Biochem Chp 8

Suggest a reason why inorganic ions, such as K1, Na1, Ca21, and Mg21, do not cross biological membranes by simple diffusion.

Biological membranes are highly nonpolar environments.

What is the structural relationship between vitamin D3 and cholesterol?

Cholesterol is a precursor of vitamin D3; the conversion reaction involves ring opening.

List an important chemical property of vitamin E.

Vitamin E is an antioxidant.

What are the main structural features of leukotrienes?

Leukotrienes are carboxylic acids with three conjugated double bonds.

What role does phosphorylation of tyrosine residues play in the action of receptor proteins?

Phosphorylation of tyrosine residues can activate or deactivate a receptor protein, depending on the specific system.

A membrane consists of 50% protein by weight and 50% phosphoglycerides by weight. The average molecular weight of the lipids is 800 Da, and the average molecular weight of the proteins is 50,000 Da. Calculate the molar ratio of lipid to protein.

A 100-g sample of membrane contains 50 g of protein and 50 g of phosphoglycerides. 50 g lipid 3 1 mol lipid 800 g lipid 5 0.0625 mol lipid 50 g protein 3 1 mol protein 50,000 g protein 5 0.001 mol protein The molar ratio of lipid to protein is 0.0625/0.001 or 62.5/1.

Define glycoprotein and glycolipid.

A glycoprotein is formed by covalent bonding between a carbohydrate and a protein, whereas a glycolipid is formed by covalent bonding between a carbohydrate and a lipid.

Define omega-3 fatty acid.

An omega-3 fatty acid has a double bond at the third carbon from the methyl end.

Write the structural formula for a triacylglycerol, and name the component parts.

Any combination of fatty acids is possible.

Suggest a reason why the cell membranes of bacteria grown at 20°C tend to have a higher proportion of unsaturated fatty acids than the membranes of bacteria of the same species grown at 37°C. In other words, the bacteria grown at 37°C have a higher proportion of saturated fatty acids in their cell membranes.

At the lower temperature, the membrane would tend to be less fluid. The presence of more unsaturated fatty acids would tend to compensate by increasing the fluidity of the membrane compared to one at the same temperature with a higher proportion of saturated fatty acids.

What happens to the human growth hormone receptor when it binds hGH?

Binding of the hormone causes a movement of the transmembrane protein that is transmitted to the intracellular side. This causes a movement of the kinase and catalytic domains, activating the kinase activity of the receptor.

What structural features do a sphingomyelin and a phosphatidyl choline have in common? How do the structures of these two types of lipids differ?

Both sphingomyelins and phosphatidylcholines contain phosphoric acid esterified to an amino alcohol, which must be choline in the case of a phosphatidylcholine and may be choline in the case of a sphingomyelin. They differ in the second alcohol to which phosphoric acid is esterified. In phosphatidylcholines, the second alcohol is glycerol, which has also formed ester bonds to two carboxylic acids. In sphingomyelins, the second alcohol is another amino alcohol, sphingosine, which has formed an amide bond to a fatty acid. (See Figure 8.6.) 5.

Which statements are consistent with the known facts about membrane transport? (a) Active transport moves a substance from a region in which its concentration is lower to one in which its concentration is higher. (b) Transport does not involve any pores or channels in membranes. (c) Transport proteins may be involved in bringing substances into cells.

Charged ions tend to be excluded from such environments rather than dissolving in them, as they would have to do to pass through the membrane by simple diffusion. Statements (a) and (c) are correct; statement (b) is not correct because ions and larger molecules, especially polar ones, require channel proteins.

Egg yolks contain a high amount of cholesterol, but they also contain a high amount of lecithin. From a diet and health standpoint, how do these two molecules complement each other?

Cholesterol is not very water-soluble, but lecithin is a good natural detergent, which is actually part of lipoproteins that transport the blood

A health-conscious friend asks whether eating carrots is better for the eyesight or for preventing cancer. What do you tell your friend? Explain. 8-7 Prostaglandins and Leukotrienes

Eating carrots is good for both. VitaminA, which is an abundant carrot, plays a role in vision. Diets that include generous amounts of vegetables are associated with a lower incidence of cancer.

What is the energetic driving force for the formation of phospholipid bilayers?

Hydrophobic interactions among the hydrocarbon tails are the main energetic driving force in the formation of lipid bilayers.

Suppose that you are studying a protein involved in transporting ions in and out of cells. Would you expect to find the nonpolar residues in the interior or the exterior? Why? Would you expect to find the polar residues in the interior or the exterior? Why?

In a protein that spans a membrane, the nonpolar residues are the exterior ones; they interact with the lipids of the cell membrane. The polar residues are in the interior, lining the channel through which the ions enter and leave the cell.

What structural features do a triacylglycerol and a phosphatidylethanolamine have in common? How do the structures of these two types of lipids differ?

In both types of lipids, glycerol is esterified to carboxylic acids, with three such ester linkages formed in triacylglycerols and two in phosphatidyl ethanolamines. The structural difference comes in the nature of the third ester linkage to glycerol. In phosphatidyl ethanolamines, the third hydroxyl group of glycerol is esterified not to a carboxylic acid but to phosphoric acid. The phosphoric acid moiety is esterified in turn to ethanolamine. (See Figures 8.2 and 8.5.)

What are isoprene units? What do they have to do with the material of this chapter?

Isoprene units are five-carbon moieties that play a role in the structure of a number of natural products, including fat-soluble vitamins.

What is JAK2 and why is it important to the effect of hGH? 8-6 Lipid-Soluble Vitamins and Their Functions

JAK2 stands for Janus Kinase 2. The hormone binding to the outside of the receptor causes an activation of the kinase on the inside of the cell. Activation of this and many kinases is the key signaling event in many pathways. Such kinases produce a second messenger that begins a cascade of molecular changes that effect the overall changes seen when hormones are released. 8-6 Lipid-Soluble Vitamins and Their Functions

Why does the American Heart Association recommend the use of canola oil or olive oil rather than coconut oil in cooking?

Less heart disease is associated with diets low in saturated fatty acids.

How can fluorescence techniques be used to monitor the motion of lipids and proteins in membranes?

Lipids can be "tagged" with a fluorescent moiety to observe their motion in membranes. Proteins have intrinsic fluorescence and can be monitored directly.

Crisco is made from vegetable oils, which are usually liquid. Why is Crisco a solid? Hint: Read the label.

Many of the double bonds have been saturated. Crisco contains "partially hydrogenated vegetable oils."

Briefly discuss the structure of myelin and its role in the nervous system.

Myelin is a multilayer sheath consisting mainly of lipids (with some proteins) that insulates the axons of nerve cells, facilitating transmission of nerve impulses.

Suggest a reason why the same protein system moves both sodium and potassium ions into and out of the cell.

Nature chooses what works. This is an efficient use of a large protein and of the energy of ATP.

Suggest a reason why partially hydrogenated vegetable oils are used so extensively in packaged foods.

Partially hydrogenated vegetable oils have the desired consistency for many foods, such as oleomargarine and components of TV dinners.

Which is more hydrophilic, cholesterol or phospholipids? Defend your answer.

Phospholipids are more hydrophilic than cholesterol. The phosphate group is charged, and the attached alcohol is charged or polar. These groups interact readily with water. Cholesterol has only a single polar group, an !OH.

List two classes of compounds derived from arachidonic acid. Suggest some reasons for the amount of biomedical research devoted to these compounds.

Prostaglandins and leukotrienes are derived from arachidonic acid. They play a role in inflammation and in allergy and asthma attacks.

Outline a possible connection between the material in this chapter and the integrity of blood platelets.

Prostaglandins in blood platelets can inhibit their aggrega- tion. This is one of the important physiological effects of prostaglandins.

What are the main structural features of prostaglandins?

Prostaglandinsarecarboxylicacidsthatincludeafive-membered ring in their structure.

Do all proteins associated with membranes span the membrane from one side to another?

Proteins that are associated with membranes do not have to span the membrane. Some can be partially embedded in it, and some associate with the membrane by noncovalent interactions with its exterior.

List the fat-soluble vitamins, and give a physiological role for each.

See Table 8.5.

Proteins, nucleic acids, and carbohydrates are grouped by common structural features found within their group. What is the basis for grouping substances as lipids?

Solubility properties (insoluble in aqueous or polar solvents, soluble in nonpolar solvents). Some lipids are not at all structurally related.

What structural features does a sphingolipid have in common with proteins? Are there functional similarities?

Sphingolipids contain amide bonds, as do proteins. Both can have hydrophobic and hydrophilic parts, and both can occur in cell membranes, but their functions are different.

Which of the following statements is (are) consistent with what is known about membranes? (a) A membrane consists of a layer of proteins sandwiched between two layers of lipids. (b) The compositions of the inner and outer lipid layers are the same in any individual membrane. (c) Membranes contain glycolipids and glycoproteins. (d) Lipid bilayers are an important component of membranes. (e) Covalent bonding takes place between lipids and proteins in most membranes.

Statements (c) and (d) are consistent with what is known about membranes. Covalent bonding between lipids and proteins [statement (e)] occurs in some anchoring motifs, but is not widespread otherwise. Proteins "float" in the lipid bilayers rather than being sandwiched between them [statement (a)]. Bulkier molecules tend to be found in the outer lipid layer [statement (b)].

Which statements are consistent with the fluid-mosaic model of membranes? (a) All membrane proteins are bound to the interior of the membrane. (b) Both proteins and lipids undergo transverse (flip-flop) diffusion from the inside to the outside of the membrane. (c) Some proteins and lipids undergo lateral diffusion along the inner or outer surface of the membrane. (d) Carbohydrates are covalently bonded to the outside of the membrane. (e) The term mosaic refers to the arrangement of the lipids alone. 8-5 The Functions of Membranes

Statements (c) and (d) are correct. Transverse diffusion is only rarely observed [statement (b)], and the term mosaic refers to the pattern of distribution of proteins in the lipid bilayer [statement (e)]. Peripheral proteins are also considered part of the membrane [statement (a)]. 8-5 The Functions of Membranes

How does the structure of steroids differ from that of the other lipids discussed in this chapter?

Steroids contain a characteristic fused-ring structure, which other lipids do not.

What does the hydrolysis of GTP have to do with the actions of receptor proteins?

The action of a number of receptor proteins requires binding of GTP to a specific subunit, followed by subsequent hydrolysis.

What is the role in vision of the cis-trans isomerization of retinal?

The cis-trans isomerization of retinal in rhodopsin triggers the transmission of an impulse to the optic nerve and is the primary photochemical event in vision.

Suggest a reason why animals that live in cold climates tend to have higher proportions of polyunsaturated fatty acid residues in their lipids than do animals that live in warm climates.

The higher percentage of unsaturated fatty acids in membranes in cold climates is an aid to membrane fluidity.

In the preparation of sauces that involve mixing water and melted butter, egg yolks are added to prevent separation. How do the egg yolks prevent separation? Hint: Egg yolks are rich in phosphatidylcholine (lecithin).

The lecithin in the egg yolks serves as an emulsifying agent by forming closed vesicles. The lipids in the butter (frequently triacylglycerols) are retained in the vesicles and do not form a separate phase.

Why might some food companies find it economically advantageous to advertise their product (for example, triacylglycerols) as being composed of polyunsaturated fatty acids with trans-double bonds?

The public is attuned to the idea of polyunsaturated fats as healthful. The trans configuration gives a more palatable consistency. Recently, however, concerns have arisen about the extent to which such products mimic saturated fats.

Are the relative amounts of cholesterol and phophatidylcholine the same in all the kinds of membranes found in a typical mammalian cell? 8-4 Membrane Proteins

The relative amounts of cholesterol and phosphatidylcholine can vary widely in different types of membranes in the same cell (see Table 8.3). 8-4 Membrane Proteins

When water birds have had their feathers fouled with crude oil after an oil spill, they are cleaned by rescuers to remove the spilled oil. Why are they not released immediately after they are cleaned? 8-3 Biological Membranes

The removal of the oil also removes the natural oils and waxes on the feathers. These oils and waxes must regenerate before the birds can be released. 8-3 Biological Membranes

In the produce department of supermarkets, vegetables and fruits (cucumbers are an example) have been coated with wax for shipping and storage. Suggest a reason why this is done.

The surface wax keeps produce fresh by preventing loss of water.

In lipid bilayers, there is an order-disorder transition similar to the melting of a crystal. In a lipid bilayer in which most of the fatty acids are unsaturated, would you expect this transition to occur at a higher temperature, a lower temperature, or the same temperature as it would in a lipid bilayer in which most of the fatty acids are saturated? Why?

The transition temperature is lower in a lipid bilayer with mostly unsaturated fatty acids compared with one with a high percentage of saturated fatty acids. The bilayer with the unsaturated fatty acids is already more disordered than the one with a high percentage of saturated fatty acids.

Succulent plants from arid regions generally have waxy surface coatings. Suggest why such a coating is valuable for the survival of the plant.

The waxy surface coating is a barrier that prevents loss of water.

You have just isolated a pure lipid that contains only sphingosine and a fatty acid. To what class of lipids does it belong?

This lipid is a ceramide, which is one kind of sphingolipid.

Which of the following lipids are not found in animal membranes? (a) Phosphoglycerides (b) Cholesterol (c) Triacylglycerols (d) Glycolipids (e) Sphingolipids

Triacylglycerols are not found in animal membranes.

Why is it possible to argue that vitamin D is not a vitamin?

Vitamin D can be made in the body. Lipid-soluble vitamins accumulate in fatty tissue, leading to toxic effects.

Why can some vitamin-K antagonists act as anticoagulants?

Vitamin K plays a role in the blood-clotting process. Blocking its mode of action can have an anticoagulant effect.

Why are many vitamin supplements sold as antioxidants? How does this relate to material in this chapter?

Vitamins A and E are known to scavenge free radicals, which can do oxidative damage to cells.

Give a reason for the toxicity that can be caused by overdoses of lipid-soluble vitamins.

Water-soluble vitamins are excreted, drastically reducing the chances of an overdose.

What are the structural features of waxes? What are some common uses of compounds of this type?

Waxes are esters of long-chain carboxylic acids and long-chain alcohols. They tend to be found as protective coatings.