Professor .Orhan CANBOLAT ; MD , PhD

Biologic Membranes Professor .Orhan CANBOLAT ; MD , PhD

BiomedicalImportance • Plasmamembranes form closedcompartmentsaroundcellularprotoplasmtoseparateonecellfromanotherandthuspermitcellularindividuality. • Theplasmamembrane has selectivepermeabilitiesandactsas a barrier, therebymaintainingdifferences in compositionbetweenthe inside andoutside of thecell. • Membranesalso form specializedcompartmentswithinthecell. • Mitochondria, ER, sarcoplasmicreticulum, Golgicomplexes, secretorygranules, lysosomes, andthenuclearmembrane.

Membranes ; cell membranes

Biomedical Importance • Changes in membranestructure can affectwaterbalanceandionfluxandthereforeeveryprocesswithinthecell. • Specificdeficienciesoralterations of certainmembranecomponentsleadto a variety of diseases • Normal cellularfunctiondepends on normal membranes.

Biomedical Importance • The selective permeabilities are provided mainly by channels and pumps for ions and substrates. • The plasma membrane also exchanges material with the extracellular environment by exocytosis and endocytosis, • In addition, the plasma membrane plays key roles in cell-cell interactions and in transmembrane signaling. • Membranes localize enzymes, function as integral elements in excitation-response coupling, and provide sites of energy transduction,such as in photosynthesis and oxidative phosphorylation.

Cell mebrane • Membrane are generaly impermeable to polar molecules or ions • Membranes are flexible but durable • Membrane are about 40 A thick • Membrane are associated with proteins

Nature of molecules • Permeabletomembrane; gases , ethanol, eter , water , anticancerdrugs • İmpermeabletomembrane; glucose , ions , aminoacids , nucleotides Cells ; import; watersolublemolecules ; sugar , aminoacids export ; wasteproductsandcontrolionconcentrations

MembraneLipids Form Bilayers The closed bilayer provides one of the most essential properties of membranes. It is impermeable to most water-soluble molecules, since they would be insoluble in the hydrophobic core of the bilayer.

Membrane Lipids Are Amphipathic • All major lipids in membranes contain both hydrophobic and hydrophilic regions and are therefore termed "amphipathic." • Conversely, if the hydrophilic region were separated from the rest of the molecule, it would be insoluble in oil but soluble in water. • Thus, the polar head groups of the phospholipids and the hydroxyl group of cholesterol interface with the aqueous environment • The polar head group is hydrophilic, and the hydrocarbon tails are hydrophobic or lipophilic

TheMajorLipids in MammalianMembranesAre Phospholipids Glycosphingolipids Cholesterol MajorLipids

Phospholipidstwomajorphospholipidclassespresent in membranes • Glycerophospholipids; glycerolbackbone • Essentialformembranestructure • Mostabundantmembranelipids • No geneticdefects in human Phosphatidylinositol ; cellsinglanig • Sphingolipids ; spingosinebackbone Sphingomyelin is prominent in myelinsheaths.

Glycosphingolipids • The glycosphingolipids (GSLs) are sugar-containing lipids built on a backbone of ceramide • they include galactosyl- and glucosylceramide (cerebrosides) and the gangliosides. • They are mainly located in the plasma membranes of cells

Sterols • The most common sterol in membranes is cholesterol which resides mainly in theplasma membranes of mammalian cells • Largely hydrophobic • İt has one polar group , a hydroxil , • making it amphipatic • The hydroxil group of cholestrol form hydrogen bonds with polar phospholipids head group • Modulates membrane fludity • Can be found in lesser quantities in mitochondria, Golgi complexes, and nuclear membranes.

Carbohydrates • Carbohydrates have a carbon backbone bearing hydroxyl groups with either an aldehyde or ketone at one carbon • Simple sugars in aqueous solution usually form cyclic structures • Connected with proteins

SCHINDLER DISEASE • Schindler disease (also called lysosomal α-N-acetylgalactosaminidase [-NAGA] deficiency , • Deficiency or mutation of α-NAGA leads to an abnormal accumulation of some glycosphingolipids trapped in the lysosomes of many tissues of the body. • Children develop normally until 8–15 months of age, when they begin to lose previously acquired skills requiring coordination of physical and mental activities (developmental regression). • Other symptoms include decreased muscle tone (hypotonia) and weakness; involuntary, rapid eye movements (nystagmus); visual impairment; and seizures. .

Proteins Membranes contain proteins, and proteins are also amphipathic molecules that can be inserted into the correspondingly amphipathic lipidbilayer. Classified as Peripheral ; are on the membrane surface İntegral ; embeded in the Membrane Trans membrane segments have an very hydrophpbic carecter

Proteins • Proteins can be amphipathic and form an integral part of the membrane by having • hydrophilic regions protruding at the inside and outside faces of the membrane but connected by • a hydrophobic region traversing the hydrophobic core of the bilayer. • Hydrophobic amino acids have positive values; • Polar amino acids have negative values. • Another aspect of the interaction of lipids and proteins is that some proteins are anchored to one leaflet of the bilayer by covalent linkages to certain lipids. • Palmitate and myristate are fatty acids involved in such linkages to specific proteins. • A number of other proteins are linked to glycophosphatidylinositol (GPI) structures.

Different Membranes Have Different Protein • The number of different proteins in a membrane Proteins are the major functional moleculesof membranes and consist of enzymes, pumps , channels, structural components, antigens and receptorsfor various molecules.

Transfer of material and information accross membranes Crossmemranemovement of smallmolecules 1.Diffussion ; CO2, O2, H2O , 2.PassiveandFacilitated transport 3.Active transport Crossmemranemovemet of largemolecules 1.Endocytosis 2.Exocytosis Signaltransmissionacrossmemranes 1. Signaltransduction ; glukagon , cAMP 2 Movementtointracellularreceptors, Steroidhormones 3.Intracellularcontact

Passive Diffusion • Thesimplepassivediffusionof a soluteacrossthemembrane is limitedbythethermalagitation of thatspecificmolecule, bytheconcentrationgradientacrossthemembrane, • Thefollowingfactorsaffect net diffusion of a substance: • (1) Itsconcentrationgradientacrossthemembrane. Solutesmovefromhightolowconcentration. • (2) Theelectricalpotentialacrossthemembrane. Solutesmovetowardthesolutionthat has theoppositecharge. The inside of thecellusually has a negativecharge. • (3) Thepermeabilitycoefficientof thesubstanceforthemembrane. • (4) Thehydrostaticpressuregradientacrossthemembrane. Increasedpressurewillincreasethe rateandforce of thecollisionbetweenthemoleculesandthemembrane. • (5) Temperature. Increasedtemperaturewillincreaseparticlemotionandthusincreasethefrequency of collisionsbetweenexternalparticlesandthemembrane.

Symport - Antiport- Uniport • Symport - A type of active transport protein found in cell membranes which moves two different ions or molecules, e.g., a Na+ ion and a glucose, in the same direction across the membrane; • often one ion or molecule can be moved in opposition to its concentration gradient because the movement of the other ion or molecule, moving in accord with its concentration gradient, is driving the process; ultimately, the movement is powered by ATP hydrolysis, but that energy source may be directly (primary active transport) or indirectly (secondary active transport) associated with the active transport protein. • antiport - A type of active transport protein found in cell membranes which moves two different ions or molecules, e.g., a Na+ ion and a glucose, in opposite directions across the membrane; often one ion or molecule can be moved in opposition to its concentration gradient because the movement of the other ion or molecule, moving in accord with its concentration gradient, is driving the process; ultimately, the movement is powered by ATP hydrolysis, but that energy source may be directly (primary active transport) or indirectly (secondary active transport) associated with the active transport protein..

Symport - Antiport- Uniport Besides ATP-powered pumps , cells have a second, discrete class of proteins that import or export ions and small molecules, such as glucoseand amino acids against a concentration gradient. SymportandAntiportsystems 1. A symport= cotransportmoves two solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+-sugar transporters (for glucose and certain other sugars) and Na+-amino acid transporters in mammalian cells. Many cells, such as those lining the small intestine and the kidney tubules, need to concentrate glucose against a very large concentration gradient. Such cells utilize a two Na+/one-glucosesymporter; a protein that couples transmembrane movement of oneglucose molecule to the transport of two Na+ ions:

2. Antiport = countertransportsystems move two molecules in oppositedirections Chloride – bicarbonate exchanger in renal cells .This anion antiporter catalyzes the one-for-one exchange of Cl− and HCO3− across the plasmamembrane, • The plasma membranemost cells contains one or more types of antiporters, which couple movement of a cotransported ion (often Na+) down its electrochemical gradient to movement of a different molecule in the opposite direction against a concentration gradient . • In cardiac muscle cells, for example, a Na+/Ca2+antiporter, plays the principal role in maintaining a low concentration of Ca2+ in the cytosol

Uniport The rate of facilitated diffusion, a uniport system, can be saturated; A "Ping-Pong" mechanism explains facilitated diffusion. In the "pong" state, it is exposed to high concentrations of solute, and molecules of the solute bind to specific sites on the carrier protein. Transport occurs when a conformational change exposes the carrier to a lower concentration of solute

FacilitatedDiffusion Hormonesregulatefacilitateddiffusionbychangingthenumber of transportersavailable. Insulinincreasesglucose transport in fatandmusclebyrecruitingtransportersfrom an intracellularreservoir. Insulinalsoenhances amino acid transport in liverandothertissues. One of thecoordinatedactions of glucocorticoidhormonesis toenhance transport of amino acidsintoliver, wherethe amino acidsthenserve as a substrateforgluconeogenesis. Growthhormoneincreases amino acid transport in allcells, andestrogens do this in theuterus. Thereare at leastfivedifferentcarriersystemsfor amino acids in animalcells.

Glucose Transport • In adipocytes and muscle, glucose enters by a specific transport system that is enhanced by insulin. • Glucose and Na+ bind to different sites on the glucose transporter. 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 symport is dependent on gradients generated by the Na+K+ ATPase, which maintains a low intracellular Na+ concentration.

Glucose Transport • The transcellular movement of sugars involves one additional component: • a uniport that allows the glucose accumulated within the cell to move across a different surface toward a new equilibrium; this occurs in intestinal cells, for example, and involves a glucose uniporter (GLUT2). • The treatment of severe cases of diarrhea (such as is found in cholera) makes use of the above information. • In cholera, massive amounts of fluid can be passed as watery stools in a very short time, resulting in severe dehydration and possibly death. • Oral rehydration therapy, consisting primarily of NaCl and glucose, has been developed by the World Health Organization (WHO). • The transport of glucose and Na+ across the intestinal epithelium forces (via osmosis) movement of water from the lumen of the gut into intestinal cells, resulting in rehydration. Glucose alone or NaCl alone would not be effective.

Amino acid Cotransporters. • Amino acid uptake into epithelial cells of the intestinal lumen is mediated by Na+/amino acid cotransporters. • a. This symport mechanism is specific only for the L-amino acids derived from digestion of dietary proteins. • b. The energy for this concentrative mechanism of amino acid transport comes directly from the Na+ electrochemical gradient across the brush border membrane. • c. There are seven transport systems tailored to chemically similar groups of amino acids, eg, there is one for neutral amino acids with small or polar side chains such as alanine, serine, and threonine.

HARTNUP DISORDER Hartnup disorder is a rare condition caused by impaired resorption of neutral amino acids (especially tryptophan, alanine, threonine, glutamine, and histidine) in the renal tubules and malabsorption in the intestine, resulting from mutations that lead to defective function of a neutral amino acid transporter.

HARTNUP DISORDER • Hartnup disorder exhibits symptoms similar to pellagra (niacin deficiency), characterized by three of the “four D’s”: diarrhea, dermatitis (a red, scaly rash), dementia (intermittent ataxia), and death (rarely). • Patients show signs of tryptophan deficiency despite a healthy diet as well as elevated urinary and fecal excretion of the neutral amino acids.

CYSTINURIA • Cystinuria, also called cystine urolithiasis, arises from impaired reabsorptive transport of cystine and the cationic amino acids from the fluid within the renal proximal tubules. • The biochemical defect is a deficiency or mutation of the gene that encodes the common membrane transporter for cystine and the dibasic amino acids. • • The disease is characterized by excessive excretion of cystine and the dibasic amino acids arginine, lysine, and ornithine by the kidneys that may lead to precipitation of some of these compounds in the form of kidney stones.

Active Transport • Molecules are transported away from thermodynamic equilibrium; hence, energy is required. a. The ATPase is an integral membrane pump that exchanges three Na+ ions for two K+ ions. • b. ATP is hydrolyzed to ADP + Pi via a catalytic site on the intracellular face of the protein. • c. The action of the pump also serves to maintain a net negative electrical potential toward the inside of the cell • Ouabain or digitalis inhibits this ATPase by binding to the extracellular domain. Inhibition of the ATPase by ouabain can be antagonized by extracellular K+.

Active Transport

Nerve Impulses Are Transmitted Up & Down Membranes • The membrane forming the surface of neuronal cells maintains an asymmetry of inside-outside voltage (electrical potential) and is electrically excitable. • When appropriately stimulated by a chemical signal mediated by a specific synaptic membrane receptor, • gates in the membrane are opened to allow the rapid influx of Na+ or Ca2+ (with or without the efflux of K+), • so that the voltage difference rapidly collapses and that segment of the membrane is depolarized. • However, as a result of the action of the ion pumps in the membrane, the gradient is quickly restored.

Nerve Impulses Are Transmitted Up & Down Membranes • When large areas of the membrane are depolarized in this manner, the electrochemical disturbance propagates in wave-like form down the membrane, generating a nerve impulse. • Myelin sheets, formed by Schwann cells, wrap around nerve fibers and provide an electrical insulator • The myelin membrane is composed of phospholipids, cholesterol, proteins, and GSLs. • Certain diseases, eg, multiple sclerosis and the Guillain-Barré syndrome, are characterized by demyelination and impaired nerve conduction.

Endocytosis- Exocytosis • The process by which cells take up large molecules is called "endocytosis." Some of these molecules ; eg, polysaccharides, proteins, and polynucleotides • Endocytosis is responsible for the entry of DNA into the cell. • Cells also release macromolecules by Exocytosis. • Endocytosis and exocytosis both involve vesicle formation with or from the plasma membrane.

Endocytosis Endocytosis requires • (1) energy, usually from the hydrolysis of ATP; • (2) Ca2+ in extracellular fluid; and • (3) contractile elements in the cell , probably themicrofilament system

Phagocytosis - Pinocytois There are two general types of endocytosis ; Phagocytosis - Pinocytois Phagocytosis occurs only in specialized cells such as macrophages and granuloytes. Phagocytosis involves the ingestion of large particles such as viruses, bacteria, cells, or debris.

Pinocytosis • Pinocytosis is a property of all cells and leads to the cellular uptake of fluid and fluid contents. • 1. Fluid-phase pinocytosis • 2. Absorptive pinocytosis • Fluid-phase pinocytosis is a nonselective process in which the uptake of a solute by formation of small vesicles is simply proportionate to its concentration in the surrounding extracellular fluid. The formation of these vesicles is an extremely active process.

Pinocytosis • The other type of pinocytosis, absorptive pinocytosis,is a receptor-mediated selective process primarily responsible for the uptake of macromolecules for which there are a finite number of binding sites on the plasma membrane. • These high-affinity receptors permit the selective concentration of ligands from the medium, minimize the uptake of fluid or soluble unbound macromolecules, and markedly increase the rate at which specific molecules enter the cell.

Absorptivepinocytosis • The low-density lipoprotein (LDL) molecule and its receptor are internalized • These endocytotic vesicles containing LDL and its receptor 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 metabolized.

Exocytosis • Most cells release macromolecules to the exterior by exocytosis. • This process is also involved in membrane remodeling, when the components synthesized in the Golgi apparatus are carried in vesicles to the plasma membrane. • The signal for exocytosis is often a hormone which, when it binds to a cell-surface receptor, induces a local and transient change in Ca2+ concentration. Ca2+ triggers exocytosis.

Exocytosis • Molecules released by exocytosis have at least three fates: • (1) They can attach to the cell surface and become peripheral proteins, eg, antigens. • (2) They can become part of the extracellular matrix, eg, collagen and glycosaminoglycans. • (3) They can enter extracellular fluid and signal other cells. Insulin, parathyroid hormone, and the catecholamines are all packaged in granules and processed within cells, to be released upon appropriate stimulation

Some Signals Are Transmitted Across Membranes • Specific biochemical signals such as neurotransmitters, hormones, and immunoglobulins bind to specific receptors(integral proteins) exposed to the outside of cellular membranes and transmit information through these membranes to the cytoplasm. • This process, called transmembrane signaling, involves the generation of a number of signals, including cyclic nucleotides, calcium, phosphoinositides, and diacylglycerol.

MutationsAffectingMembraneProteinsCauseDiseases • In view of the fact that membranes are located in so many organelles and are involved in so many processes, it is not surprising that mutations affecting their protein constituents should result in many diseases or disorders. • Proteins in membranes can be classified as receptors, transporters, ion channels, enzymes, and structural components. Members of all of these classes are often glycosylated, so that mutations affecting this process may alter their function. • these mainly reflect mutations in proteins of the plasma membrane, with one affecting lysosomal function (I-cell disease). • Over 30 genetic diseases or disorders have been ascribed to mutations affecting various proteins involved in the transport of amino acids, sugars, lipids, urate, anions, cations, water, and vitamins across the plasma membrane.

Mutations • Mutations in genes encoding proteins in other membranes can also have harmful consequences. For example, mutations in genes encoding mitochondrial membrane proteins involved in oxidative phosphorylation can cause neurologic and other problems ; eg, Leber's hereditary optic neuropathy; LHON • Membrane proteins can also be affected by conditions other than mutations. Formation of autoantibodies to the acetylcholine receptor in skeletal muscle causes myasthenia gravis. • Ischemia can quickly affect the integrity of various ion channels in membranes. Abnormalities of membrane constituents other than proteins can also be harmful. • With regard to lipids, excess of cholesterol (eg, in familial hypercholesterolemia), of lysophospholipid (eg, after bites by certain snakes, whose venom contains phospholipases), or of glycosphingolipids (eg, in a sphingolipidosis) can all affect membrane function.

Professor .Orhan CANBOLAT ; MD , PhD