Cell Biology And Human Genetics 2023-2024 PDF

CELL BIOLOGY AND HUMAN GENETICS Ve más allá Academic Year 2023-2024 DAVID BALLESTEROS PLAZA Department of Pre-clinical Dentistry (Building A) E-mail address: davidalberto.ballesteros @universidadeuropea.e s Ve más allá Academic Year 2023-2024 Lesson 2 BIOLOGICAL MEMBRANES PART I: STRUCTURE, COMPONEN...

CELL BIOLOGY AND HUMAN GENETICS Ve más allá Academic Year 2023-2024 DAVID BALLESTEROS PLAZA Department of Pre-clinical Dentistry (Building A) E-mail address: davidalberto.ballesteros @universidadeuropea.e s Ve más allá Academic Year 2023-2024 Lesson 2 BIOLOGICAL MEMBRANES PART I: STRUCTURE, COMPONENTS AND BASIC FUNCTIONS 1.General characteristics and functions 2. Structure and chemical composition. Factors influencing the fluidity of the membrane 3 The carbohydrate fraction 4 Types and functions of membrane proteins. Lipid rafts https://www.youtube.com/watch?v=QQtqPG8cCpE GENERAL CHARACTERISTICS AND FUNCTIONS SINGLE MEMBRANE Endosome Peroxisome DOUBLE MEMBRANE Golgi ER Mitochondria Nucleus Pasma Membrane Chloroplast (plants) Lisosome CELL MEMBRANE FUNCTIONS Cell membranes are fascinating supramolecular aggregates that not only form a BARRIER between compartments but also harbor many CHEMICAL REACTIONS essential for the existence and functioning of a cell.  Provide boundaries for different cell compartments and intra/extracellular space (1,2).  Facilitate the exchange of substances (selective permeability) and the transport between the cell exterior and interior (3).  Receive extracellular signals and trigger intracellular responses through receptorligand interactions. (4).  Establish Intercellular communication (5).  Produce and maintain electrochemical gradients between the two sides of the membrane. Physical Properties of Biological Membranes  Biological membranes show a laminar structure with two lipid layers.  Biological membranes are flexible structures : they allow changes in the cellular shape and morphology.  Biological membranes are semipermeable : only small uncharged molecules can diffuse freely through the lipid bilayer  Biological membranes are self-sealing thus allowing fusion events (endocytosis, exocytosis, cell division)  Most cell membranes are electrically polarized (transmembrane potential).  Biological membranes are not passive barriers but rather sustain important cellular processes such as specific solute transport, signal transduction, ATP synthesis … STRUCTURE OF THE PLASMA MEMBRANE The plasma membrane is a thin, 75 Å wide layer that completely surrounds the cell. It is a constant component of all cells. Electron micrographs of plasma membrane showed 3 layers of different electron density (2 dense layers of 20 Å, 1 less dense lamina of 35 Å) 1 Å (Ångström)= 0,1 nm The characteristic “railroad track” appearance indicates the presence of two polar layers, consistent with the phospholipid bilayer structure. TECHNIQUE : Electron micrographs after cryofracture of cells. It is a specialized technique that opens up the membrane and splits the lipid bilayer in its two leafs. Extracellular layer Plasma membrane Cytoplasmic layer Protrusions indicate the presence of proteins within the lipid bilayer FLUID MOSAIC MODEL OF BIOMEMBRANES (Singer and Nicholson 1972) ① The membrane is a fluid mosaic made of a lipid bilayer with proteins embedded in it. These proteins interact with each other and with lipids. ② Membranes are dynamic structures where protein and lipids can move in the plane of the membrane and continuously reorganize to support cellular processes ③ Membranes are asymmetrical structures. They show specific and different distribution of proteins, lipids and carbohydrates in each of the two monolayers. http://www.susanahalpine.com/anim/Life/memb.swf BIOLOGICAL MEMBRANES ARE FLUID STRUCTURES Protein Mobility Various experiments have shown that many integral membrane proteins, like phospholipids, are free to diffuse laterally and seem to float quite freely in a sea of lipids. According to this concept, known as the fluid mosaic model, the membrane is viewed as a two-dimensional mosaic of laterally mobile phospholipid and protein molecules. MEMBRANE ASYMMETRY  Carbohydrates in glycoproteins or glycolipids are only present on the extracellular side of the plasma membrane  Peripheral proteins are bound to one side or the other.  The lipid composition of each bilayer is different : some lipids are preferentially found on one side or the other. Each membrane presents a characteristic lipid and protein composition, as well as carbohydrate composition. Examples:Human myelin Sheath (30% porteins and 49% lipids), rat hepatocytes (45% prot and 52% lipids) Bacterial membranes, in which many metabolic processes occur, contain more proteins than lipids. (ex. E.coli 75% proteins, 25% lipids) MEMBRANE COMPOSITION The relative proportions of proteins, lipids and sugars vary in different organisms and cell types. LIPIDS : 43% human erythrocyte, 52% mouse liver PROTEINS : 49% human erythrocyte, 44% mouse liver SUGARS : 8% human erythrocyte, 4%mouse liver MEMBRANE LIPIDS PHOSPHOGLYCERYDES SPHINGOLIPIDS PHOSPHOSPHINGOLIPIDS GLYCOSPHINGOLIPIDS STEROIDS: Cholesterol/Phytosterols VARIABLE LIPID COMPOSITION The lipid composition varies depending on the cell type, the type of membrane (organelle) and also between the inner and outer monolayers in each membrane (membrane asymmetry). Lipid composition is determined by the biological function of the cell or organelle. LIPID DISTRIBUTION IS CHARACTERISTIC OF SPECIES, CELL TYPE AND ORGANELLE LIPID ASYMMETRY Phosphatidylcholine and Sphingomyelin are mostly found in the outer layer. Phosphatidylserine and phosphatidylethanolamine are enriched in the inner layer. Phosphatidylinositol is important in signal transductions and is located in the cytosol layer. Cholesterol is found on both sides. MEMBRANE FLUIDITY BIOLOGICAL RELEVANCE OF MEMBRANE FLUIDITY: IS IMPORTANT FOR CELLS TO LIVE, GROW AND REPRODUCE CELL SIGNALING: membrane fluidity enables protein diffusion in the plane of the lipid bilayer and facilitates interaction with other proteins. TRANSPORT OF LIPIDS AND PROTEINS: membrane fluidity facilitates the transport of membrane proteins and lipis to other regions of the cell. CELL DIVISION: membrane fluidity ensures that membrane molecules are distributed evenly between daugther cells when a cell divides. MEMBRANE FUSION : membrane fluidity allows membranes to fuse and mix their protein and lipid components. MEMBRANE FLUIDITY Membrane fluidity is determined by : Length of fatty acid hydrocarbon chain Presence of double bonds (unsaturations) Cholesterol content Temperature  LENGHT OF HYDROCARBON CHAINS Lipids with short fatty acyl chains have a lower melting point and therefore tend to increase membrane fluidity. This is due to the fact that short chains have less surface area to form van der Waals interactions with one another. LONGER CHAINS SHORTER CHAINS LOWER FLUIDITY HIGHER FLUIDITY + FLUIDITY - MEMBRANE FLUIDITY  PRESENCE OF UNSATURATIONS (double bonds) : UNSATURATED Hydrocarbon chains SATURATED Hydrocarbon chains MORE UNSATURATIONS SATURATED CHAIN Hydrocarbon chains with trans double bonds behave like saturated chains and do not increase membrane fluidity. HIGHER FLUIDITY LOWER FLUIDITY MEMBRANE FLUIDITY  PRESENCE OF CHOLESTEROL Polar head Polar head Steroid ring structure Rigid planar Region more rigid with the presence CHOLESTEROL Hydrocarbon tail nonpolar Region more fluid Cholesterol restricts the random movement of the polar heads of the fatty acyl chains, but it separates and disperses their tails, causing the inner regions of the bilayer to become slightly more fluid. CHOLESTEROL ACTS AS REGULATOR OF MEMBRANE FLUIDITY At high temperature, cholesterol tends to make the membrane less fluid. However, at low temperature, cholesterol keeps the membrane in a fluid state. Therefore, cholesterol mitigates changes in membrane fluidity caused by changes in temperature. MEMBRANE FLUIDITY  TEMPERATURE : BELOW NORMAL PHYSIOLOGICAL TEMPERATURES NORMAL PHYSIOLOGICAL TEMPERATURES SEMISOLID STATE Motion of individual lipid molecules is strongly constrained. ORDERED LIQUID STATE There is some thermal motion in the acyl chains of the lipid bilayer,. HIGHER TEMPERATURE HIGHER FLUIDITY ABOVE NORMAL PHYSIOLOGICAL TEMPERATURES DISORDERED LIQUID STATE Individual hydrocarbon chains of fatty acids are in constant motion MEMBRANE DYNAMICS : LIPID MOVEMENT 1 Rotation around their own axis 2 Lateral Diffusion 3 Transbilayer movement : Flip-Flop Spontaneous Flip-Flop movements are rare and slow. Such movements require enzymatic activities (Flippases, Floppases, Scramblases) and energy from ATP hydrolysis. FLIPPASES FLOPPASES SCRAMBLASE Down their gradient and activated by calcium CARBOHYDRATES IN CELL MEMBRANES  Many membrane proteins and some lipids are covalently linked to carbohydrate chains.  These glycoproteins and glycolipids are ALWAYS located in the exoplasmic leaf of the membrane, with the carbohydrate chain protruding from the cell surface (asymmetry).  They are specially abundant in the plasma membrane of eukaryotic cells. They are absent from inner mitochondrial membrane and other intracellular membranes. THE GLYCOCALIX The carbohydrate moieties of lipis and proteins forms a fuzz-like coating on the cell surface : the Glycocalix. These carbohydrates can interact with components of the extracellular matrix, as Microvilli GLYCOCALIX well as lectins, growth factors, and antibodies. They have several physiological roles: Protection of cell surface proteolytic enzymes or lesions. from and Cellular recognition differentiation Relationship Cell-Extracellular Matrix Provides viscosity movement and facilitates Recruitment and binding of bacteria, virus and molecules for endocitosis. MEMBRANE PROTEINS  In animal cells proteins constitute about 50% of the mass of most plasma membranes.  Proteins are asymmetrically distributed in the membrane.  Membrane proteins are specific of the species and the specialized functions of each membrane Plasma membrane proteins can diffuse laterally in the lipid bilayer. Based on the type and strength of association with lipid bilayer membrane proteins can be calssified as : Integral membrane proteins : strongly bound to the membrane through hydrophobic interactions: They require strong agents for extraction (detergents, organic solvents...). Peripheral membrane proteins : attached to the membrane through non-covalent interactions with other proteins. They can be extracted with mild reagents (pH, high salt....). Peripheral and Integral Membrane Proteins INTEGRAL PROTEINS (1,2,3,4,5,6) PHERIPHERAL PROTEINS (7,8)  Integral Membrane proteins :  Transmembrane proteins (1, 2, 3) : with one or more transmembrane domains  Anchored to the cytosolic surface through an amphiphilic α-helix (4)  Lipid-anchored proteins (5, 6): proteins covalently attached to a lipid chain.  Peripheral proteins (7, 8): bound through noncovalent interactions (electrostatic or Hydrogen bonds) to other integral membrane proteins.. TRANSMEMBRANE PROTEINS They contain one or more membrane-spanning α-helical domains with hydrophobic amino acids protruding outwards that establish HYDROPHOBIC INTERACTIONS with the lipids in the bilayer. In addition, these proteins (cytosolic and extracellular domains) also establish IONIC INTERACTIONS with the polar head groups of the phospholipids. They can be extracted from the membrane with detergents or organic solvents. EXTRACELLULAR DOMAIN TRANSMEMBRANE DOMAIN CYTOSOLIC DOMAIN Transmembrane proteins can also form hydrophilic CHANNELS. Beta- Barrel : curved β sheets Multiple amphiphilic α-helices FUNCTIONS OF INTEGRAL MEMBRANE PROTEINS LIPID-ANCHORED PROTEINS These proteins are COVALENTLY BOUND to a lipid : Fatty acid Isoprenoid Glycosylated phospholipid (GPI) The hydrophobic carbon chain of the attached lipid is embedded in one leaf of the membrane and anchors the protein to the membrane. The polypeptide chain itself DOES NOT enter the phospholipid bilayer. GPI-proteins are found on the outer side of the membrane (outer leaf). All other lipidanchored proteins are bound to the inner side of the membane. Fatty acids Isoprenoid GPI-proteins (Glycosylated derivatives of phosphatidylinositol) GPI proteins can be extracted with Phospholipase C PERIPHERAL PROTEINS These proteins ssociate to transmembrane proteins through NON-COVALENT INTERACTIONS : Electrostatic interactions and Hydrogen bonds They can be dissociated from the membrane by changes in pH or high concentration of salts. They can be localized to the cytosolic face of the plasma membrane (cytoskeletal proteins spectrin and actin in erythrocytes for example) or to the outer (exoplasmic) surface of the plasma membrane (extracellular matrix proteins). E P lipid PHOSPHATIDYLINOSITOL PHOSPHATIDYLSERINE ESPHINGOMIELYNE PHOSPHATIDYLETANOLAMINE CHOLESTEROL GLUCOLIPID GLUCOPROTEIN E P 1 + 2 3 4 5 6 7 8 MICRODOMAINS IN PLASMA MEMBRANES: LIPID RAFTS Lipids and proteins are not randomly distributed in the membrane. Cholesterol and sphingolipids tend to cluster with certain proteins to form MEMBRANE MICRODOMAINS called LIPID RAFTS. This clustering occurs in the outer leaf of the membrane and results in a thicker and less fluid (more rigid) patch in the membrane. 1.- sphingolipids 2.- cholesterol 3.- glycoproteins 4.- proteins anchored by GPI Extracellular space 1 Thicker and more rigid region 2 4 Intracellular space Lipid raft 3 LIPID RAFTS Lipid- Rafts are enriched in different cell-surface receptors, as well as in signaling proteins than bind to receptors. That´s why these membrane microdomains are thought to facilitate cell signalling events and endocytosis. Some proteins are only present in rafts transiently (in response to a stimulus for example) while others are preferentially found in rafts. Region of membrane with lipid rafts (red). Yellow picks are proteins anchored by GPI. https://www.youtube.com/watch?v=QQtqPG8cCpE https://www.youtube.com/watch?v=Ym3mTa5WEOY Liposomes containing Phosphatidylcholine and Sphingomyelin ithout (A) or with Cholesterol (B)