1. Membrane Structure and Function.pdf

MEMBRANES Structure and Function Asst. Prof. Betül ÖZBEK [email protected] 2023-2024 The Body’s Internal Water Is Compartmentalized by the Cell Membrane o Water makes up about 60% of the lean body mass of the human body and is distributed in two large compartments....

MEMBRANES Structure and Function Asst. Prof. Betül ÖZBEK [email protected] 2023-2024 The Body’s Internal Water Is Compartmentalized by the Cell Membrane o Water makes up about 60% of the lean body mass of the human body and is distributed in two large compartments. Which is separated by a cell membrane. Intracellular Fluid (ICF): This compartment constitutes two-thirds of total body water and provides a specialized environment for the cell. Extracellular Fluid (ECF): This compartment contains about one-third of total body water and is distributed between the plasma and interstitial compartments. 2 The Ionic Compositions of Intracellular & Extracellular Fluids 3 Properties of the Cell Membranes o Membranes are structures consisting of an asymmetric lipid bilayer with distinct inner and outer surfaces. o Membranes are dynamic, highly fluid structures consisting of a lipid bilayer and associated proteins. The plasma membrane also exchanges material with the extracellular environment by exocytosis and endocytosis due to their flexibility, and there are special areas of membrane structure—gap junctions—through which adjacent cells may communicate by exchanging material. o The plasma membrane has selective permeabilities. o Selective membrane molecular permeability is generated through the action of specific transporters and ion channels. 4 Properties of the Cell Membranes o Plasma membrane plays key roles in cell–cell interactions and in transmembrane signaling. o Membranes also form specialized compartments (organelles) within the cell. For example mitochondria, endoplasmic reticulum (ER), Golgi, lysosomes, and the nucleus. o Membranes localize enzymes. o Changes in membrane components can affect water balance and ion flux. Inhibition of Na+-K+ ATPase by digoxins or digitals. o Specific deficiencies or alterations of certain membrane components (eg, caused by mutations in genes encoding membrane proteins) lead to a variety of diseases. 5 Properties of the Cell Membranes o Membranes are structures consisting of an asymmetric lipid bilayer with distinct inner and outer surfaces. The choline-containing phospholipids (phosphatidylcholine and sphingomyelin) are located mainly in the outer leaflet; The aminophospholipids (phosphatidylserine and phosphatidylethanolamine) are preferentially located in the inner leaflet. External location of the carbohydrates attached to membrane proteins 6 Properties of the Cell Membranes o Membranes are dynamic structures, formed by hydrophobic interactions and do not contain covalent bonds. Phospholipids can rotate around themselves or move between the inner and outer surfaces of the membrane in a so-called flip-flop action (with the help of flippase enzymes). 7 Properties of the Cell Membranes o Membrane structures and surfaces are protein-studded. o Membranes are amphipathic, both have hydrophobic (lipid tale) and hydrophilic compartments. Triacylglycerols (TAGs) and cholesterol-esters are pure hydrophobic molecules. Therefore they are not present in membranes. o The individual lipid and protein units in a membrane form a fluid mosaic model. Amphipathic structure; S: 8 Saturated FA, U: Unsaturated FA Properties of the Cell Membranes o The lipid bilayer is impermeable to most water-soluble molecules since such charged molecules would be insoluble in the hydrophobic core of the bilayer. o Gases such as oxygen, CO2, and nitrogen—small molecules with little interaction with solvents—readily diffuse through the hydrophobic regions of the membrane. o In general, the permeability coefficients of small molecules in a lipid bilayer correlate with their solubilities in nonpolar solvents. The permeability coefficient measures a molecule’s ability to diffuse across a permeability barrier. Molecules that move rapidly through a given membrane are said to have a high permeability coefficient. 9 Properties of the Cell Membranes o H2O can easily pass o Na+ (sodium) is the slowest one to pass 10 Plasma Membrane o Membranes include A. Lipid B. Carbohydrate and C. Protein parts o The major lipids in mammalian membranes are Phospholipids, Glycosphingolipids and Sterols Most common sterol in animal cell membranes is Cholesterol (unesterified cholesterol) 11 A. Membrane Lipids Blue colored areas are hydophilic, other areas are hydrophobic parts of the membrane lipids. Both hydrophilic and hyrophobic = Amphipathic 12 A. Membrane Lipids 1) Phospholipids 1. Glycerophospholipids: Glycerol-phosphate backbone, Phosphatidate derivative I. Phosphatidic acid and an alcohol II. Cardiolipin (Diphosphatidylglycerol) III. Plasmalogens IV. Platelet-activating factor 2. Sphingolipids: Sphingosine backbone, Ceramide derivative 2) Glycolipids 1. Glycosphingolipids (GSLs): Sphingosine backbone, Ceramide derivative I. Cerebrozides II. Sulphatides III. Globoside IV. Gangliozides 2. Galactolipids: Not present in mammalians, present in plants 3) Archaeal ether lipids: Not present in mammalians, present in archeas 4) Sterols: Most common streol in membranes of animal cell is Cholesterol 13 Terminology o Phosphorilation of glycerol backbone at 3rd position = Glycerol 3-Phospate o Glycerol 3-Phospate + 2 Fatty Acid = 1,2-diacylglycerol 3-phosphate = Phosphatidic acid (Phosphatidate) o Serine amino acid + Palmitic acid = Sphingosine o Sphingosine + 1 Fatty acid = Ceramide 14 A. Membrane Lipids 1) Phospholipids 1. Glycerophospholipids: Glycerol-phosphate backbone, Phosphatidate derivative I. Phosphatidic acid and an alcohol o Derived from phosphatidic acid (phosphatidate) and an alcohol o Alcohol groups such as choline, ethanolamine, glycerol, inositol, or serine give phospholipid’s name 15 Phospholipids. The O— shown shaded in phosphatidic acid is substituted by the substituents shown to form the phospholipids: (A) 3-phosphatidylcholine, (B) 3-phosphatidylethanolamine, (C) 3-phosphatidylserine, (D) 3-phosphatidylinositol, and (E) Cardiolipin (diphosphatidylglycerol) 16 1) Phospholipids 1. Glycerophospholipids: Glycerol-phosphate backbone, Phosphatidate derivative I. Phosphatidic acid and an alcohol Phosphatidylcholine (lecithin): o Are the most abundant phospholipids of the cell membrane. o Represent a large proportion of the body’s store of choline. o Choline is important in nervous transmission, as acetylcholine, and as a store of labile methyl groups. o Dipalmitoyl lecithin is a very effective surface-active agent and a major constituent of the surfactant preventing adherence, due to surface tension, of the inner surfaces of the lungs. Its absence from the lungs of premature infants causes respiratory distress syndrome (RDS). LCAT (lecithin cholesterol acyl transferase) activity of HDL 17 1) Phospholipids 1. Glycerophospholipids: Glycerol-phosphate backbone, Phosphatidate derivative I. Phosphatidic acid and an alcohol Phosphatidylinositol: o It is a precursor of second messengers. o Play an important part in cell signaling and membrane trafficking. o Most common phosphoinositide is phosphatidylinositol 4,5-bisphosphate (PIP2). With a suitable hormone agonist stimulation phosphatidylinositol 4,5-bisphosphate (PIP2), is cleaved into diacylglycerol and inositol trisphosphate, and both of these act as internal signals or second messengers. 18 A. Membrane Lipids 1) Phospholipids 1. Glycerophospholipids: Glycerol-phosphate backbone, Phosphatidate derivative II. Cardiolipin (Diphosphatidylglycerol) o Two molecules of phosphatidic acid esterified through their phosphate groups to an additional molecule of glycerol form cardiolipin. o In eukaryotes, cardiolipin is found only in mitochondria and it is the major lipid of the inner mitochondrial membrane. Has a key role in mitochondrial structure and function, and is also thought to be involved in programmed cell death (apoptosis). 19 o Cardiolipin is found in membranes of procaryotes (bacteria) and eukaryotic mitochondria. o Cardiolipin is an antigenic lipid. It is recognized by antibodies (Ab) raised against Treponema pallidum, the bacterium that causes syphilis (Wasserman test). 20 Phospholipases 1. Phospholipase A1 2. Phospholipase A2 3. Phospholipase C 4. Phospholipase D 23 A. Membrane Lipids 1) Phospholipids 2. Sphingolipids: Sphingosine backbone, Ceramide derivative o The only significant sphingophospholipid in human is sphingomyelin. o Alcohol group is choline in sphingomyelin. o Sphingomyelins are found in the outer leaflet of the cell membrane lipid bilayer. o Sphingomyelins are found in large quantities in the myelin sheath that surrounds nerve fibers (The myelin sheath is a layered, membranous structure that insulates and protects neuronal axons of the central nervous system). 24 A. Membrane Lipids 2) Glycolipids o Glycolipids are lipids with an attached carbohydrate or carbohydrate chain. o They are particularly distributed in nervous tissue such as the brain. o They occur particularly in the outer leaflet of the plasma membrane, where they contribute to cell surface carbohydrates that form the glycocalyx. o The major glycolipids found in animal tissues are glycosphingolipids. 25 A. Membrane Lipids 2) Glycolipids 1. Glycosphingolipids (GSLs): Sphingosine backbone, Ceramide derivative Glycosphingolipid Cerebroside Sulfatide Globoside Ganglioside S S S S f f f f i i i i n n n n g g g g o Fatty acid o Fatty acid Fatty acid Fatty acid o o s s s s i i i i n n n n e e e e Monosaccaride Monosaccaride SO4- Di, tri, tetrasaccaride Oligosaccaride Sialic acid The green areas of glycoshingolipids are hydrophilic, other areas are hydrophobic. 26 A. Membrane Lipids 2) Glycolipids 1. Glycosphingolipids (GSLs): Sphingosine backbone, Ceramide derivative I. Cerebrozides S f i n g o Fatty acid s i n e Monosaccaride Monosaccaride part of cerebroside: If glucose: Glucosylceramide is the predominant simple glycosphingolipid of extraneural tissues If galactose: Galactosylceramide a major glycosphingolipid of brain and other nervous tissue 27 A. Membrane Lipids 2) Glycolipids 1. Glycosphingolipids (GSLs): Sphingosine backbone, Ceramide derivative II. Sulfatides S f i n g o Fatty acid s i n e Monosaccaride SO4- o Galactosylceramide can be converted to sulfogalactosylceramide (sulfatide) which has a sulfo group attached to the galactose and is present in high amounts in myelin. 28 A. Membrane Lipids 2) Glycolipids 1. Glycosphingolipids (GSLs): Sphingosine backbone, Ceramide derivative IV. Gangliosides S f i n g o Fatty acid s i n e Oligosaccaride Sialic acid o Gangliosides are complex glycosphingolipids derived from glucosylceramide that contain in addition one or more molecules of a sialic acid bind to galactose or N-acetyl galactosamine. o Neuraminic acid is the principal sialic acid found in human tissues. o Gangliosides present in nervous tissues in high concentration. 30 A. Membrane Lipids 4) Sterols: Most common sterol in membranes of an animal cell is Cholesterol o The majority of cholesterol resides within plasma membranes. o Cholesterol is amphipathic, It intercalates among the phospholipids of the membrane, with its hydrophilic hydroxyl group at the aqueous interface and the remainder of the molecule buried within the lipid bilayer leaflet. Cholesterol acts as a buffer to modify the fluidity of membranes. 31  Factors affecting the fluidity of membranes 1. Directly proportional with temperature. As the temperature increases, the hydrophobic side chains undergo a transition from the ordered state (more gel-like or crystalline phase) to a disordered one, taking on a more liquid-like or fluid arrangement. The temperature at which membrane structure undergoes the transition from ordered to disordered (ie, melts) is called the “transition temperature” (Tm). 2. Directly proportional to the degree of unsaturated fatty acids. Unsaturated bonds that exist in the cis configuration tend to increase the fluidity 3. Inversely proportional to the fatty acid chain length. Hydrophobicity increases with the chain length of fatty acid. S: Saturated FA, U: Unsaturated FA High hydrophilic/hydrophobic values of a phospholipid show its good interaction with water. 4. Inversely proportional to the amount of cholesterol. While unesterified-cholesterol increases the fluidity in the membrane at values below the transition temperature, it decreases the fluidity at values above the transition temperature. 32 B. Carbohydrates In Cell Membranes Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids o On almost every eukaryotic cell, specific oligosaccharide chains attached to components of the plasma membrane form a carbohydrate layer (the glycocalyx), that serves as an information-rich surface that the cell shows to its surroundings. o These oligosaccharides are central players in cell-cell recognition and adhesion, cell migration during development, blood clotting, the immune response, wound healing, and other cellular processes. 33 B. Carbohydrates In Cell Membranes Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids o The informational carbohydrate is covalently joined to a protein or a lipid (they do not occur as free entities) to form a glycoconjugate, which is a biologically active molecule. o Carbohydrate chains are attached to the amino-terminal portion outside the external surface. o Carbohyrate presence on the outer surface of the plasma membrane (the glycocalyx) has been shown with the use of plant lectins (proteins that bind specific glycosyl residues). o Glycophorin is a major integral membrane glycoprotein of human erythrocytes which increases the flip-flop frequency of membrane phospholipids. o Carbohydrates are also present in apoprotein B of plasma lipoproteins. 34 C. Membrane Proteins o Proteins are the major functional molecules of membranes and consist of enzymes, pumps and transporters, channels, structural components, antigens (eg, for histocompatibility), and receptors for various molecules. o Membrane proteins are asymmetrically distributed in the lipid bilayer. o They are loosely bound to the membrane so can move through it. o Membrane proteins are classified into two types: 1. Integral 2. Peripheral 35 C. Membrane Proteins 1) Integral Proteins: o They interact extensively with the phospholipids. o They generally span the bilayer as a bundle of α-helical transmembrane segments (top right figure). transporter molecules, ion channels, various receptors, and G proteins, etc. span the bilayer many times glycophorin A (a simple membrane protein) spans the membrane only once o Usually globular and are themselves amphipathic. They consist of two hydrophilic ends separated by an intervening hydrophobic region that traverses the hydrophobic core of the bilayer. 36 C. Membrane Proteins o Several families of integral proteins in the plasma membrane provide specific points of attachment between cells or between a cell and proteins of the extracellular matrix (surface adhesion). a. Integrins: Surface adhesion proteins that mediate a cell’s interaction with the extracellular matrix and with other cells. Integrins also carry signals in both directions across the plasma membrane, integrating information about the extracellular and intracellular environments. b. Cadherins: Involved in surface adhesion. Interacts with identical cadherins in an adjacent cell. c. Selectins: In the presence of Ca2+, bind specific polysaccharides on the surface of an adjacent cell. They are an essential part of the blood-clotting process. 37 C. Membrane Proteins o Transmembrane proteins can be classified under integral proteins. o Several transmembrane proteins serve as a channel for water-soluble molecules or have receptor functions. These channels serve as transporters and allow the movement of ions and small lipid- insoluble molecules to traverse the lipid bilayer (membrane). o Detergents solubilize (release) integral and transmembrane proteins. 38 C. Membrane Proteins 2) Peripheral proteins: o They do not interact directly with the hydrophobic cores of the phospholipids in the bilayer. o They bind to the hydrophilic regions of specific integral proteins and head groups of phospholipids and can be released from them. o Can be released from the membrane by treatment with salt solutions of high ionic strength 39 C. Membrane Proteins o Peripheral proteins generally bind to cell membranes with loose hydrogen bonds. o However, some peripheral proteins bind to membrane lipids by covalent linkages. This process is called protein lipidation. isoprenylation (farnesyl), cholesterylation, glycosylphosphatidylinositol (GPI): exclusively on the outer face of the membrane myristoylation and internal cysteine S-prenylation and S-acylation (palmitic acid). Mutation resulting in deficient attachment of the GPI anchor to certain proteins of erythrocytes results in paroxysmal nocturnal hemoglobinuria. 40 C. Membrane Proteins 41 Membrane Markers 42 MEMBRANES ARE COMPLEX STRUCTURES COMPOSED OF LIPIDS, PROTEINS, & CARBOHYDRATE-CONTAINING MOLECULES Various cellular membranes have different lipid and protein compositions. o Highest membrane lipid content: Myelin, an electrical insulator found on many nerve fibers. o Highest membrane protein content: Mitochondrial inner membrane because; 1. The electron transport chain (ETC) is localized in the mitochondrial inner membrane and this system contains oxidative enzymes that produce ATP. Except for coenzyme Q, all complexes are in protein structure. 2. The mitochondrial inner membrane is the most selectively permeable membrane known. Except for a few substances, substance transport is carried out by means of carrier proteins localized in the membrane. These factors are the reasons of high protein content of the mitochondrial inner membrane. 43 Some Diseases or Pathologic States Resulting From or Attributed to Abnormalities of Membranes 44 Specialized Features of Plasma Membranes Plasma membranes contain certain specialized structures: 1. Lipid Rafts, 4. Gap Junctions 7. Microvilli 2. Caveolae 5. Desmosomes 3. Tight Junctions 6. Adherens junctions 1. Lipid Rafts: Specialized areas of the exoplasmic (outer) leaflet of the lipid bilayer enriched in cholesterol, sphingolipids, and certain proteins. They are involved in signal transduction. Lipid rafts are stabilized through interactions (direct and indirect) with the actin cytoskeleton 45 Specialized Features of Plasma Membranes 2. Caveolae: o Caveolae may derive from lipid rafts. o A caveola is an flask, or tube-shaped invagination in the plasma membrane into the cytosol. o Proteins detected in caveolae include various components of the signal transduction system (eg, the insulin receptor and some G proteins), the folate receptor, and endothelial nitric oxide synthase (eNOS). 46 Specialized Features of Plasma Membranes 3. Tight junctions: o Tight junctions found in surface membranes. o They are often located below the apical surfaces of epithelial cells and prevent the diffusion of solute macromolecules. o There are 2 types of tight junction i. Zonula ocludens ii. Zonula adherens 47 Specialized Features of Plasma Membranes 4. Gap junctions: o Structures that permit direct transfer of small molecules from one cell to its neighbor. o Gap junctions are composed of a family of proteins called connexins. o Six connexins form a connexin hemichannel (hemiconnexon) and join to a similar structure in a neighboring cell to make a complete, membrane-spanning connexon channel. 48 Specialized Features of Plasma Membranes 49 Membrane Lipid Degradation Related Diseases 50 Transport Systems in Membranes  Transfer of Material and Information Across Membranes A. Small molecules cross the membrane by B. Large molecules cross the membrane by 1. Passive transport (diffusion) 1. Endocytosis a) Simple passive diffusion 2. Exocytosis i. Pass directly from membrane ii. via Ion channels b) Facilitated passive diffusion i. via Transporters (carrier protein) 2. Active transport via a specific transporter (pump) 52 Classes of Transport System o 2 types 1) Uniport 2) Cotransport i. Symport ii. Antiport o Transporters differ in the number of solutes (substrates) transported and the direction in which each solute moves. o This classification tells us nothing about whether these are energy- requiring (active transport) or energy-independent (passive transport) processess. 53 Classes of Transport System o A uniport system moves one type of molecule bidirectionally. o In cotransport systems, the transfer of one solute depends on the stoichiometric simultaneous or sequential transfer of another solute. A symport moves two solutes in the same direction. Examples, Na+-glucose transporters and Na+-amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions Examples, Na+-Ca2+ exchanger (Na+ in and Ca2+ out) or Na+-K+ ATPase. 54 Classes of Transport System Chloride- Bicarbonate Exchanger in Erythrocytes A.1. Passive Diffusion (Transport) o 2 types a) Simple passive diffusion i. Pass directly from the membrane ii. via Ion channels b) Facilitated passive diffusion i. via Transporters o Many small, uncharged molecules or ions (ion channels) pass freely through the lipid bilayer by simple diffusion. o Larger, uncharged molecules, and some small uncharged molecules, are transferred by specific carrier proteins (transporters) or through channels or pores. o Passive transport is always down an electrochemical gradient toward equilibrium (from higher to lower concentration). o Does not require energy. 57 A.1. Passive Diffusion o Gases, and steroid hormones can enter the cell by diffusing down an electrochemical gradient across the membrane and do not require metabolic energy (simple diffusion). o Water has a high permeability coefficient. Easily and rapidly pass membranes (simple diffusion). o Sodium (Na+) (electrolytes are poorly soluble in lipids) has a low permeability coefficient. Cannot pass the membranes and needs a channel. 58 A.1. Passive Diffusion a) Simple diffusion Simple diffusion of a solute across the membrane is limited by three factors: 1. the thermal agitation of that specific molecule; temperature increases diffusion 2. the concentration gradient across the membrane; higher gradient difference faster diffusion 3. the solubility of that solute (the permeability coefficient (PC)) in the hydrophobic core of the membrane bilayer; high PC or solubility easier, and faster diffusion 59 Ion Channels o They are composed of transmembrane protein subunits (pore-like structures). o They allow impermeable ions to cross membranes at rates approaching diffusion limits. o Ion channels are very selective, in most cases permitting the passage of only one type of ion (Na+, Ca2+, etc.) a few are non-selective. o Most cells have a variety of specific channels for Na+, K+, Ca2+, and Cl−. o Mutations in genes encoding them can cause specific diseases (Cystic fibrosis-Cl channel mutation). o Their activities are affected by certain drugs (digitals, digoxin). 60 A.1. Passive Diffusion b) Facilitated diffusion o Involves certain transporters o Different properties of facilitated diffusion from those of simple diffusion: The rate of facilitated diffusion, a uniport system, can be saturated. A “ping-pong” mechanism helps explain facilitated diffusion. A conformational change occurs in the carrier protein. This process is completely reversible, and net flux across the membrane depends on the concentration gradient. 62 A.1. Passive Diffusion b) Facilitated diffusion o Hormones can regulate facilitated diffusion. Insulin via a complex signaling pathway increases glucose uptake in fat and muscle by glucose transporter 4 (GLUT4) stimulation. o Transfer of glucose into organs other than fat and muscle tissue is also done with the help of other glucose transporters (GLUT1, GLUT2,… etc) 63 Transport of Glucose o Glucose Transport: Facilitated diffusion and secondary active transport o Several different glucose transporters (GLUTs) are involved, varying in different tissues. Glucose transport by GLUTs is an example of facilitated diffusion. o In adipocytes and skeletal muscle, glucose enters to the cells by GLUT4 that is enhanced by insulin. o Glucose transport by SGLT-1 (Sodium Glucose Transporter-1) system is an example of secondary active transport. SGLT system appears in Epithelial cells of enterocytes Choroid plexus Renal tubules 64 A Specific Transporter in Facilitated Diffusion: Aquaporins o In certain cells (eg, red cells and cells of the collecting ductules of the kidney), the movement of water by simple diffusion is augmented by movement through water channels. o These channels are a family of integral membrane proteins, called aquaporins (AQPs). Each with a specific location and role at least 10 distinct aquaporins (AP-1 to AP-10) have been identified. o These channels permit the passage of water but exclude the passage of ions and protons. o Insensitivity of AQP2 to ADH in the proximal tubule of kidneys cause polyuria (greater urine output) and more dilute urine a pathology called nephrogenic diabetes insipitus (NDI). 65 A.2. Active Transport o The process of active transport differs from diffusion in that molecules are transported against concentration gradients; hence, energy (ATP) is required. o The maintenance of electrochemical gradients in biological systems is so important that it consumes approximately 30% of the total energy expenditure in a cell. o 2 types of active transport 1. Primary active transport 2. Secondary active transport 66 A.2. Active Transport: Transmembrane Pumps Na+-K+-ATPase (the most important enzymatic marker of the plasma membrane) o Cells maintain a low intracellular Na+ concentration and a high intracellular K+ concentration, along with a net negative electrical potential inside. o The Na+-K+-ATPase pumps three Na+ out and two K+ into the cells. 67 A.2. Active Transport: Transmembrane Pumps Na+-K+-ATPase o This differential ion transport creates a charge imbalance between the inside and the outside of the cell, making the cell interior more negative (an electrogenic effect). o Two clinically important cardiac drugs ouabain and digitalis, inhibit the Na+-K+-ATPase. o The Na+-K+-ATPase can be coupled to various other transporters, such as those involved in the transport of glucose. 68 A.2. Secondary Active Transport: Na+-glucose symport Glucose transport in the small intestine ✓ Glucose and Na+ bind to different sites on a Na+-glucose symporter located at the apical surface. ✓ Na+ moves into the cell down its electrochemical gradient and “drags” glucose with it. Therefore, the greater the Na+ gradient, the more glucose enters; and if Na+ in extracellular fluid is low, glucose transport stops. ✓ To maintain a steep Na+ gradient, this Na+- glucose symporter is dependent on gradients generated by the Na+-K+-ATPase, which maintains a low intracellular Na+ concentration. 69 The Transcellular Movement Of Glucose In An Intestinal Cell 70 Transmembrane Pumps 71 Similarities of Transporter Mediated Facilitated Diffusion and Active Transport 1) Both types of transport involve specific carrier proteins (transporters) and both show specificity for ions, sugars, and amino acids. 2) There is a specific binding site for the solute. 3) The carrier is saturable, so it has a maximum rate of transport (Vmax) 4) There is a binding constant (Km) for the solute, and so the whole system has a Km. 5) Structurally similar competitive inhibitors block transport. 73 B. Macromolecule Transport Macromolecule transport occurs in 2 ways: 1. Endocytosis a) Phagocytosis b) Pinocytosis i. Fluid-phase pinocytosis ii. Absorptive pinocytosis or receptor-mediated endocytosis 2. Exocytosis o The process by which a cell takes up large molecules is called endocytosis. o Procedure of the release of macromolecules (ex: hormones) from the cell is called exocytosis. o Endocytosis and exocytosis both involve vesicle (bilayer lipid membrane) formation. 74 B. Macromolecule Transport 1. Endocytosis o In endocytosis cells sometimes take up large molecules such as polysaccharides, proteins, or polynucleotides and hydrolyze inside the cell to produce cellular functions. o Most endocytotic vesicles fuse with primary lysosomes to form secondary lysosomes, which contain hydrolytic enzymes. o Endocytosis requires: 1) Energy, usually from the hydrolysis of ATP; 2) Ca2+ and, 3) Contractile elements in the cell (likely the microfilament system) 75 B. Macromolecule Transport 1. Endocytosis a) Phagocytosis Occurs only in specialized cells such as macrophages and granulocytes. Phagocytosis involves the ingestion of large particles such as viruses, bacteria, cells, or cellular debris. b) Pinocytosis Pinocytosis (“cell drinking”) is a property of all cells and leads to the cellular uptake of fluid and fluid contents. There are two types. i. Fluid-phase pinocytosis ii. Absorptive pinocytosis or receptor-mediated endocytosis 76 B.1.b. Endocytosis_Pinocytosis i. Fluid-phase pinocytosis is a nonselective process in which the uptake of a solute by the formation of small vesicles is simply proportionate to its concentration in the surrounding extracellular fluid. ii. Absorptive pinocytosis or receptor- mediated endocytosis, is primarily responsible for the uptake of specific macromolecules for which there are binding sites (receptors) on the plasma membrane. 77 B.1.b. Endocytosis_Pinocytosis Absorptive pinocytosis or receptor-mediated endocytosis: o The vesicles formed during absorptive pinocytosis are derived from invaginations (pits) that are coated on the cytoplasmic side with a filamentous material and are appropriately named coated pits. o In many systems, the protein clathrin is the filamentous material and it limits the size of the vesicle. o The lipid phosphatidylinositol 4,5-bisphosphate (PIP2) also plays an important role in vesicle assembly. o In addition, the protein dynamin, which both binds and hydrolyzes GTP, is necessary for the pinching off of clathrin-coated vesicles from the cell surface. 78 B.1.b. Endocytosis_Pinocytosis Absorptive pinocytosis or receptor-mediated endocytosis: o LDL (low-density lipoprotein) uptake: The LDL molecule and its receptor are internalized using coated pits containing the LDL receptor. Endocytotic vesicles containing the LDL-bound LDL receptor complex fuse to lysosomes in the cell. The receptor is released and recycled back to the cell surface membrane, but the apoprotein of LDL is degraded and the cholesteryl esters are metabolized. o SARS-CoV-2 (the virus that causes COVID-19) enters cell with receptor-mediated endocytosis 79 B.2. Macromolecule Transport 2. Exocytosis o Most cells release macromolecules synthesized in the ER and Golgi to the exterior by exocytosis. o In “classical exocytosis” a signal (often a hormone), when it binds to a cell surface receptor, induces a local and transient change in Ca2+ concentration. Ca2+ triggers exocytosis. o Insulin, parathyroid hormone, and catecholamines are all packaged in granules and processed within cells, to be released on appropriate stimulation. 80 Membrane Transport Systems Summary 81 References 1. Harpers Illustrated Biochemistry, 31st edition 2. Lehninger Principles of Biochemistry, 8th edition 3. Lippincott Illustrated Reviews Biochemistry, 7th edition 82