BCMLch11

Biochemistry-Lehinger CH11

INCOMPLETE/UNSORTED
Home Intersection Biochemistry

369-415 Biological Membranes Func: Biological energy concservation Cell to cell communication
Properties: Selectively Permealble Membranes are flexible, self sealing and selectively permeable to polar solutes
-Flexibility permits the shape changes that accompany cell growth and movement
-Ability to break and reseal exocytose/endocytose without creating gross leaks through cellular surfaces
-Selectively permable retain ions within cells and within specific cellular compartments while excluding others Contain specilized proteins/catalyzing various cellular processes
-Transporters move specific organic solutes and inorganic ions across the membrane
-Receptors sense extracellular signals and trigger molecular changes in the cell
-Adhesion hold neighboring changes in the cell Within the cell, organize cellular processes Synthesis of lipids and certain proteins Energy transductions in mitochondria and chloroplasts Membranes are just two layers - very thin essentially two dimensional Intermollecular collisiosn fare more probable in 2d space than in its 3d space Efficnency of Ecat RXNs is vastly increased
General Composition of Membranes: Each kindom, each species, each tissue/cell type and the organelles have a characteristic set of membrane lipids and proteins Lipids: Cells control kinds and amoutns of membrane lipids and target specific lipids to particular organelles Plasma membranes enriched in cholesterol contain no detectable cardiolipin Inner mitochondrial membrane very low cholesterol and high cardiolipin (Cardiolipin - essential to function of certain proteins of inner mitochondrial membrane) Proteins: - varies more widely reflecting functional specialization Rod cell - 90% rhodopsin Erythrocytes - Less specialized 20 prominent types of proteins + minor proteins Covalently linked to complex arrays of carbohydrates Ser, Thr, Asn - glycophorin Sugars infleunce folding of proteins as well as stablities and intracellular destinations Significant role in specific binding of ligans to glycoprotein surface receptors Covalently linked to one or more lipids Serve as hydrophobic anchors that hold proteins to membrane Common Properties:
-impermeable to most polar or charged solutes
-permeable to nonpolar compounds
-thick (50-80Ang) and trilaminar
-Fluid Mosaic Model: Fluid because of noncovalent interactions Phospholipids form bilayer
-nonpolar inside
-polar outside Proteins are embedded in bilayer sheet
-held by hydrophobic interactions of protein and inner lipid Bilayer is assymetric
-individual lipids and proteins free to change constantly
-Biological membranes constructed of lipid bilayers 30ang thick Proteins protruding on each side Hydrocarbon core of fatty acyl groups impermeable to solutes
-Asymetrically distributed btw/ monolayres Choline lipids found in outer (phosphatidylcholine and sphingomyelin) P-serine, P-ethanolamine, P-linositols found in inner
-Changes in distribution have biological consequences Platelets - when P-serine is outside, it is activated to clott blood Other cells - P-serine is outside, marks cell destruction by programmed cell death
-Membrane Proteins - Integral Protiens, Peripheral Proteins
-1-Integral - firmly associated with membrane Removable by agnets that interfere with hydrophobic interactions (detergents, organic solvents and denatruants)
-2-Peripheral - associate with electrostatic interactions of H bonds with hydPHO of integral proteins and polar head groups of membrane lipids Released by relatively mild treatments that interfere with electrostatic interactions or break H bonds (carobnate @high pH) Func: serve as regulators of membrane bound enzymes or may limit the mobility of integral proteins
Unique components Each type has characteristic lipids and proteins
Membrane Proteins Span Lipid Bilayer Protein topology can be determined with protein side chains but cannot cross membranes - polar chemical reagents that react primary amines of Lys residues Like trypsin that cleave proteins cannot cross the membrane Erythrocyte has no membrane bound organelles Membrane protein in an inteact erythrocyte reacts with a membrane impermeant reagent Must have at least one domain exposed Found to cleave extracellular domains does not affect doamins buried within the bilayer unless the plasma membrane is broken to make these doamisn accessible to the enzyme
Erythrocyte glycoprotein glycophorin spans the plasma membrane Amino terminal domain is on the outer suface and is cleaved by trypsin Carboxyl terminus protrudes on the inside of the cell where it cannot react with impermeant reagents Amino terminal and carboxyl terminal contain many polar or charged amino acid residues and therefore quite hydPHI Segment in the center contains mainly hydPHO amino acid residues Glycophorin has a transmembrane segment arranged as shown Disposition I nthe membrane is asymmetric One domain of a transmembrane protein always faces out Other always faces in Glycoproteins invariably situated with their sugar residues on the outer surface of the cell Asymmetric arrangement results in functional asymmetry All the molecules have same orientation pump in the same direction
Firm atachemtn of integral proteins to membranes result of hydPHO interactions etween membrane lipids and hydPHI domains of protein Some have a single hydPHO sequence in the middle or at the amino carboxyl terminus Others have multiple hydPHO sequences when in the α helix hydPHO interations between nonpolar AA and fatty acyl groups firmly anchor the protein in the membrane 7 helices is a common motif RXN center has 4 protein subunits which contain αhelical segments that span the membrane Rich in nonpolar AA hydPHO oriented toward the outside of molecule where they interact with membrane lipids Inverse of that seen in most water soluble proteins hydPHO residues are buried w/in protein core and hydPHI residues (myoglobin and hemoglobin)
3d structure of membrane protein/topology much more difficult than determining its AA sequence by sequencing the protein or its gene Presence of unbroken seq of 20+ hydPHO residues evidence that these seq traverse the lipid bilayer acting as hydPHO anchors or forming transmembrane channels All integral proteins have at least one such seq Secondary structure α-helical 20-25 residues span 30ang (α-helix each residue 1.5ang) Polypeptide chain surrounded by lipids having no water molecules Tend to form α-helices or β-sheets which intrachain H bond is maximized Side chains are nonpolar, hydPHO surrounding lipids further stabilize the helix Relative polarity of each AA has been determined experimentally by measuring the free energy change accompanying the movement of AA side chain from hydPHO solvent into water Transfer ranges from very exergonic – charged or polar residues to very endergonic – AA with aromatic/aliphatic residues Overall hydPHO is summing the free eng of transfer for residues in the seq – hydropathy index Scan a polypeptide seq for potential membrane spanning segments Successive segments/windows – 7-20 residues A region with more than 20 residues of high hdyropathy index (high hydPHO) is presumed to be a transmembrane segment Predicts a single hydPHO helix for glycophorin and seven transmembrane segments of bacteriorhodopsin Many of the transport proteins believed to have multiple membrane spanning helical regions they are type III or type IV integral proteins Presence of Tyr and Trp residues at the interface btw lipid and water Side chains serve as membrane interface anchors able to interact simultaneously with central lipid phase and the aq phases on either side of the membrane Some integral membranes penetrate only one leaflet of the bilayer Cyclooxygenase target of asprin action do not span the whole membrane but interact strongly with acyl groups on one side of the bilayer Not all integral are composed of transmembrane α helices Another structural motif is the β-barrel 20+ transmembrane segments form β-sheets that line a cylinder Same that favor α-helix formation in hydPHO interior of lipid bilayer also stabilize β-barrels When no water molecules are available to H bond the carbon oxygen and nitrogen of the peptide bond, maximal intrachain H bonding give sthe most stable conformation Planar β-sheets do not maximize these interactions, are generally not found in the membrane interior Β-barrels do allow all possible H bonds apparently common among membrane proteins Porins: allow certain polar solutes to cross the outer membrane of G-neg bact have many stranded β-barrels Polypeptide is more extended in the β-conformaiton than in an α-helix 7-9 residues are needed to span a membrane Alternating side chains project above and below the sheet Every second residue is hydPHO and interacts with the lipid bilayer Aromatic side chains are commonly found at the lipid protein interface Other residues may or may not be hydPHI Is not useful in predicting transmembrane segments for proteins with β-barrel motifs
Covalently attached lipids anchor some membrane proteins Some proteins contain one or more covalently linked lipids of several types: long chain fatty acids, isoprenoids, sterols, or glycoysylated derivatives of phosphatidylinositol (GPI) Attached lipid provides a hydPHO anchor at the membrane surface Strenght between the hydPHO interaction between bilayer and single hydrocarbon chain linked is barely enough to anchor securely Many proteins have more than one attached lipid moiety Ionic interactions btw (+) charged Lys residues in the protein and (-) charged lipid head groups probably contribute to the stability of the attachment Association of these lipid linked proteins weaker than that for integral membrane proteins At least some cases reversible Alkaline carbonate does not release GPI linked proteins Beyond merely anchoring a protein attached lipid may have specific role Proteins with GPI anchors are exclusively on the outer face and are confined within clusters Other types of lipid linked proteins are exclusively on the inner face Attachment of a specific lipid to a newly synthesized membrane protein has a targeting function Directing the protein to its correct membrane location
Membrane Dynamics Remarkable feature of all biological membranes is their flexibility change shape without losing their integrity and becoming leaky Basis is the noncovalent interactions and the motions allowed to individual lipids not covalently anchored
Acyl groups are ordered Lipid bilayer is quite stable Invidual phospholipid and sterol molecules have freedom of motion Depend on temp and kinds of lipids present gel phase: low temps lipid bilayer form a semilsolid all types of motion of individual molecues are strongly constratined bilayer is paracrystalline liquid disordered state: high temp fatty acids are in constant motion produced by rotation about C-C bonds of long acyl chains interior is more fluid than solid liquid ordered state: intermediate temperatures less thermal motion in the acyl chains but lateral movement still takes place
temps in physiological range (20-40C) long chain saturated fatty acids pack well into liquid ordered array but kinks in unsaturated fatty acids interfere with packing favoring the liquid disorderd state Shorter chain same effect Sterol content important determinnat of lipid state Rigid planar structure of steroid molecules insterted btw fatty acyl side chains reduces the freedom of neighboring fatty acyl chains to move by rotiaotn about their C-C bonds Sterols therefore reduces the fuildity in the core of the bilayer – favoring the liquid orderd phase Increases the thickness of the lipid leaflet Cells regulate lipid compostion to achieve a constant membrane fluidity EX bact synthesize more unsaturated fatty acids and fewer saturated ones Low temps than when cultured at high temps Adjustment in lipid composition at high or low twmps have about the same degree of fluidity
Transbilayer movement of lipids requires catalysis Physiological temp, transbilayer or flipflop diffusion occurs very slowly most membranes Reuqire that a polar or charged head group leave aqueous environment and move into hydPHO interior of bilayer Large positive free energy change Situation in such movement is essential: -1-Flippases catalyze translocation of aminophospholipids phosphatidylethanolamine and phosphatidylserine from the extracellular to the cytosoloic leaflet of the plasma membrane, contributing to the asymmetric distributio of phosphohlipids p-amine and p-serine primarliy in the cytosolic - basolateral inner cytosolic leaflet Sphingolipids and phosphatidylcholine I the - apical outer leaflet Keeping p-serine out of the exTrC leaflet - triggers apoptosis/engulfment of macrophages that cary the p-serine receptors Act in the ER where they move the newly synthesized phospholipids from the site of synthesis to the cytosolic leaflet to the lumenal leaflet Consume 1ATP per phospholipid translocated Structuraly and funcitonall related to the P-type ATPases
-2-Floppases move plasmamembrane phospholipids from the cytosolic to the exTrC leaflet and are ATP dependent ABC transporter family which actively transport hydrophobic substances outward across the plasma membrane
-3-Scramblases are proteins that move any membrane phsopholipid across the bilayer down its conc gradient Is not dependent of ATP Controlled randomization of head group composition on the two faces of the bilayer Activity rises charply with the increase in cytosolic Ca conc May result from cell activition, cell injury or apoptosis
Phosphatidylinositol transfer proteins move phosphatidyllinositol lipids across lipid bilayers Have important roles in lipid signalling and membrane trafficking
Lipids and Proteins diffuse laterally in the bilayer Lipid molecules can move laterally Chaning places with neighboring layer/leaflet EX diffuse laterally so fast it circumnavigates the erythrocyte in seconds Rapid lateral diffusion bilayer tends to randomize th positions of individual molecues in a few seconds Experiementally, fluorescent probes Head groups of lipids follow the probes over time Small region of cell surface bleached by intense laser radiation Within milliseconds, region recoveres its flouresceince as unbleached lipid diffuse into the bleach patch Bleached lipid molecues diffuse away from it Rate o fluorescence recovery after photobleaching or FRAP – measure of rate of lateral diffusion in lipids Some lipids diffuse laterally by up to 1micrometer/second Another technique single particle tracking follow the movement of a single lipid molecules in plasma membrane Confirm the rapid lateral diffusion with small discrete regions Movement from one such region to a nearby region is inhibitied Lipids behave as corralled by fences occasionally jump Many membrane portiens seem to be afloat in a sea of lipids Like lipids proteins are free to diffuse laterally in the plane of the bilayer and in constant moiton Some proteins associate to form large aggregates on the surface of a cell or organelle individual protein molecules do not move relative to one another Other membrane proteins are anchored to internal structures that prevent their free diffusion Tethered to spectrin a filamentous cytoskeleton protein Possible explanation of lateral diffusion of lipid – membrane proteins immobilized by their association with spectrin are the fences that define the regions of relatively unrestricted lipid motion
Sphingolipids and cholesterol cluster in membrane rafts Diffusion of membrane lipids one bilayer leaflet into the other is very slow unless catalyzed Different lipid specieds are asymmetrically distributed Even within a single leaflet lipid distribution is not random Glycosphingolipids contain long chain saturated fatty acids form transient clusters in the outer leaflet largely exclude glycerophospholipids Typically contain one unsaturated fatty acyl group and a shorter saturated fatty acyl group Long saturated acyl groups of sphingolipids form more compat stable associaotn with long rign system of cheolesterol that can be shorter, often unsat chaings of phospholipids Cholesterolphingolipid microdomains in the outer monolayer of the plasma membrane are slightly thicker and more orderd than neighboring microdomains rich in phospholipids More difficult to dissolve with nonionic detergents Behave like liquid orderd sphingolipid rafts in a sea of liquid disordered phospholipids Remarkably enriched in two classes of integral membrane proteins anchored membrane by two covalently attached long chain saturated fatty acids and GPI anchored prteoins Lipid anchors like acyl chains of sphingolipids form more stable association with cholesterol and long acyl groups in rafts than surrounding phosphoplipids Other linked proteins are not preferentially associated with the outer leflet of sphingolipid cholesterol rafts are not rigidly separated membrane proteins can move into and out of lipid rafts of seconds Shorter time scale more relevatnt to many membrane mediate biochemical processes These proteins reside primarily in a raft Fraction of the cell surface occupied by rafts Resists detergent solubilization can be high as 50% in some cases Rafts cover half Most cells express more than 50 different kinds of plasma membrane proteins likely a single raft contains only a subset of membrane protins this segregation of membrane proteins is functionally significant Interaciton of two membrane proteins presence in a single raft would hugely increase the likelihood of their collision Membrane recotpors and signaling protins Signaling though these protins can be disrupted by manipulations that deplet the plasma membrane of cholesterol and destroy lipid rafts
Caveolins special classes of membrane rafts Caveolin is an integral membrane protein with two globular domains connected by hairpin shapted hydPHO domain Binds the protein to the cytoplasmic leaflet of the plasma membrane Three palmitoyl groups attached to the carboxyl terminal globular domain further anchor it to the membrane Binds cholesterol in the membrane and the prescence of caveolin forces lipid bilayer to curve inward Cavolae involve both leaflets of the bilayer cytoplasmic leaflet globular domains project exoplasmic leaflet Shingolipid cholesterol raft with associated GPI anchored proteins Implicated in a variety of cellular functions Membrane trafficking within cells and the transduction of external signals into cellular responses Insulin and other growth factors as well as certain GTP binding proteins Appear to be localized in rafts and perhaps in caveolae
Integral proteins mediate cell-cell interactions and adhesion Integral proteins provide specific points of attachment between celsl or between a cell and extracellular matric proteins Integrins are heterodimeric proteins (2unlike units of α& β) anchored to plasma membrane by single hydPHO transmembrane helix Large exracellular domains combine to form a speicif biding site for exTrC proteins 18 different α subunits and at least 8 different β subunits Wide variety of specificities Various combinations of α&β One common determinant is the sequence Arg-Glys-Asp (RGD) Integrisn are not merely adhesives Srve as receptors and signal transducers conveying in formation across the plasma membrane in both directions Regulate many processes: Platelet aggregation of a wound, tissue repair by tumor Mutaiton in an integrin gene encoding β subunit CD18 cuase of leukocyte adhesion defifciency in humans Rare genetic disease in leukocytes fail to pass out blood vessels to reach sites of infection Infants commonly die of infections 3 other families involved in surface adhesion Cahedrins undergo hemophilic interactions with indetnical cadherins Immunoglobulin-like prtoiens undergo either hemophilic interactions with identical counterparts on another cell or heterophilic interactions with an integrin on a neighboring cell Selectins – exTrC domains that presence of Ca bind specific polysaccharides on the surface Selectins are present primarity in the various types of blood vessels and in the endothelial cells Essential part of the blood clotting process Integral proteins play a role in many other celluar processes serve as transporters and ion channels Recpetors for hormones neurotransmitter and growthfactors Central to oxidative phosphorylation and photosynthesis and cell to cell and antigen cell recognition I nthe immune system Also important players in the membrane fusion that accompany exocytosis/endocytosis
Membrane fusion is central to many biological processes Biological membrane undergo fusion with another membrane without losing tis continuity Membranes are stable they are by no means static Membranous compartments constantly reorganize Vesicles carry newly synthesized lipids and proteins to other organlelles and to the plasma membrane Exocytosis endocytosis, cell division, fusion of egg and sperm cells entry of a membrane enveloped virus all involve membrane reorganization with the fundamental operation is fusion of two membrane segments without loss of continuity Specific fusion requires that: 1 recognize each other 2 surgaces become closely apposed require removal of water normally asosicted with the polar head groups of lipids 3 bilayer structures become locally disrupted fusion of the outer leaflet (hemifusion) 4 bilayers fusse to form a single continuous bilayer receptor mediate endocytosis or regulated secretion 5 fusion process sis triggered at the appropriate time in response to specific signal Fussion proteins mediate these events Speicifc recognition and trasiet local distortion of the bilayer strucuture that favors membrane fusion Fusion proteins are unrelated to the products Two cases of membrane fusion:
-1-Host cell enveloped virus
-2-Relase of neurotransmitters by exocytosis