Cellular Membranes PDF

Lipids  Outlines 1 Lipid classification 2 Lipid bilayers 3 Membrane proteins 4 Membrane structure & assembly  Key concepts 1 o The length and saturation of a fatty acid chain determine its physical properties. o Triacylg...

Lipids  Outlines 1 Lipid classification 2 Lipid bilayers 3 Membrane proteins 4 Membrane structure & assembly  Key concepts 1 o The length and saturation of a fatty acid chain determine its physical properties. o Triacylglycerols and glycerophospholipids contain fatty acids esterified to glycerol. o Sphingolipids resemble glycerophospholipids but may include large carbohydrate groups. o Steroids, isoprenoids, and other lipids perform a wide variety of functions. Lipids o Lipids are distinguished by their high solubility in non-polar solvents and low solubility in H2O o Diverse group of compounds o Fats, Oils, Waxes, some vitamins and hormones and most non-protein components of membranes o Lipids are amphipathic molecules polar + nonpolar o Lipids can be: (A) Major components of biological membranes o membranes define the basic unit of life (cell) and subcellular compartments (eukaryotes) o includes cholesterol (B) Major form of stored energy in biological systems o lipids are largely reduced compounds; complete oxidation of lipids generates lots of energy (ie. more than from sugars) (C) Hormones o signal transduction (communication) between cells The properties of fatty acids depends on their hydrocarbon chains carboxylic acid o Fatty acids are carboxylic acids with long-chain hydrocarbon side groups o In higher plants and animal: o C16 and C18 species are dominant o Palmitic, oleic, linoleic and stearic acids o Fatty acids with 20 carbon atoms are uncommon o Most fatty acids have an even number of carbon atoms because they are biosynthesized by the concatenation of C2 units Some C18 Fatty Acids The properties of fatty acids depends on their hydrocarbon chains o Fatty acid classes o Unsaturated (contain double bond) o Fatty acid double bonds almost always have the cis configuration o Polysaturated ( contain two or more double cis bonds) o pack together less efficiently than saturated fatty acids o Reduced van der Waals interaction o melting points decreased o Fluidity of lipids increased cis o Saturated (no double bond, fully reduced) o Highly flexible molecules → diverse conformations trans  Common Biological Fatty Acids 3 2 1  Triacylglycerols contain three esterified fatty acids glycerol o Triacylglycerols (triglycerides) o Glycerol + 3 fatty acids: nonpolar, water-insoluble ester substances o Function as energy reservoirs o Triacylglycerols differ according to the identity and placement of their three fatty acid residues o 1-palmitoyleoyl-2-linoleoyl-3-stearoylglycerol o Fats and oils are complex mixtures of triacylglycerols whose fatty acid composition vary with the organism that produced them. o e.g. plant oils are usually richer in unsaturated fatty acid than animal fats, as the lower melting points of oils imply. ester  Triacylglycerols function as energy reserves o Fats are a highly efficient form in which to store metabolic energy. o triacylglycerols are less oxidized than carbohydrates or proteins o hence yield significantly more energy per unit mass on complete oxidation. o Fats provide about 6 times the metabolic energy of an equal weight of hydrated glycogen o In animals, adipocytes are specialized for the synthesis and storage of triacylglycerols o While other types of cells have only a few small droplets of fat dispersed in their cytosol, adipocytes may be almost filled entirely with fats o The fat content of normal human (21% for men, 26% for women) allows them to survive starvation for 2-3 months o In contrast, the body’s glycogen supply can provide for the body’s energy needs for less than a day Adipocytes Containing Fat Globule  Glycerophospholipids are amphiphilic ester o Glycerophospholipids (or phosphoglycerides) are the major lipid components of biological membranes. o They consist of glycerol-3-phosphate whose C1 and C2 positions are esterified with fatty acids o In addition, the phosphoryl group is linked to another usually polar group, X o Glycerophospholipids are therefore amphiphilic molecules o “tails” : nonpolar aliphatic (hydrocarbon) o “heads" : polar phosphoryl-X “heads” o The simplest glycerophospholids, in which X=H, are phosphatidic acids o The head groups : polar group “tails” o C1: Saturated C16 or C18 fatty acids o C2: Unsaturated C16 to C20 fatty acids ester Common Glycerophospholipids  Phospholipases hydrolyze glycerophospholipids o The chemical structures-including fatty acyl chains and head groups-of glycerophospholipids can be determined from the products of the hydrolytic reaction catalyzed by enzymes known as phospholipases. o For example, phospholipase A2 hydrolytically excises the fatty acid residue at C2, leaving a lysophospholipid Phospholipases Hydrolyze Glycerophospholipids o Lysophospholipids are the powerful detergents that disrupt cell membranes, thereby lysing cells o Bee and snake venoms are rich sources of phospholipase A2 o Phospholipase A2 o Small protein ( ~ 14 kD, ~ 125 amino acids) o The enzyme binds a glycerophospholipid molecule such that its polar head group fits into the enzyme’s active site, whereas the hydrophobic tails, which extend beyond the active site, interact with several aromatic side chain o Phospholipase C generates 1,2,-diacylglycerol derived from membrane lipids → Signal molecule that activates a protein Phospholipase A2 and kinase. a Glycerophospholipid Sphingolipids are amino alcohol derivatives o Sphingolipids are also major membrane components o They were named after the Sphinx because their function in cells was at first mysterious o Most sphingolipids are derivatives of the C18 amino alcohol sphingosine, whose double bond has the trans configuration. The N-acyl fatty acid derivatives of sphingosine are known as ceramides o Ceramides are the parent compounds of the more abundant sphingolipids 1. Sphingomyelins 2. Cerebrosides 3. Gangliosides  Sphingolipids are amino alcohol derivatives 1. Sphingomyelins o the most common sphingolipids, are ceramides bearing either a phosphocholine or a phosphoethanolamine head group o Classified as sphingophospholipids o 10~20% mol % of plasma membrane lipids Myelinated Nerve Fibers o The membranous myelin sheath that surrounds and electrically insulates many nerve cell axons is particularly rich in sphingomyelins. Sphingomyelins 2. Cerebrosides o Ceramides with head groups that consist of a single sugar residue o These lipids are therefore glycosphingolipids o Galactocerebrosides and glucocerebrosides are the most common Sphingolipids are amino alcohol derivatives 3. Gangliosides o Are the most complex glycosphingolipids o They are ceramides with attached oligosaccharides that include at least one sialic acid residue o The structure of gangliosides GM1, GM2 and GM3 o Gangliosides are primarily components of cell- surface membranes and constitutes a significant fraction (6%) of brain lipids Gangliosides  Gangliosides have considerable physiological and medical significance o Their complex carbohydrate head groups, which extend beyond the surfaces of the cell membranes, o act as specific receptors for certain pituitary glycoprotein hormones that regulate a number of important physiological functions. o Gangliosides are also receptors for certain bacterial protein toxins such as cholera toxin o Specific determinants of cell-cell recognition o Important role in the growth and differentiation of tissues as well as in carcinogenesis o Disorders of ganglioside breakdown are responsible for several hereditary sphingolipid storage disease, such as Tay-Sachs disease, characterized by an invariably fatal neurological deterioration in early childhood o Genetic disorder resulting in destruction of nerve cells in the brain and spinal cord o Mutation in hexosaminidase A (HEXA) which catalyze the degradation of GM2 gangliosides o Accumulation of GM2 ganglioside in the brain and spinal cord and eventually affect the function of the nerve cells o Symptom starts 3-6 month of age, weaken muscle, seizure, vision & Ganglioside GM1 hearing loss, live only a few years General structures of sphingolipids Steroids contain four fused rings o Steroids : derivatives of cyclopentanoperhydrophenanthrene o A compound that consists of four fused, nonplanar rings (labeled A- D) sterol o cholesterol, which is the most abundant steroid in animals is further classified as a sterol because of its C3-OH group o Cholesterol is a major component of animal plasma membranes, typically constituting 30 to 40 mol % of plasma membrane lipids o Its polar OH group gives it a weak amphiphilic character o Where it is fused ring system provides it with greater rigidity than other membrane lipids o Cholesterol can be esterified to long-chain fatty acids to form cholesteryl ester, such as cholesteryl stearate Steroid Hormones in mammals o Plants contain little cholesterol but synthesize other sterols o Yeast and fungi also synthesize sterols, which differ from cholesterol in their aliphatic side chains and number of double bonds o Prokaryotes contain little sterol o In mammals, cholesterol is the metabolic precursor of steroid hormones, substances that control a great variety of physiological function through their regulation of gene expression o Steroid hormones are classified according to the physiological responses they evoke 1. Glucocorticoids, such as cortisol, affect carbohydrate, protein, and lipid metabolism and influence a wide variety of other vital functions, including inflammatory reactions and the capacity to cope with stress 2. Aldosterone and other mineralocorticoids regulate the excretion of salt and water by the kidneys 3. The androgens and estrogens affect sexual development and function. o Testosterone, C19 compound, is the prototypic androgen (male sex hormones), whereas ß-esteradiol, c18 compound, is an estrogen (female sex hormone)  Key concepts 2 o Certain amphiphilic molecules form bilayers o The bilayer is a fluid structure in which lipids rapidly diffuse laterally.  Bilayer formation is driven by the hydrophobic effect Aggregates of Single-Tailed Lipids o Micelle o Globular aggregates whose hydrocarbon groups are out of contact with water o In aqueous solutions, amphiphilic molecules such as soaps and detergents form micelles o To eliminate unfavorable contacts between water and the hydrophobic tails of the amphiphiles and yet permits the solvation of the polar head groups o Approximate size and shape of a micelle can be predicted from geometrical consideration o Single-tailed amphiphiles form spheroidal or ellipsoidal micelles because of their tapered shapes o Too few lipid molecules would expose the hydrophobic core of the micelle to water o Too many lipid molecules give the micelle an energetically unfavorable hollow center  Bilayer formation is driven by the hydrophobic effect o Two hydrocarbon tails of glycerophospholipids and sphingolipids give these amphiphiles a somewhat rectangular cross section → disk-like micelles that are really extended bimolecular leaflets o These lipid bilayers are ~60 Å thick, as measured by electron microscopy and x-ray diffraction techniques Bilayer Formation by Phospholipids o A suspension of phospholipids can form liposome-closed, self- sealing solvent filled vesicle that are bounded by only a single bilayer o Once formed, liposomes are quite stable and can be purified by dialysis, gel filtration chromatography, or centrifugation o Liposomes serve as models of biological membranes, and hold promise as vehicles for drug delivery since they are absorbed by many cells through fusion with the plasma membrane Liposome  Lipid bilayers have fluidlike properties o Transverse diffusion or a flip-flop o The transfer of a lipid molecule across a bilayer o extremely rare event o It requires hydrated, polar head group of the lipid to pass through the anhydrous hydrocarbon core of the bilayers o It takes several days or more to diffuse o Lateral diffusion o The movement of lipid in a lateral fashion o Lipids are highly mobile in the plane of the bilayer. o It takes ~ 1s to diffuse o Considered as "two-dimensional fluid" Diffusion in Phospholipid Bilayer  Lipid bilayers have fluidlike properties o The interior of the lipid bilayer is in constant motion due to rotations around the C-C bonds of the lipid tails o Interior of the bilayer has the viscosity of light machine oil o Molecular dynamic simulation - time dependent position of atoms are predicted from calculation of the forces acting on them o Viscosity of the bilayer increases dramatically closer to the lipid head groups, whose rotation is limited and whose lateral mobility is more constrained by interaction between the polar or charged groups o Hydrophobic tail are not stiffly, rather bend and Bilayer Core is Fluid interdigitate  The fluidity of a lipid bilayer is temperature-dependent o As a lipid bilayer cools below a characteristic transition temperature, it undergoes a sort of phase change in which it becomes a gel-like solid Above transition temperature Below transition temperature o Above the transition temperature, the highly mobile lipids are in state known as liquid crystal because they are ordered in some directions but not in others. o The transition temperature of bilayer increases with the chain length and the degree of saturation of its component fatty acid o Transition temperature of most biological membrane o 10~40 °C Liquid crystal Gel-like o Cold-blooded animal modify (through lipid synthesis and degradation) the fatty acid composition with ambient temperature o Cholesterol, which by itself does not form a bilayer, decreases membrane fluidity because its rigid steroid ring system interferes with the motion of the fatty acid chain  Key concepts 3 o Integral membrane proteins contain a transmembrane structure consisting of α helices or a β-barrel with a hydrophobic surface. o Lipid-linked proteins have a covalently attached prenyl group, fatty acyl group, or glycosylphosphatidylinositol group. o Peripheral membrane proteins interact noncovalently with proteins or lipids at the membrane surface.  Integral membrane proteins interact with hydrophobic lipids o Integral or intrinsic proteins associate tightly with membranes through hydrophobic interactions and can be separated from membranes only by treatment with agents that disrupt the membranes o For example, detergents such as sodium dodecyl sulfate solubilize membrane proteins by replacing the membrane lipids that normally surround the protein o The hydrophobic portions of the detergent molecules coat the hydrophobic region of the protein, and the polar head groups render the detergent-protein complex soluble in water o Although membrane protein comprise ~ 30% of all proteins, the Integral Proteins are Tightly Membrane consequent difficulty of crystallizing them has limited the number Associated of membrane protein X-ray structure to 2.5% of all known structures. SDS (Sodium dodecyl sulfate) Integral proteins are asymmetrically oriented amphiphiles o Integral proteins are amphiphiles; the protein segments immersed in a membrane’s nonpolar interior have predominantly hydrophobic surface residues, whereas those portions that extend into the aqueous environment are by and large sheathed with polar residues o Glycophorin A has three domains o Externally located N-terminal domain o 16 carbohydrate chains o Hydrophobic residues o Spans the erythrocyte cell membrane o Cytoplasmic c-terminal domain o High proportion of charged and polar residues o Thus, glycophorin A is a transmembrane (TM) protein; that is, it completely spans the membrane  Transmembrane proteins may contain α Helices o For a polypeptide chain to penetrate or span the lipid bilayer, it must have hydrophobic side chains that contact the lipid tails and it must shield its polar backbone groups. o Consequently, all known TM segments of integral membrane proteins consist of either α helices or βsheet o The existence of such TM helices can be predicted with reasonable reliability by plotting the average hydropathy index Hydropathy index o Electron crystallography to determine the structure of the integral membrane protein bacteriorhodopsin o 7 transmembrane (α-helical rods, ~25 residues) o Light-driven proton pump o Proton concentration gradient across the cell membrane that powers ATP synthesis o Covalently bound retinal is the protein’s light- absorbing group Bacteriorhodopsin  Key concepts 4 o The dynamic arrangement and interactions of membrane lipids and proteins are described by the fluid mosaic model. o The membrane skeleton gives the cell shape yet is flexible. o Lipids are not distributed uniformly throughout a membrane and may form rafts. o The secretory pathway describes the transmembrane passage of membrane and secreted proteins. o Different types of coated vesicles transport proteins between cellular compartments. o SNAREs bring membranes together and help mediate vesicle fusion.  The fluid mosaic model accounts for lateral diffusion Plasma Membrane o The demonstrated fluidity of artificial lipid bilayers suggests that biological membranes have similar properties o Fluid mosaic model : integral proteins are visualized as “icebergs” floating in a two-dimensional lipid “sea” in a random or mosaic distribution o A key element of the model is that integral proteins can diffuse laterally in the lipid matrix unless their movements are restricted by association with other cell components.  The fusion of mouse and human cells o The model of membrane fluidity explained the earlier experimental results of Michael Edidin, who fused cultured cells and observed the intermingling of their differently labeled cell- surface proteins o The accompanying photomicrograph were taken through filters that allowed only red or green light to reach the camera Process Diagram: Fusion of Cells  Fluorescence recovery after photobleaching (FRAP) o In this technique, a fluorophore (fluorescent FRAP Measures Rates of Diffusion in group) is specifically attached to a membrane Membranes component in an immobilized cell or in an artificial membrane system. o An intense laser pulse focused on a very small area (~3μm2) destroys (bleaches) the fluorophore there. The rate at which the bleached area recovers its fluorescence, as monitored by fluorescence microscopy, indicates the rate at which unbleached fluorophore- labeled molecules laterally diffuse into and out of the bleached area Photobleaching : the photochemical alteration of a dye or a fluorophore molecule such that it is permanently unable to fluoresce  The human erythrocyte membrane skeleton o A dense and irregular protein meshwork that underlies the erythrocyte plasma membrane. o Spectrin (α,β) & Ankyrin Membrane Skeleton  The membrane skeleton helps define cell shape Membrane Skeleton Helps Define Cell Shape: Spectrin o Spectrin o ~75% of the erythrocyte membrane skeleton o Two similar polypeptide chain (α,β) o These large polypeptides are loosely intertwined to form a flexible α,β dimer, ~1000 Å long. o A defect or deficiency in spectrin synthesis causes hereditary spherocytosis, in which erythrocyte are spheroidal and relatively fragile and inflexible resulting in anemia  The membrane skeleton helps define cell shape o Ankyrin o which binds to an integral membrane ion channel protein o Anchors the membrane skeleton to the membrane o 24 tandem repeats known as ankyrin repeats o The entire assembly forms an elongated Ankyrin Anchors Membrane Skeleton concave surface that is postulated to bind various integral protein as well as spectrin  Gates and fences model o The interaction of membrane components with the underlying skeleton helps explain why integral membrane protein exhibit different degrees of mobility within the membrane o A : interacts tightly with the underlying cytoskeleton o B: free to rotate within the confines of the cytoskeletal "fence” o C: diffuse freely by traveling through “gates” in the cytoskeleton Membrane Skeleton Influences Integral Protein Mobilities