Ch. 7 and Ch. 8 Exam 3 Review

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.

0.0625/0.001 or 62.5/1

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.

C. Membranes contain glycolipids and glycoproteins. D. Lipid bilayers are an important component of membranes.

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.

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.

Explain why the 2nd phase of the chemotrypsin mechanism is slower than the 1st mechanism.

Serine 195 is a stronger nucleophile than water, and water must diffuse to the proper place to perform it's nucleophilic attack which takes time.

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.

What are the 2 essential amino acids in the active site of chymotrypsin?

Serine and Histidine

What group is often attacked by a nucleophile in biochem mechanisms?

The carbon of a carbonyl group

Which of the following lipids are not found in animal membranes? a. phosphoglycerides b. cholesterol c. triacylglycerols d. glycopipids e. sphingolipids

c. triacylglycerols

Briefly describe the role of nucleophilic catalysis in the mechanism of the chymotrypsin reaction.

1st step: the serine hydroxyl is the nucleophile that attacks the substrate peptide bond; 2nd step: water is the nucleophile that attacks the acyl-enzyme intermediate.

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.

Explain how a researcher makes an abzyme.

An abzyme is created by injecting a host animal with a transition-state analogue of a reaction of interest. The host animal makes antibodies to the foreign molecule, and these antibodies have specific binding points that mimic an enzyme surrounding a transition state.

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

What is meant by general acid catalysis with respect to enzyme mechanisms?

General acid catalysis is the part of an enzyme mechanism in which an amino acid or other molecule donates a hydrogen ion to another molecule.

Explain how the pKa for histidine 57 is important to its role in the mechanism of chymotrypsin action.

His 57 exists in both the protonated and unprotonated form during thr chymotrypsin rxn. It's pKa of 6.0 makes this possible in the physiological pH range.

Explain the function of histidine 57 in the mechanism of chymotrypsin.

His 57 performs a series of steps involving general base catalysis followed by general acid catalysis.

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 phosphatidyl ethanolamine 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.

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 are coenzymes related to vitamins?

Most coenzymes are derivatives of compounds we call vitamins. For example, nicotinamide adenine dinucleotide is produced from the B vitamin niacin. Flavin adenine dinucleotide comes from riboflavin.

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

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.

Explain the difference between an SN1 reaction mechanism and SN2 reaction mechanism.

SN1 stands for unimolecular nucleophilic substitution, and the unimolecular part means that it obeys 1st order kinetics. SN2 stands for bimolecular nucleophilic substitution. The rate displays 2nd order kinetics.

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.

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.

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.

What is the relationship between a transition-state analog and the induced-fit model of enzyme kinetics?

The induced-fit model assumes that the enzyme and substrate must both move and change to conform to each other perfectly. Thus, the true fit is not between the enzyme and substrate but between the enzyme and the transition state of the substrate on its way to product. A transition-state analogue fits the enzyme nicely in this model.

Why does the enzyme reaction for chymotrypsin proceed in 2 phases?

The initial phase releases the 1st product and involves and acylenzyme intermediate, and is faster than the 2nd part. The 2nd part involves water entering the active site and breaks the acyl-enzyme bond.

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.

What is the purpose of an abzyme?

The purpose is to create an antibody with catalytic activity.

Are the relative amounts of cholesterol and phophatidylcholine the same in all the kinds of membranes found in a typical mammalian cell?

The relative amounts of cholesterol and phosphatidylcholine can vary widely in different types of membranes in the same cell.

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.

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.

What type of rxn uses vitamin B6?

Vitamin B6 is the source of pyridoxal phosphate, which is used in transamination reactions.

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.