1. Lipids and Proteins Are Associated in Biological Membranes Feb 18, 2016 CHEM 281

2. What is a Lipid  Lipids: a heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties  insoluble in water, but soluble in aprotic organic solvents including diethyl ether, chloroform, methylene chloride, and acetone  Amphipathic in nature  Lipids include:  Open Chain forms  fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, glycolipids,  lipid-soluble vitamins  prostaglandins, leukotrienes, and thromboxanes  Cyclic forms  cholesterol, steroid hormones, and bile acids

3. Fatty Acids  Fatty acid: an unbranched-chain carboxylic acid, most commonly of 12 - 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or phosphodiacylglycerols of biological membranes  In the shorthand notation for fatty acids  the number of carbons and the number of double bonds in the chain are shown by two numbers, separated by a colon

4. Fatty Acids (Cont’d) Length of fatty acid plays a role in its chemical character Usually contain even numbers of carbons (can contain odd, depending on how they are biosynthesized) FA that contain C=C, are unsaturated: If contain only C-C bonds, they are saturated

5. Fatty Acids (Cont’d)  In most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare  Unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point

6. Triacylglycerols  Triacylglycerol (triglyceride): an ester of glycerol with three fatty acids  natural soaps are prepared by boiling triglycerides (animal fats or vegetable oils) with NaOH, in a reaction called saponification (Latin, sapo, soap)

7. Soaps  Soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water)  Reactions with acids/bases as catalysts  Salts formed by saponification

8. Phosphoacylglycerols (Phospholipids)  When one alcohol group of glycerol is esterified by a phosphoric acid rather than by a carboxylic acid, phosphatidic acid produced  Phosphoacylglycerols (phosphoglycerides) are the second most abundant group of naturally occurring lipids, and they are found in plant and animal membranes

9. Waxes  A complex mixture of esters of long-chain carboxylic acids and alcohols  Found as protective coatings for plants and animals

10. Sphingolipids  Contain sphingosine, a long-chain amino alcohol sphingosine  Found in plants and animals  Abundant in nervous system  Bares structural similarity to phospholipids

11. Glycolipids  Glycolipid: a compound in which a carbohydrate is bound to an -OH of the lipid  In most cases, sugar is either glucose or galactose  many glycolipids are derived from ceramides  Glycolipids with complex carbohydrate moiety that contains more than 3 sugars are known as gangliosides

13. Steroids  Steroids: a group of lipids that have fused-ring structure of 3 six-membered rings, and 1 five-membered ring.

14. Cholesterol  The steroid of most interest in our discussion of biological membranes is cholesterol

15. Biological Membranes  Every cell has a cell membrane (plasma membrane)  Eukaryotic cells also have membrane-enclosed organelles (nuclei, mitochondria…etc)  Molecular basis of membrane structure is in lipid component(s):  polar head groups are in contact with the aqueous environment  nonpolar tails are buried within the bilayer  the major force driving the formation of lipid bilayers is hydrophobic interaction  the arrangement of hydrocarbon tails in the interior can be rigid (if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids)

16. Lipid Bilayers  The polar surface of the bilayer contains charged groups  The hydrophobic tails lie in the interior of the bilayer

17. Biological Membranes  Plant membranes have a higher percentage of unsaturated fatty acids than animal membranes  The presence of cholesterol is characteristic of animal rather than plant membranes  Animal membranes are less fluid (more rigid) than plant membranes  The membranes of prokaryotes, which contain no appreciable amounts of steroids, are the most fluid

18. Membrane Layers  Both inner and outer layers of bilayer contain mixtures of lipids  Compositions on inside and outside of lipid bilayer can be different  This is what distinguishes the layers

19. Effect of Double Bonds on the Conformations of Fatty Acids  Kink in hydrocarbon chain  Causes disorder in packing against other chains  This disorder causes greater fluidity in membranes with cis-double bonds vs. saturated FA chains

20. Cholesterol reduces Fluidity  Presence of cholesterol reduces fluidity by stabilizing extended chain conformations of hydrocarbon tails of FA  Due to hydrophobic interactions

21. Temperature Transition in Lipid Bilayer With heat, membranes become more disordered; the transition temperature is higher for more rigid membranes; it is lower for less rigid membranes Mobility of the lipid chains increases dramatically

22. Membrane Proteins  Functions: transport substances across membranes; act as receptor sites, and sites of enzyme catalysis  Peripheral proteins  bound by electrostatic interactions  can be removed by raising the ionic strength  Integral proteins  bound tightly to the interior of the membrane  can be removed by treatment with detergents or ultrasonification  removal generally denatures them

23. Proteins Can be Anchored to Membranes  N-myristoyl- and S-palmitoyl anchoring motifs  Anchors can be via N-terminal gly  Thioester linkage with Cys

24. Fluid Mosaic Model  Fluid: there is lateral motion of components in the membrane;  proteins, for example, “float” in the membrane and can move along its plane  Mosaic: components in the membrane exist side-by-side as separate entities  the structure is that of a lipid bilayer with proteins, glycolipids, and steroids such as cholesterol embedded in it  no complexes, as for example, lipid-protein complexes, are formed

25. Fluid Mosaic Model of Membrane Structure

26. Membrane Function: Membrane Transport  Passive transport  driven by a concentration gradient  simple diffusion: a molecule or ion moves through an opening  facilitated diffusion: a molecule or ion is carried across a membrane by a carrier/channel protein  Active transport  a substance is moved against a concentration gradient  primary active transport: transport is linked to the hydrolysis of ATP or other high-energy molecule; for example, the Na + /K + ion pump  secondary active transport: driven by H + gradient

27. Passive Transport  Passive diffusion of species (uncharged) across membrane dependent on concentration, presence of carrier protein

28. 1˚ Active transport  Movement of molecules against a gradient directly linked to hydrolysis of high-energy yielding molecule (e.g. ATP)

30. 2˚ Active Transport

31. Membrane Receptors  Membrane receptors  generally oligomeric proteins  binding of a biologically active substance to a receptor initiates an action within the cell

33. Lipid-Soluble Vitamins  Vitamins are divided into two classes: lipid-soluble and water- soluble

34. Vitamin A  Vitamin A (retinol) occurs only in the animal world  Extensively unsaturated hydrocarbon (  -carotene)  Vitamin A is found in the plant world in the form of a provitamin in a group of pigments called carotenes  enzyme-catalyzed cleavage of  -carotene followed by reduction gives two molecules of vitamin A

35. Vitamin A  The best understood role of Vitamin A is its participation in the visual cycle in rod cells  the active molecule is retinal (vitamin A aldehyde)  retinal forms an imine with an -NH 2 group of the protein opsin to form the visual pigment called rhodopsin  the primary chemical event of vision in rod cells is absorption of light by rhodopsin followed by isomerization of the 11-cis double bond to the 11- trans double bond

39. Vitamin D  A group of structurally related compounds that are involved in the regulation of calcium and phosphorus metabolism  the most abundant form in the circulatory system is vitamin D 3

40. Vitamin E  The most active of vitamin E is  -tocopherol  Vitamin E is an antioxidant; traps HOO and ROO radicals formed as a result of oxidation by O 2 of unsaturated hydrocarbon chains in membrane phospholipids

41. Vitamin K  Vitamin K has an important role in the blood-clotting process  Long unsaturated hydrocarbon side consists of repeating isoprene units

42. Prostaglandins  Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid  First detected in seminal fluid from prostate  The metabolic precursor is arachidonic acid (20 carbon atoms: 4 double bonds)  Production of prostaglandins from arachidonic acid occurs in several steps.

43. NSAIDs  Prostaglandins are potent mediators of inflammation  Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit the COX enzyme  Animals possess two forms: COX-1 (physiological) and COX-2 (inflammatory response)  “COX-2 inhibitors” (e.g., Celebrex) bind to COX-2 but not COX-1  COX-1 has a bulkier isoleucine at position 523 which prevents binding of NSAIDs  COX-2 has valine, which accommodates the NSAIDs

44. NSAIDs Garrett and Grisham, Biochemistry 4th ed.

45. Arachodonic Acid and Some Prostoglandins

46. Leukotrienes Compounds also derived from arachidonic acid Found in white blood cells (leukocytes) Consists of 3 conjugated double bonds An important property is constriction of smooth muscles, especially in the lungs

47. Leukotrienes (Cont’d) Leukotriene C