiled polypeptide backbone. They are embedded in the membranes by hydrophobic interactions with the lipid interior of the bilayer and also by the ionic interactions with the polar head groups of phospholipids. The hydrophobic helix forms Van der Waals interactions with the fatty acyl chains. This hydrophobic helix is prevented from sliding across the membrane by particular amino acids (positively charged) that occur at different positions relative to the surface or interior of the bilayer in transmembrane segments of integral proteins. This positively charged amino acids interact with the negatively charged phospholipid head groups and thus help to stabilize the α –helices. Leucine, isoleucine, alanine, valine are those amino acid residues with aliphatic side-chains that predominate in the middle of the bilayer.
Tyrosine and tryptophan are common near the membrane surface while Lysine and arginine are often at the lipid/water interface, with the positively charged groups at the ends of their aliphatic side chains that extend towards the polar membrane surface.
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Glycophorin, a homodimeris protein present in the erythrocyte plasma membrane is an example of an integral protein whose intramembrane domains consist mainly of transmembrane α -helices.
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Bacteriorhodopsin, a protein present in the photosynthetic bacterium is characterized by seven membrane-spanning α –helices.
Transmembrane α-helices lining a water-filled channel may possibly have polar amino acid R-groups (side-chains) facing the lumen, and non-polar amino acid R-groups facing lipids or other hydrophobic α-helices. β-strands: Transmembrane α-helices are the most common structural design for integral proteins. In contrast, a family of bacterial outer envelope channel proteins called porins consists of β barrel structures. Various types of porins are present in the outer membrane of the bacteria like E.coli. A β barrel is a β -sheet rolled up to form a cylindrical pore. Almost all the porins are trimeric transmembrane proteins. Each subunit (monomer) is barrel-shaped containing the β strands as the wall and a transmembrane pore at the centre. In a β -sheet, amino acid R-groups alternately point above and below the sheet. The primary structure of a porin mainly consists of alternating polar and non-polar amino acids. Polar residues face the aqueous lumen and the non-polar ones are in contact with membrane lipids.
Protease enzymes and monoclonal antibodies are used to test the transmembrane topology. These are all impermeant probes, added on one side of a membrane.
Protease enzymes degrade the protein segment with their addition and this enables exposure to the aqueous phase on the side of the membrane to which the protease was added.
Monoclonal antibodies are raised to peptides which are on par with the individual segments of the protein. Binding of these antibodies shows surface exposure of that protein segment on the side to which they were added.
From these observations, it has been identified that all copies of a given type of integral protein have the same orientation relative to the bilayer membrane and flip-flopping of integral proteins does not occur. However in some natural membranes, occasionally flip-flopping of integral proteins from one leaflet of the membrane to the other happens. This is catalyzed by some enzymes called as flippases. But when considering energetic barriers, such reaction are highly unfavourable.
Solubilization of integral proteins: Amphipathic detergents are necessary for solubilization of integral proteins. Hydrophobic domains of detergents serve as substitute for lipids and polar domains of detergents interact with water. If detergents are removed, purified integral proteins tend to aggregate and come out of solution.
Proteins with Lipid anchor: Some proteins bind to membranes through a covalently attached lipid anchor that inserts into the bilayer. A very common example of this type of protein is Glycosylphosphatidylinositol. It contains two fatty acyl groups, N-acetylglucoasamine, mannose and inositol. Glycosylphosphatidylinositol can be abbreviated GPI. It is a complex glycolipid and attaches some proteins to the outer surface of the plasma membrane. Alkaline phosphatase is a common enzyme belonging to this type of proteins. GPI-linked proteins may be released from the outer cell surface by phospholipases which have an important role in the degradation of aged and damaged cell membranes. In another group of lipid-anchored cytosolic proteins, a fatty acyl group like palmitate or myristate is linked through an amide bond to the N-terminal glycine residue. Palmitate is usually attached via an ester linkage to the thiol of a cysteine residue.
4. Fluid Mosaic Model of Biological Membrane The cell membrane forms a boundary around the cell and it allows the regulation of molecules moving in or out from the cell. This cell membrane is composed of a lipid bilayer with protein molecules implanted throughout in an arbitrary manner. Several experiments have shown that many integral membrane proteins are floating freely in the two-dimensional space of the natural membrane. Based on this concept fluid mosaic model is raised in which the membrane is observed as a two-dimensional mosaic consisting of laterally mobile proteins and phospholipids. Fluid: The lipid bilayer is fluid in nature and lipid molecules are free to move. Mosaic: The embedded proteins found in the bilayer are scattered in various places and protruding in varying amounts to produce a mosaic pattern. The fluid-mosaic model helps the researchers to explain the behaviour and function of cell membranes as it is reliable with the restrictions forced by thermodynamics.
An overview of the biological molecules associated with cell membranes are as follows: Proteins: Membrane proteins have many different functions. Protein molecules characterized by their tertiary structure serve as channels, pumps, proteins and receptors for hormones or other signal molecules, and enzymes.
Phospholipids: Phospholipids form the most abundant lipid components of the membrane. The hydrophilic heads of the molecule face the outside and inside of the cell (aqueous region) and the hydrophobic tails face each other in the middle of the membrane. Phosphatidylcholine is a common phospholipid.
Sphingolipids: Sphingolipids lack a glycerol backbone and contain sphingosine which is an amino alcohol with a long unsaturated hydrocarbon chain. Sphingomyelin is an example of a glycolipid.
Cholesterol: This is a significant lipid with the four fused rings structure acts to strengthen the membrane and control its fluidity. Cholesterol and its derivatives represent an important group of membrane lipids called the “steroids”. Carbohydrates: Carbohydrate chains are found attached to membrane lipids and proteins on outer surface of the cells. They covalently bound to the proteins as components of glycoproteins or to the lipids as glycolipids. FRAP: Using Fluorescence Recovery After Photobleaching (FRAP) technique, the lateral movement of proteins and lipids on the surface of the membrane can be quantified. In this method, diffusion coefficient (the rate at which the surface lipid or protein move) can be determined.
Several similar experiments have shown that nearly 30-90 percent of all integral proteins present in the plasma membrane are free to move. The percent varies depending upon the type of the cells.
Points to Remember: •
Lipids are hydrophobic compounds, soluble in organic solvents.
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Most of the lipids present in the membrane are amphipathic, which means having both the non-polar end and polar end.
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Fatty acids are made up of a hydrocarbon chain having a carboxylic acid at one end.
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Fatty acids can be saturated (no double bonds in the acyl chain) or unsaturated (having double bonds). Monounsaturated fatty acids contain one double bond and Polyunsaturated contains multiple double bonds.
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Saponification is the process in which soap is produced from the reaction between lipids and a strong base.
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Triacylglycerides, glycerophospholipids, and sphingolipids are the three major saponifiable lipids.
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Non Saponifiable Lipids do not undergo saponification and the most common examples of cholesterol, vitamin D2, and α– tocopherol.
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Phosphatidylinositol is a glycerophospholipid with inositol as polar head group. It is a significant membrane lipid and serves an important role in cell signaling.
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Phosphatidylcholine is another glycerophospholipid with choline as its polar head group. It is a common membrane lipid.
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Sphingolipids are derivatives of the lipid sphingosine which has a long hydrocarbon tail.
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Cerebrosides and gangliosides, are collectively called glycosphingolipids.
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The phospholipid bilayer forms the fundamental structure of all cellular membranes. It consists of two layers of phospholipid molecules.
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The fatty acyl tails of the phospholipid molecules form the hydrophobic interior of the bilayer and their hydrophilic head line the peripheral surface of the bilayer on both sides.
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The major portion of cholesterol molecule is hydrophobic but only the hydroxyl (OH) group is hydrophilic.
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Membrane Proteins may be classified as Integral membrane proteins and Peripheral membrane proteins
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Peripheral proteins occur on the surface of the membrane. They are watersoluble, with mostly hydrophilic surfaces and are commonly referred as extrinsic proteins.
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Integral membrane proteins are also called as intrinsic proteins. They possess one or more domains that extend into the hydrocarbon core of the membrane.
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In all transmembrane proteins, the membrane-spanning domains are found to be α -helix or β-strands.
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Some integral proteins are anchored into the membrane by covalently attached lipid anchors to one of the leaflets of the phospholipid bilayer.
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Fluid mosaic model is viewed as the membrane is a two-dimensional mosaic consisting of laterally mobile proteins and phospholipids.
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Fluorescence Recovery after Photobleaching (FRAP) is a technique through which the lateral movement of proteins and lipids on the surface of the membrane can be quantified.
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