Lipids and Membranes

1. Lipids and Membranes Chapter 9

2. Lipids Lipids fats oils…. Greasy molecules, mmmmm donuts. Several levels of complexity: • Simple lipids - a lipid that cannot be broken down to smaller constituents by hydrolysis. • Fatty acids, waxes and cholesterol • Complex lipids - a lipid composed of different molecules held together mostly by ester linkages and susceptible to cleavage reactions. • acylglycerols - mono, di and triacyl glycerols ( fatty acids and glycerol) • phospholipids (also known as glycerophospholipids) - lipids which are made of fatty acids, glycerol, a phosphoryl group and an alcohol. Many also contain nitrogen • glycolipids (also known as glycosphingolipids): Lipids which have a spingosine and different backbone than the phospholipids

3. Phospholipids: • Two fatty acids covalently linked to a glycerol, which is linked to a phosphate. • All attached to a “head group”, such as choline, an amino acid. • Head group POLAR – so hydrophilic (loves water) • Tail is non-polar –hydrophobic • The tail varies in length from 14 to 28 carbons.

4. Several levels of complexity: • Precursor and derived lipids - these include several of the initial long chain hydrocarbons and other molecules that make up lipids, such as ketone bodies. Lipids are defined as those molecules soluble in organic solvents and not water

5. Properties of fatty acids • Typically found as esters , mono, di and tri -acylglycerols (good old fashioned fat) • Free fatty acids are found associated with carrier proteins such as albumin in blood • These are amphipathic molecules with various carbon chain lengths • Can be up to 30 carbons long but generally less than 20 • Most common 12-24 carbons long

6. Properties of fatty acids • Saturated fatty acids are saturated with hydrogen. No double bonds. • 1/2 of animal fat has one desaturation • Unsaturated fatty acids contain at least one double bond separated by a methyl group. These are typically cis- in nature. Trans unsaturated fatty acids have been linked to cancer and heart disease. (the strength of the proof is not clear). • Most double bonds are separated by two carbons • Polyunsaturated fatty acids - duh

7. Nomenclature • Trivial (greek) system, IUPAC system, omega system • Common name most commonly used (trivial system) • The systematic name for saturated acids ends in -anoic. Unsaturated fatty acids ends in -enoic. • The number one carbon is the carboxyl carbon • Carbon #1 = alpha carbon • Carbon #2 = beta carbon • Carbon #3 = gamma carbon • Omega vs. delta fatty acids • Double bonds counted from the first carbon are delta desaturations • Double bonds counting from the methyl end are the omega desaturations • Omega nomenclature helps to identify physiological important fats w3 - w6 (can not be intraconverted in body)

8. Nomenclature • Common name most commonly used • The systematic name for saturated acids ends in -anoic. Unsaturated fatty acids ends in -enoic. • The number one carbon is the carboxyl carbon • Carbon #1 = alpha carbon • Carbon #2 = beta carbon • Carbon #3 = gamma carbon • Omega vs. delta fatty acids • Double bonds counted from the first carbon are delta desaturations • Double bonds counting from the methyl end are the omega desaturations Trivial IUPAC Carboxyl Omega Oleic Acid

9. Nomenclature • Common name most commonly used • The systematic name for saturated acids ends in -anoic. Unsaturated fatty acids ends in -enoic. • The number one carbon is the carboxyl carbon • Carbon #1 = alpha carbon • Carbon #2 = beta carbon • Carbon #3 = gamma carbon • Omega vs. delta fatty acids • Double bonds counted from the first carbon are delta desaturations • Double bonds counting from the methyl end are the omega desaturations Trivial IUPAC Carboxyl Omega Oleic Acid 9-octadecenoic acid

10. Nomenclature • Common name most commonly used • The systematic name for saturated acids ends in -anoic. Unsaturated fatty acids ends in -enoic. • The number one carbon is the carboxyl carbon • Carbon #1 = alpha carbon • Carbon #2 = beta carbon • Carbon #3 = gamma carbon • Omega vs. delta fatty acids • Double bonds counted from the first carbon are delta desaturations • Double bonds counting from the methyl end are the omega desaturations Trivial IUPAC Carboxyl Omega Oleic Acid - 9-octadecenoic acid 18:1 9

11. Nomenclature • Common name most commonly used • The systematic name for saturated acids ends in -anoic. Unsaturated fatty acids ends in -enoic. • The number one carbon is the carboxyl carbon • Carbon #1 = alpha carbon • Carbon #2 = beta carbon • Carbon #3 = gamma carbon • Omega vs. delta fatty acids • Double bonds counted from the first carbon are delta desaturations • Double bonds counting from the methyl end are the omega desaturations Trivial IUPAC Carboxyl Omega Oleic Acid - 9-octadecenoic acid 18:1 9 18:1 9

12. Nomenclature • Common name most commonly used • The systematic name for saturated acids ends in -anoic. Unsaturated fatty acids ends in -enoic. • The number one carbon is the carboxyl carbon • Carbon #1 = alpha carbon • Carbon #2 = beta carbon • Carbon #3 = gamma carbon • Omega vs. delta fatty acids • Double bonds counted from the first carbon are delta desaturations • Double bonds counting from the methyl end are the omega desaturations Trivial IUPAC Carboxyl Omega Oleic Acid - 9-octadecanoic acid 18:1 9 18:1 9 Linoleic Acid - 9, 12-octadecenoic acid 18:2 9,12 18:2 6

13. Nomenclature • Common name most commonly used • The systematic name for saturated acids ends in -anoic. Unsaturated fatty acids ends in -enoic. • The number one carbon is the carboxyl carbon • Carbon #1 = alpha carbon • Carbon #2 = beta carbon • Carbon #3 = gamma carbon • Omega vs. delta fatty acids • Double bonds counted from the first carbon are delta desaturations • Double bonds counting from the methyl end are the omega desaturations Trivial IUPAC Carboxyl Omega Oleic Acid - 9-octadecenoic acid 18:1 9 18:1 9 Linoleic Acid - 9, 12-octadecenoic acid 18:2 9,12 18:2 6 Essential FA- those that humans can not make but are required to Sustain growth - Linolenic 3 and Arachadonic 6

14. Common FA Names Carbon # Trivial Name IUPAC 12 Lauric acid Dodecanoate 12:0 14 Myristic acid Tetradecanate 14:0 16 Palmitic acid Hexadecanoic acid 16:0 18 Stearic acid Octadecanoic acid 18:0 16 Palmitoleic acid 9-Hexadecenoic acid 16:1 9 18 Oleic acid 9,12 Octadecadienoic acid 18:2 9,12 20 Arachidonic acid 5,8,11,14 Eicosatetraenoic 20:4 5,8,11,14

15. Melting points and membrane fluidity. Both length and level of unsaturation determine the stability of the hydrophobic interactions and thus shift the transition phase - melting point of membranes • Double bonds - usually cis. This results in bends the chain. • Reductions in hydrophobic effect reduce energy required to disrupt the crystalin structure of a membrane or oil. Think of animal fat with lower unsaturated fatty acids - butter and plant oils that are polyunsaturated, corn oil. • The longer the fatty acids the higher the melting point. • Again the more hydrophobic interactions effects the more the energy it takes to break the order. Decreases in the packing efficiency decreases the mp • The van der Waals forces then come apart more easily at lower temperatures. • Animal alter the length and unsaturated level of the fatty acids in lipids (cholesterol too) to deal with the cold temps

16. Esterification • The general name for a chemical reaction in which two reactants (typically an alcohol and an acid) form an ester as the reaction product. Esters are common in organic chemistry and biological materials, and often have a characteristic pleasant, fruity odor. • This leads to their extensive use in the fragrance and flavour industry. • Esterification is a reversible reaction. Hydrolysis- literally "water splitting" involves adding water and a catalyst (commonly NaOH) to an ester to get the sodium salt of the carboxylic acid and alcohol. • As a result of this reversibility, many esterification reactions are equilibrium reactions and therefore need to be driven to completion • Esterifications are among the simplest and most often performed organic transformations.

17. Lipids II Triacylglycerols (Triglycerides) • Esterification of glycerol takes place at each of the alcohol moieties • This is the major form of fatty acid storage. Found in adipose tissue in specialized cells. These fat cells are filled with TAGs and can grow in size. • Release (hydrolysis) of fatty acids occurs under the control of hormone sensitive lipase • Fatty acids released into the bloodstream are carried via albumin.

18. Triglycerides • The acylated acyl chains are typically different at each position of glycerol. • Each of the three carbons on glycerol are different. The 1 and 3 carbons are steriochemically different and recognized as such by enzymes. The carbons are labeled sn1, sn2, sn3

19. Triacylglycerols (TAGS) • TAGs DAG and MAG. The sn3 carbon is typically removed first. • Saponification - hydrolysis by bases (KOH - potash) cleaves the fatty acids from glycerol. Using KOH from wood ashes and animal fat creates old-fashioned soaps. (divalent cations in hard water cause the ppt of soaps) • There is a large amount of energy available in the storage of TAGs. These are highly reduced molecules with low amounts of water associated. The result is a molecule that can undergo repeated oxidation steps transferring the energy to form ATP. The low water content increases the gram per gram energy available vs. carbohydrates • Polar bears can go for up to 8 months without eating. Most of energy and water comes from the fats produced by the bear and the fat ingested from seals.

20. Phospholipids • Phospholipids are the largest constituent of the membrane • Phospholipids serve two important purposes (different than TAGs) structural and signaling. Although DAG is an important signaling molecule as well • You should know the different signaling lipids and there functions • There are two general classes of phospholipids • Glycerophospholipids, phospholipids or phosphate esters (same thing) and the sphingolipids.

21. Glycerophospholipids - major lipid comonent of biological membranes • The sn 1 and sn2 positions on glycerol are esterified (red). The sn2 carbon is typically unsaturated. Differences occur between tissues and organism • The sn3 position is phosphorylated (blue) • The phosphoryl group can be • modified with several different alcohols. • Common classes are shown in Table 9.2 pp249 of 3rd ed • LEARN THEM

22. Phosphatidic acid is the base glycerophospholipid but is not found in high concentrations • Common head groups (the alcohol derivatives) are • serine, choline, ethanolamine, glycerol and inositol. • This divides glycerophospholipids into • basic • neutral • acidic lipids. • The nomenclature is 1-acyl-2-acyl-3-phosphatidyl "head group". • Biological use –lung surfactant

23. phospholipases • These are a set of hydrolytic enzymes which act on both the fatty acid chains and the head group • Phospholipase A2 excises C2 fatty acid group (red) forming a Lysophospholipid - recognizes the sn-2 acyl bond • Powerful detergent which lyses cells by disrupting membranes • Bee and snake venoms rich in this enzyme • Inflammatory responses

24. phospholipases • Phospholipase C cleave phospholipids just before the phosphate group • plays an important role in eukaryotic cell physiology, particularly lipid signaling pathways in a calcium-dependent manner • proliferation, differentiation, apoptosis, cytoskeleton remodeling, vesicular trafficking, ion channel conductance, endocrine function and neurotransmission.

25. Phospholipase C • Phospholipase C cleave phospholipids just before the phosphate group generating inositol triphosphate (IP3) and diacylglycerol (DAG). • IP3 is soluble, and diffuses through the cytoplasm and interacts with IP3 receptors on the endoplasmic reticulum, causing the release of calcium and raising the level of intracellular calcium. • DAG remains tethered to the inner leaflet of the plasma membrane due to its hydrophobic character, where it recruits protein kinase C (PKC), which becomes activated in conjunction with binding calcium ions

26. phospholipases • Phospholipase D cleave phospholipids just after the phosphate group, form phosphatidic acid (PA), releasing the soluble choline headgroup into the cytosol • Phosphatidic is extremely short lived and is rapidly hydrolised by the enzyme PA phosphohydrolase to form diacylglycerol (DAG).

27. Plasmalogens • Different than the other phosphoglycerides - • Has an ether lipid where the first position of glycerol (sn1)binds a vinyl residue (from a vinyl alcohol) with the double bond next to the ether bond. • The second carbon has a typical ester-linked fatty acid, and the third carbon usually has a phospholipid head group like choline • Phospholipids preferentially hydrolyze substrates that are located in bilayer membreases. They carry our interfacial catalysis at the boundry of water and a lipid phase These are typically involved in platelet aggregation & vasiodialation. Protect the heart

28. Sphingolipids • Another class of phospholipids • Do not contain glycerol as a backbone instead an amino alcohol called sphingosine. • Add one fatty acid and it is ceramide • regulates differentiation, proliferation, programmed cell death • Add an head group (choline or ethanolamine) and it is a sphingophospholipid • These are very different in location and concentration than the glycerolipids

29. Eicosanoids • These are a diverse group of hormonelike molecules produced in nearly all mammalian cells • Can be formed from the action of Phospholipase A2 • Stem from the greek word eikosi meaning twenty • Because they act on the same organ they are produced in they are autocrines, vs a paracrine which act distal to the site of origin • Most are derived from arachidonic acid 20:4 D5,8,11,14

30. Leukotrienes are hydroxylated fatty acid derivatives of arachidonate • Initially found in leukocytes - white blood cells • Often congugated • Secreted by damaged cells during anaphylaxis • Promotes bronchioconstriction and vasioconstriction • Acts as a chemotractant to bring white blood cells to fight infection

31. Prostoglandins have a cyclopentane ring and is hydroxylated at various carbons. • There are several versions each with a different effect. • Different types and concentrations of prostaglandins are found in different tissues. • Induce inflammation and cause fever and pain • Ovulation and uterine contraction during conception and labor • Antiplatelet aggregation • Vasiodialation • Smooth muscle contraction • Aspirin inhibits the formation of cyclic ecosanoids • The Serine in the active site of the enyme cyclooxygenase is acetlyated by aspirin

32. Thromboxanes (TxB) produced by platelets to cause aggregation at sites of cardiovascular injury. • Leads to clot formation and foam cell formation (platelets differentiate into cells that cause plauques in arteries) • Thromboxanes produced by platelets Alterations in double bonds in arachidonate still leads to TxB formation but theyare less able to aggregate platelets • These fatty acids are found in some cold fish oils omega 3 fatty acids

33. Prostacyclin produced by blood vessels and inhibit platelet aggregation • These are antagonistic to thromboxanes • The omega 3 fatty acids leads to a more potent antiaggregant activity

34. Steroids • Cholesterol plays an important role in membrane fluidity and the starting material for steroids and vitamin D • Six rings three cyclohexane and one cyclopentane • Modified with hydrophobic functional groups on carbons 10,13 and 17 • Inflexible hydrophobic bulky molecule • -OH on the 3rd carbon - makes the molecules ampipathic • Found in all tissues in the membranes often acylated at the OH

35. Function in membranes • Cholesterol broadens the phase change from solid to fluid or oil like state. • The stiff ring decreases coiling and movement of the fatty acid tails in the phospholipids. This can have two effects on fluidity • Below the melting point - cholesterol is too bulky to fit into the rigid crystal state. This increases the melting point • Above the melting point the cholesterol still restricts the movement of the fatty acid tails. Thus the melting point is decreased

36. Atherosclerosis - heart disease where plaque form blockages in blood flow. Specialized cells macrophages are converted to foam cells where they are filled primarily with cholesterol and cholesterol esters. Eventually these cells can calcify and harden ultimately blocking the flow of blood to the heart. • Precursor for the steroids

37. Steroid hormones - do not act by binding receptors like other hormones. These are lipid soluble hormones that are transported through the plasma and interstitial fluids bound to steroid carrier proteins (SCP). Once to the cells, the steroid crosses the membrane and binds specifically to a cytosolic steroid receptor. The complex often travels to the nucleus and acts at the level of altering DNA-> RNA ->Protein production. (transcription)

38. Steroid hormones

39. Glucocorticoids - involved in reducing inflammation, pain and carbohydrate metabolism. • Can bind to proteins which act then bind to DNA and alter gene expression and protein production

40. Mineralcorticoids - required for normal kidney function in the regulation of K+ and Na+ ion filtration

41. Glucocorticoids, mineralcorticoids and some androgens are produced in adrenal cortex - small organ just above the kidney. Over 50 varieties found in this tissue

42. Regulation of blood pressure and volume When sodium ions levels are high- so a high blood volume a glucocorticord (cortisol) makes the heart release atrial natriuretic hormone (ANH) Kidneys excrete sodium ions and water follows. Volume and pressure return to normal. When sodium ions levels are low- a low blood volume makes kidneys secrete renin (ENZYME) Adrenal cortex secretes a mineralocorticoid (Aldosterone), which makes the kidneys reabsorb sodium ions and thus water. Volume and pressure return to normal.

43. Sex steroids; Androgens and estrogens. Structures are related and formed from the other. Effect of these are hormonal. Source is generally from the gonadal tissues.

44. Membrane dynamics - • Formation of lipid bilayers - these structures form due to the insolubility in water.

45. When enough lipids are present, micelles form. Eventually these can fill out enough to form a bilayer where the polar heads face the aqueous phase and the hydrophobic tails are buried in the nonpolar phase • Lipids easily diffuse lateraly but rarely "flip" due to the changes in entropy changes that must accompany such a switch.

46. New - Lipid rafts and caveolae • Lipid rafts - Lipid rafts are specialized membrane domains enriched in certain lipids cholesterol and proteins. • three types of lipid rafts; caveolae, glycosphingolipid enriched membranes (GEM), and polyphospho inositol rich rafts. • The main role of rafts is in signal transduction where the rafts act as an anchor for signaling protiens to assemble into “scafolds” and the raft also acts as a binding site for actin related proteins. • Caveolae - Caveolae are flask shaped invaginations on the cell surface that are a type of membrane raft, these are cave shaped and associated with proteins called caveolin. Involved in receptor internalization and cell signaling The fatty-acid chains of lipids within the rafts tend to be extended and so more tightly packed, creating domains with higher order. It is therefore thought that rafts exist in a separate ordered phase that floats in a sea of poorly ordered lipids. Glycosphingolipids, and other lipids with long, straight acyl chains are preferentially incorporated into the rafts.