Bài giảng Biochemistry 2/e - Chapter 9: Membranes and Cell Surfaces

Chapter 9Membranes and Cell Surfacesto accompanyBiochemistry, 2/ebyReginald Garrett and Charles GrishamAll rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline9.1 Membranes9.2 Structure of Membrane Proteins9.3 Membrane and Cell-Surface Polysaccharides9.4 Glycoproteins9.5 Proteoglycans9.1 MembranesStructures with many cell functions Barrier to toxic molecules Help accumulate nutrients Carry out energy transduction Facilitate cell motion Assist in reproduction Modulate signal transduction Mediate cell-cell interactions Spontaneously formed lipid structuresHydrophobic interactions all! Very few lipids exists as monomers Monolayers arrange lipid tails in the air! Micelles bury the nonpolar tails in the center of a spherical structure Micelles reverse in nonpolar solvents Spontaneously formed lipid structuresHydrophobic interactions all! Lipid bilayers can form in several ways unilamellar vesicles (liposomes) multilamellar vesicles (Alex Bangham) The Fluid Mosaic ModelS. J. Singer and G. L. Nicolson The phospholipid bilayer is a fluid matrix The bilayer is a two-dimensional solvent Lipids and proteins can undergo rotational and lateral movement Two classes of proteins: peripheral proteins (extrinsic proteins) integral proteins (intrinsic proteins)Motion in the bilayerLipid chains can bend, tilt and rotate Lipids and proteins can migrate ("diffuse") in the bilayer Frye and Edidin proved this (for proteins), using fluorescent-labelled antibodies Lipid diffusion has been demonstrated by NMR and EPR (electron paramagnetic resonance) and also by fluorescence measurementsMembranes are AsymmetricLateral Asymmetry of Proteins: Proteins can associate and cluster in the plane of the membrane - they are not uniformly distributed in many cases Lateral Asymmetry of Lipids: Lipids can cluster in the plane of the membrane - they are not uniformly distributed Membranes are AsymmetricTransverse asymmetry of proteins Mark Bretscher showed that N-terminus of glycophorin is extracellular whereas C-terminus is intracellular Transverse asymmetry of lipids In most cell membranes, the composition of the outer monolayer is quite different from that of the inner monolayer FlippasesA relatively new discovery! Lipids can be moved from one monolayer to the other by flippase proteins Some flippases operate passively and do not require an energy source Other flippases appear to operate actively and require the energy of hydrolysis of ATP Active flippases can generate membrane asymmetriesMembrane Phase TransitionsThe "melting" of membrane lipids Below a certain transition temperature, membrane lipids are rigid and tightly packed Above the transition temperature, lipids are more flexible and mobile The transition temperature is characteristic of the lipids in the membrane Only pure lipid systems give sharp, well-defined transition temperatures 9.2 Structure of Membrane ProteinsSinger and Nicolson defined two classes Integral (intrinsic) proteins Peripheral (extrinsic) proteins We'll note a new one - lipid-anchored proteins Peripheral ProteinsPeripheral proteins are not strongly bound to the membrane They can be dissociated with mild detergent treatment or with high salt concentrations Integral Membrane ProteinsIntegral proteins are strongly imbedded in the bilayer They can only be removed from the membrane by denaturing the membrane (organic solvents, or strong detergents) Often transmembrane but not necessarily Glycophorin, bacteriorhodopsin are examples GlyophorinA single-transmembrane-segment proteinOne transmembrane segment with globular domains on either end Transmembrane segment is alpha helical and consists of 19 hydrophobic amino acids Extracellular portion contains oligosaccharides and these constitute the ABO and MN blood group determinants Mark Bretscher showed that glycophorin was a transmembrane proteinBacteriorhodopsinA 7-transmembrane-segment (7-TMS) protein Found in purple patches of Halobacterium halobium Consists of 7 transmembrane helical segments with short loops that interconnent the helices Note the symmetry of packing of bR (see Figure 9.15) bR is a light-driven proton pump!Lipid-Anchored ProteinsA relative new class of membrane proteins Four types have been found: Amide-linked myristoyl anchors Thioester-linked fatty acyl anchors Thioether-linked prenyl anchors Glycosyl phosphatidylinositol anchors Amide-Linked Myristoyl AnchorsAlways myristic acid Always N-terminal Always a Gly residue that links Examples: cAMP-dependent protein kinase, pp60src tyrosine kinase, calcineurin B, alpha subunits of G proteins, gag protein of HIV-1Thioester-linked Acyl AnchorsBroader specificity for lipids - myristate, palmitate, stearate, oleate all found Broader specificity for amino acid links - Cys, Ser, Thr all found Examples: G-protein-coupled receptors, surface glycoproteins of some viruses, transferrin receptor triggers and signalsThioether-linked Prenyl AnchorsPrenylation refers to linking of "isoprene"-based groups Always Cys of CAAX (C=Cys, A=Aliphatic, X=any residue) Isoprene groups include farnesyl (15-carbon, three double bond) and geranylgeranyl (20-carbon, four double bond) groups Examples: yeast mating factors, p21ras and nuclear laminsGlycosyl Phosphatidylinositol AnchorsGPI anchors are more elaborate than others Always attached to a C-terminal residue Ethanolamine link to an oligosaccharide linked in turn to inositol of PI See Figure 9.20 Examples: surface antigens, adhesion molecules, cell surface hydrolases Lipid Anchors are Signaling DevicesRecent evidence indicates that lipid anchors are quite transient in nature Reversible anchoring and de-anchoring can control (modulate) signalling pathwaysSimilar to phosphorylation/ dephosphorylation, substrate binding/ dissociation, proteolytic cleavage triggers and signalsBacterial Cell WallsComposed of 1 or 2 bilayers and peptidoglycan shell Gram-positive: One bilayer and thick peptidoglycan outer shell Gram-negative: Two bilayers with thin peptidoglycan shell in between Gram-positive: pentaglycine bridge connects tetrapeptides Gram-negative: direct amide bond between tetrapeptides More Notes on Cell WallsNote the gamma-carboxy linkage of isoglutamate in the tetrapeptide Peptidoglycan is called murein - from Latin "murus", for wall Gram-negative cells are hairy! Note the lipopolysaccharide "hair" in Figures 9.23 and 9.24 Cell Surface PolysaccharidesA host of important functions! Animal cell surfaces contain an incredible diversity of glycoproteins and proteoglycans These polysaccharide structures regulate cell-cell recognition and interaction The uniqueness of the "information" in these structures is determined by the enzymes that synthesize these polysaccharides9.4 GlycoproteinsMany structures and functions! May be N-linked or O-linked N-linked saccharides are attached via the amide nitrogens of asparagine residues O-linked saccharides are attached to hydroxyl groups of serine, threonine or hydroxylysine See structures in Figure 9.26O-linked Saccharides of GlycoproteinsFunction in many cases is to adopt an extended conformation These extended conformations resemble "bristle brushes" Bristle brush structure extends functional domains up out of the glycocalyx See Figure 9.27N-linked OligosaccharidesMany functions known or suspected Oligosaccharides can alter the chemical and physical properties of proteins Oligosaccharides can stabilize protein conformations and/or protect against proteolysis Cleavage of monosaccharide units from N-linked glycoproteins in blood targets them for degradation in the liver - see pages 287-2899.5 ProteoglycansGlycoproteins whose carbohydrates are mostly glycosaminoglycans Components of the cell membrane and glycocalyx Consist of proteins with one or two types of glycosaminoglycan See structures, Figure 9.319.5 ProteoglycansExample: syndecan - transmembrane protein - inside domain interacts with cytoskeleton, outside domain interacts with fibronectinProteoglycan FunctionsModulation of cell growth processes Binding of growth factor proteins by proteoglycans in the glycocalyx provides a reservoir of growth factors at the cell surface Cushioning in joints Cartilage matrix proteoglycans absorb large amounts of water. During joint movement, cartilage is compressed, expelling water!