1. Biochemistry Mary K. Campbell PowerPoint by William H. Brown
Beloit College
Copyright © 1999 by Harcourt Brace & Company
All rights reserved.
Requests for permission to make copies of any part of the work should be mailed to:Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida

2. Lipids and Membranes

3. Lipids
Lipids: a heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties
they are insoluble in water, but soluble in aprotic organic solvents, including diethyl ether, chloroform, methylene chloride, and acetone
Lipids include
triacylglycerols, phosphodiacylglycerols, sphingolipids, glycolipids, lipid-soluble vitamins, and prostaglandins
cholesterol, steroid hormones, and bile acids

4. Fatty Acids
Fatty acid: a long, unbranched chain carboxylic acid, most commonly of 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

5. Fatty Acids

6. Fatty Acids Among the fatty acids most abundant in plants and animals
nearly all have an even number of carbon atoms, most between 12 and 20, in an unbranched chain
the three most abundant are palmitic (16:0), stearic acid (18:0), and oleic acid (18:1)
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

7. 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)

8. Soaps
Soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water)

9. Phosphoacylglycerols
Phosphoacylglycerols (phosphoglycerides) are the second most abundant group of naturally occurring lipids
found almost exclusively in plant and animal membranes, which typically consist of 40% -50% phosphoacylglycerols and 50% - 60% proteins
the most abundant phosphoacylglycerols are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid
the three most abundant fatty acids in phosphatidic acids are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1)

10. Phosphoacylglycerols
A phosphatidic acid
further esterification with a low-molecular weight alcohol gives a phosphoacylglycerol
among the most common of these low-molecular-weight alcohols are

11. Phosphoacylglycerols

12. Phosphoacylglycerols

13. Phosphoacylglycerols
A lecithin

14. Waxes Esters of long-chain fatty acids and alcohols
from the Old English word weax = honeycomb

15. Sphingolipids

16. Glycolipids
Glycolipid: a compound in which a carbohydrate is bound to an -OH of the lipid
many glycolipids are derived from ceramides

17. Steroids
Steroids: a group of plant and animal lipids that have this tetracyclic ring structure
The features common to the ring system of most naturally occurring steroids are illustrated on the next screen

18. Steroids

19. Cholesterol

20. Androgens Androgens - male sex hormones synthesized in the testes
responsible for the development of male secondary sex characteristics

21. Estrogens Estrogens - female sex hormones synthesized in the ovaries
responsible for the development of female secondary sex characteristics and control of the menstrual cycle

22. Biological Membranes
In aqueous solution, phosphoglycerides spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers (Figure 6.8)
polar head 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)

23. Biological Membranes the presence of cholesterol increases rigidity
with heat, membranes become more disordered; the transition temperature is higher for more rigid membranes; it is lower for less rigid membranes

24. Membrane Proteins
Functions: transport substances across membranes, 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
removed by treatment with detergents or ultrasonification
removal generally denatures them

25. 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 basic 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

26. Membrane Transport Passive transport Active transport
driven by a concentration gradient
simple diffusion: a molecule or ion moves through an opening created by a channel protein
facilitated diffusion: molecule or ion is carried across a membrane by a carrier protein
Active transport
a molecule or ion is moved against a concentration gradient
see the Na+/K+ ion pump (Figs )

27. Membrane Receptors Membrane receptors generally oligomeric proteins
binding of a biologically active substance to a receptor initiates an action within the cell
see the low-density-lipoprotein (LDL) receptor (Fig. 6.21)
see the Neuromuscular Junction (Figs )

28. Lipid-Soluble Vitamins
Vitamins are divided into two broad classes on the basis of their solubility
those that are lipid-soluble (and hence classified as lipids)
those that are water-soluble
The lipid-soluble vitamins include A, D, E, and K

29. Vitamin A Vitamin A, or retinol, occurs only in the animal world
Vitamin A is found in the plant world in the form of a provitamin in a group of pigments called carotenes (tetraterpenes)
enzyme-catalyzed cleavage of b-carotene followed by reduction gives two molecules of vitamin A

30. Vitamin A

31. 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), which forms an imine with an -NH2 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 configuration

32. Vitamin A

33. Vitamin D
A group of structurally related compounds that play a role in the regulation of calcium and phosphorus metabolism
the most abundant form in the circulatory system is vitamin D3

36. Vitamin E
Vitamin E is a group of compounds of similar structure; the most active is a-tocopherol
an antioxidant; traps HOO• and ROO• radicals formed as a result of oxidation by O2 of unsaturated hydrocarbon chains in membrane phospholipids

37. Vitamin K
The name of this vitamin comes from the German word Koagulation, signifying its important role in the blood-clotting process

39. Prostaglandins
Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid

40. Prostaglandins
Prostaglandins are not stored in tissues as such, but are synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers
one such polyunsaturated fatty acid is arachidonic acid

41. Prostaglandins
among those synthesized from arachidonic acid are

42. Prostaglandins
Research on the involvement of PGs in reproductive physiology has produced several clinically useful derivatives
15-Methyl-PGF2a is used as a therapeutic abortifacient

43. Prostaglandins
the PGE1 analog, misoprostol, is used for prevention of ulceration associated with the use of aspirin-like nonsteroidal antiinflammatory drugs (NSAIDs)

44. Leukotrienes Leukotrienes: derived from arachidonic acid
found in white blood cells (leukocytes)
an important property is constriction of smooth muscles, especially in the lungs

45. Thromboxanes derived from arachidonic acid
contain a four-membered cyclic ether within a six-membered ring
induce platelet aggregation and smooth muscle contraction

46. End
Chapter 6