Phospholipids and Membrane Phospholipids • Phospholipids – Are composed of • Glycerol backbone • 2 fatty acids • Phosphate – Are soluble in water – Are manufactured in our bodies so they are not required in our diet Phospholipids O Each glycerophospholipid includes a polar region: glycerol, carbonyl O of fatty acids, Pi, & the polar head group (X) non-polar hydrocarbon tails of fatty acids (R1, R2). H2C O R1 C O O CH H2C C R2 O O glycerophospholipid P O− O X Sphingolipids Sphingolipids (SPLs) also have a polar head group and two non-polar tails but do not contain glycerol • Instead, the backbone is sphingosine, a long-chain amino alcohol • Some derivatives are Ceramide, Sphingomyelin, and Glycosphingolipids Sphingosine is a fatty amine, a glycerol molecule is never seen! O CH3 H3C Sphingomyelin has a phosphocholine or phosphoethanolamine head group. + N H2 C H2 C CH3 O P − O OH O phosphocholine H2C sphingosine Sphingomyelins are common constituent of plasma membranes Sphingomyelin H C CH NH CH O C fatty acid R HC (CH2 )12 CH3 Sphingomyelin, with a phosphocholine head group, is similar in size and shape to the glycerophospholipid phosphatidyl choline. Glycosylated sphingolipids A cerebroside is a sphingolipid (ceramide) with a monosaccharide such as glucose or galactose as polar head group. A ganglioside is a ceramide with a polar head group that is a complex oligosaccharide, including the acidic sugar derivative sialic acid. Cerebrosides and gangliosides, collectively called glycosphingolipids, are commonly found in the outer leaflet of the plasma membrane bilayer, with their sugar chains extending out from the cell surface. Sterols • Compounds containing 4 carbon ring structure with any of a variety of side chains • Many important body compounds are sterols – cell membranes, bile acids, sex & adrenal hormones, vit D & cholesterol. • Sterols are found in plant & animal foods – Manufactured in bodies so non-essential Sterols • • • Compounds containing 4 carbon ring structure with any of a variety of side chains Many important body compounds are sterols – cell membranes, bile acids, sex & adrenal hormones, vit D & cholesterol. Sterols are found in plant & animal foods – Manufactured in bodies so non-essential Cholesterol Sterols • Metabolic precursors of steroid hormones – – – – Regulate physiological functions Androgens (testosterone) Estrogens (β-estradiol) Glucocorticoids (cortisol) • Insoluble in water • Bind to proteins for transport to target tissue Steroid Hormones Derived from Cholesterol Bile Acid Fat-Soluble Vitamins • A vitamin is a compound that is essential to the health of humans but cannot be synthesized internally and must be obtained through diet. • Vitamins A and D are precursors of hormones: – Vitamin D is a cholesterol derivative that is converted into a hormone regulating calcium metabolism in the intestine, kidney, and bone (deficiency leads to ricketts) – Vitamin A (aka. retinol) derivatives can absorb light (retinal + opsin = rhodopsin, a vision thing), and control epithelial tissue development Fat-Soluble Vitamins • Vitamin K – a cofactor essential for activating prothrombin, needed to promote blood clotting • Vitamin E – group of lipids called tocopherols, which as antioxidants, protect unsaturated fatty acids, and scavenge damaging free radicals • Warfarin is a synthetic compound that inhibits the formation of active prothrombin (excellent rat poison or human anticoagulant!) • Ubiquinone (aka Coenzyme Q) and Plastiquinone funcation as lipophilic electron carriers in the redox reactions that drive ATP synthesis Several vitamins are lipids. Vitamins. Definition - Organic compound that are required in small amounts. Say a few words about the lipid vitamins (the fat soluble vitamins). Lipid vitamins Water soluble vitamins: Vitamin A Vitamin D Vitamin E Vitamin K B1, B2, B3, B5, B6, B7, B9, B12 (we will discuss water soluble vitamins later in the course) Vitamin A • • • Collective term for retinol, retinal, retinoic acid Formed from oxidative cleavage of β-carotene in liver Function – Aldehyde: visual cycle/process, component of rhodopsin (visual pigment) – Alcohol, carb acid: growth, reproduction • Deficiency – Night blindness – Xerophthalmia • • • • Dryness in eyes No tear production Damage to cornea Leads to blindness Vitamin A - Retinol Retinol (vitamin A) Sources in diet - Many plants, also meat, especially liver. Vitamin A is fat soluble. You can get too much, or too little if absorption is a problem. Some uses of vitamin A: Vision (11-cis-retinol bound to rhodopsin detects light in our eyes). Regulating gene transcription (retinoic acid receptors on cell nuclei are part of a system for regulating transcription of mRNAs for a number of genes). Vitamin D refers to a group of similar lipid-soluble molecules (major forms are D2 and D3, also D1, D4, D5). Vitamin D3 (cholecalciferol) Vitamin D2 (ergocalciferol) Vitamin D can be obtained in the diet, or derived from cholesterol in a reaction that requires UV light. Vitamin D binds to a “vitamin D binding protein” (VDP) for transport to target organs. Why is VDP needed? Vitamin D is not active itself; it is modified to yield biologically active forms, such as calcitriol. Calcitriol (derived from vitamin D) is a transcription factor, influencing expression of proteins involved in calcium absorption and transport. Deficiency causes bone loss. Vitamin D is also important for immune system function. Vitamin D • Sterol derivatives – Open B rings • Function – Regulate Ca and P absorption during bone growth • Sources – Diet: D2 (milk additive, plant sources) and D3 (animal sources) – Precursor: intermediate in cholesterol synthesis – Formed in skin non-enzymatically from 7-dehydrocholesterol • Deficiency – Soft bones, impaired growth and skeletal deformities in children Inactive form Vitamin D production requires UV light (sunlight) Sometime after the first humans migrated north out of Africa about 50,000 years ago, mutations appeared that reduced melanin (pigment) production in the skin, permitting vitamin D production with less sunlight. Disadvantages of less melanin production are skin that is easily damaged by the sun, skin cancer risk, and loss of folic acid due to UV damage. The melanin-reducing mutations helped early humans make vitamin D in northern europe in winter. Vitamin E α-tocopherol • Function – Antioxidant: prevents cell damage from oxidation of polyunsaturated FAs in membranes by O2 and free radicals • Deficiency – Associated with defective lipid transport/absorption Vitamin K • • Phylloquinone or menaquinone Function O CH3 – Synthesis of blood clotting proteins • Sources – K1 = plants; K2 = animals – Bacteria in intestine • O CH3 O CH3 Toxicity – Jaundice from large doses of vit. K, toxic effects on membrane of red blood cells, cells die, lead to increased levels of yellow bilirubin (formed from heme) CH3 Vitamin K1 (phylloquinone) Deficiency – Unlikely due to synthesis and wide distribution in food – Injection for infants – Hemolytic anemia = destruction of red blood cells • CH3 CH3 O CH3 CH3 Vitamin K2 (menaquinone) Other Lipids • Classified on basis of physical properties – Solubility – Hydrophobicity – Amphiphilicity • Fat-soluble vitamins – Vitamins A, E, K (and D) – Isoprenoids • Eicosanoids – Prostaglandins – Thromboxanes – Leukotrienes Terpenes • Simple lipids, but lack fatty acid component • Formed by the combination of 2 or more molecules of 2-methyl-1,3-butadiene (isoprene) • Monoterpene (C-10) – made up of 2 isoprene units • Sesquiterpene (C-15) – made up of 3 isoprene units • Diterpene (C-20) – made up of 4 isoprene units Monoterpenes CHO OH limonene citronellal menthol camphene Monoterpenes are readily recognized by their characterisitic flavors and odors ( limonene in lemons, citronellal in roses and geraniums, pinene in turpentine, and menthol from peppermint) Lipid Derivatives Active roles for lipids derivatives • Lipids as Signals, Cofactors & Pigments – Intracellular signaling – Phosphatidylinositol – Paracrine hormones – Eicosanoids – Steroid hormones –polar cholesterol derivatives – Vitamins – A, D, E, and K • Provide crucial parameters of membrane fluidity Eicosanoids • Hormones involved in production of pain, fever, inflammatory reactions – Prostaglandins – Thromboxanes – Leukotrienes • Metabolites of arachidonic acid (a polyunsatruated FA) • Synthesis inhibited by NSAIDs – e.g. acetylsalicylic acid (aspirin) – Acylate Ser residue, preventing access to active site Eicosanoids • Eicosanoids are paracrine hormones that act on cells near their point of synthesis to affect inflammation, blood clotting, gastric acid secretion, etc. • They are derived from arachidonic acid (20:4(Δ5, 8, 11, 14)) liberated from particular membrane phospholipids by phospholipase A2 • This group includes prostaglandins, thromboxanes and leukotrienes Pathways for Eicosanoids Synthesis Diet Body LTB4 LTC4 LA AA LNA EPA DHA LA AA LNA EPA DHA AA E,D E,D LTB5 LTC5 L L E,D EPA DHA AA + EPA, DHA C C PGI2 TXA2 PGI3 TXA3 Mechanisms of action of EPA and DHA L C LTB4, LTC4 <-------------- AA -----------------> PGI2, TXA2 LTB5, LTC5 <---------- EPA, DHA -----------> PGI3, TXA3 * LTB4, LTC4: increase adhesion of Leucocytes to endothelium and inflamatory effects * LTB5, LTC5: less effective than LTB4, LTC4 *PGI2: vasodilator, inhibit platelet aggregration *PGI3: help effect of PGI2 *TXA2: increase platelets aggregation, vasoconstriction *TXA3: less potent than TXA2 N3 fatty acids compete with N6 for elongase, desaturase, cyclooxyenase and lipoxigenase Leukotrienes Leukotrienes are derived from arachidonic acid via the enzyme 5-lipoxygenase which converts arachidonic acid to 5-HPETE (5-hydroperoxyeicosatetranoic acid) and subsequently by dehydration to LTA4 OH OH COOH COOH H C5H11 H S C5H11 Cys gGlu LEUKOTRIENE F4 (LTF4) Peptidoleukotrienes S Cys Gly gGlu LEUKOTRIENE C4 (LTC4) Biological activities of leukotrienes 1. LTB4 2. LTC4 3. LTD4 4. LTE4 - potent chemoattractent - mediator of hyperalgesia - growth factor for keratinocytes - constricts lung smooth muscle - promotes capillary leakage 1000 X histamine - constricts smooth muscle; lung - airway hyperactivity - vasoconstriction - 1000 x less potent than LTD4 (except in asthmatics) Lipids are the major component of membranes. Membranes separate cells from their environment. Also, membranes separate intercellular regions (such as mitochondria, vesicles, nucleus). Lipid bilayer: Phospholipids Lipid bilayer forms sphere in aqueous solution Forms barrier defining inside and outside spaces “Fluid” means the membrane interior is liquid-like (the lipids are mobile). “Mosaic” means the membrane is a mosaic of different types of components (phosphate groups, carboxylic acids, proteins, sugars). Cell Membrane-more complex Contains a variety of lipids, proteins, and carbohydrates Outer leaflet Lipids Lipid bilayer 5 nm Multiple types of lipids are found in membranes Inner leaflet Protein Cytosol (inside) Three Types of Membrane Lipid Molecules all amphipathic Phospholipids serine Sterols (cholesterol) Glycolipids (sugar lipid) ECB 11-7 phosphatidylserine galactocerebroside Type B blood - terminal sugar is galactose. Type A blood - terminal sugar is N-acetylated galactose. Type O blood - terminal sugar is neither of above. Fuzzy stuff on the surface is the sugar part of the glycolipids. Membrane Fluidity (viscosity) Describes the physical state of the membrane Pure lipid bilayer - two states Liquid state Hydrophobic tails free to move Gel state Movement is greatly restricted (crystalline gel) Transition Liquid at temperatures Above the transition temp. temperature Crystalline gel at temperatures below the transition temp Living cells require a fluid membrane, but not too fluid: Membrane fluidity is regulated by the cell Membrane fluidity is governed by FA length and saturation 1. Fatty acid length - shorter the FA, the lower the transition temperature (melting point), favors liquid state 2. Fatty acid saturation - the more saturated, the higher the transition temperature, favors gel state Melting points of 18-carbon Fatty Acids Fatty Acid Stearic acid Oleic acid α-Linoleic acid Linolenic acid Double bonds 0 1 2 3 Melting point (˚C) 70 13 -9 -17 3. Presence of cholesterol - broadens the temperature over which transition occurs. Membrane fluidity: The interior of a lipid bilayer is normally highly fluid. liquid crystal crystal In the liquid crystal state, hydrocarbon chains of phospholipids are disordered and in constant motion. At lower temperature, a membrane containing a single phospholipid type undergoes transition to a crystalline state in which fatty acid tails are fully extended, packing is highly ordered, & van der Waals interactions between adjacent chains are maximal. Kinks in fatty acid chains, due to cis double bonds, interfere with packing in the crystalline state, and lower the phase transition temperature. Lipid composition varies in inner and outer leaflet Glycolipids in outer leaflet Phospholipids Influence of FA saturation on lipid bilayer order ordered Saturated straight hydrocarbon chains (no double bonds) less ordered Unsaturated hydrocarbon chains (with double bonds) Less ordered state increases membrane fluidity Polar head group Rigid Planar Steroid ring Nonpolar hydrocarbon tail ECB 11-16 Mainly in animal cells, Not in plants Cholesterol stiffens lipid bilayers Polar head group Stiffened region Fluid region HO Cholesterol Cholesterol in membrane Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase & its hydrophobic ring system adjacent to fatty acid chains of phospholipids. The OH group of cholesterol forms hydrogen bonds with polar phospholipid head groups. Interaction with the relatively rigid cholesterol decreases the mobility of hydrocarbon tails of phospholipids. Cholesterol in membrane But the presence of cholesterol in a phospholipid membrane interferes with close packing of fatty acid tails in the crystalline state, and thus inhibits transition to the crystal state. Phospholipid membranes with a high concentration of cholesterol have a fluidity intermediate between the liquid crystal and crystal states. Lipids in membranes are mobile. 1. Spin (fast) 2. Lateral movement (less fast) 3. Flip-flop Almost never Two strategies by which phase changes of membrane lipids are avoided: Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-chain saturated fatty acids. In the absence of cholesterol, such membranes would crystallize at physiological temperatures. The inner mitochondrial membrane lacks cholesterol, but includes many phospholipids whose fatty acids have one or more double bonds, which lower the melting point to below physiological temperature. Membranes are permeable to small non-polar uncharged molecules (includes O2 and N2 and CO2). Impermeable to ions and most other water-soluble molecules; these need specific transporters to get across the membrane. Lipid Bilayer Permeability Small hydrophobic Molecules O2, CO2, N2, benzene Small Uncharged polar molecules H2O, glycerol, ethanol Large, uncharged Polar molecules Amino acids, glucose, nucleotides IONS H+, Na+, HCO3-, K+, Ca2+, Cl-, Mg2+ ECB Fig.12-2 Ions such as Na+ and K+ can NOT pass through lipid membranes (except through special pores) The red blood cell glucose transporter (only lets glucose through, passive transport). The red blood cell glucose transporter (only lets glucose through, passive transport). Uniport - specific for one type of molecule. Symport - transports 2 molecules together. Antiport - transports 2 molecules in opposite directions. Active transport requires energy (ATP hydrolysis). Can work against a concentration gradient. Example of active transport: Na+/K+ pump (Na+ conc is higher outside cells). 3 Na+ ions bind to transporter protein inside cell. ATP phosphorylates protein, causes conformational change. The 3 Na+ ions are released outside cell ; 2 K+ ions bound. Triggers dephosphorylation of protein. Protein goes back to original state ; K+ released inside cell. This is an “antiport” ; two ions moving in opposite directions through the some transporter. A good animation of the Na+/K+ pump: google: “animation sodium potassium pump” Porins - Relatively simple transporters located in bacterial outer membranes, mitochondria and chloroplasts. Porin proteins are trimeric, a group of 3 beta-barrels. Core of barrel has narrow aqueous channel. Small molecules with MW less than about 600 can pass through. OmpF trimer & view of one beta-barrel (from text) The E. coli OmpF protein is a porin. OmpF stands for “outer membrane protein F”. Ion channels - Found in neurons and other eukaryotic proteins, as well as bacteria. A well-known ion channel is the potassium channel (bacterial). Allows potassium to pass, but not sodium. The core of the potassium ion channel has an arrangement of backbone carbonyl atoms that has the correct geometry to coordinate potassium ions, but not sodium. Membrane potential Many cells have a charge imbalance across the membrane, typically excess negative charge inside the cell, and excess positive charge outside (due to different Na+ and K+ inside and outside cell). This charge imbalance results in a voltage across the membrane. Typical animal cells have a membrane potential of about 70 mVolts, due to difference in Na+ and K+ inside and outside cells. When an axon of a nerve cell is stimulated, Na+ channels in the membrane open, allowing Na+ to cross, and altering the membrane potential, typically from -70 mV to +50 mV, within a millisecond. This triggers the opening of a nearby K+ channel, which returns the potential to -70 mV, and stimulates other Na+ channels farther along the neuron. This propagating voltage change is called an “action potential”. Action potentials propagate rapidly along an axon (in milliseconds).