Question 1

HIV is the virus that causes AIDS.In the mid-1990s,researchers discovered an enzyme in HIV called protease.Once the enzyme's structure was known,researchers began looking for drugs that would fit into the active site and block it.If this strategy for stopping HIV infections were successful,it would be an example of what phenomenon?
A)vaccination
B)poisoning
C)allosteric regulation
D)competitive inhibition

Accepted Answer

Competitive inhibition involves blocking the active site of an enzyme with a molecule that resembles the substrate, thereby preventing the substrate from binding and the reaction from occurring. In this case, the researchers are looking for drugs that can block the active site of the HIV protease enzyme, which would prevent the enzyme from working and stop the virus from replicating. This is a form of competitive inhibition. Vaccination involves stimulating the immune system to produce antibodies against a pathogen, poisoning involves introducing a toxin into the body, and allosteric regulation involves controlling enzyme activity through the binding of a molecule at a site other than the active site.

Question 2

Consider the HIV enzyme called protease.The amino acid residues at the active site are highly hydrophobic.In designing a drug that would bind to the active site and jam it,researchers should use which type of molecule?
A)hydrophobic
B)polar
C)charged
D)acidic

Accepted Answer

Since the amino acid residues at the active site of the HIV protease enzyme are highly hydrophobic, a drug molecule that is also hydrophobic would have the greatest chance of binding to the active site and jamming it. Polar, charged, or acidic molecules would not be as effective in this case.

Question 3

An enzyme has a total of four active sites.When you denature the molecule and study its composition,you find that each active site occurs on a different polypeptide.Which of the following hypotheses does this observation support?
A)The enzyme is subject to allosteric regulation.
B)The enzyme requires a cofactor to function normally.
C)The protein's structure is affected by temperature and pH.
D)The protein has quaternary structure.

Accepted Answer

The observation that each active site occurs on a different polypeptide supports the hypothesis that the protein has quaternary structure, which means it is composed of multiple polypeptide chains that come together to form a functional protein. Denaturing the protein would disrupt the intermolecular forces that hold the subunits together, revealing that each active site is on a different polypeptide chain. This is a characteristic of proteins with quaternary structure. The other hypotheses (A, B, and C) do not necessarily require multiple polypeptide chains to be true.

Question 4

If the primary structure of a protein is incorrect
A)the secondary structure will be correct.
B)the tertiary structure will be correct.
C)any quaternary structure will be correct.
D)any higher-level folding of the protein will be incorrect.

Accepted Answer

The primary structure of a protein refers to the amino acid sequence. If this sequence is incorrect or altered, it will affect the higher-level folding of the protein. The secondary, tertiary, and quaternary structures all depend on the correct primary structure. Therefore, if the primary structure is incorrect, any higher-level folding of the protein will also be incorrect.

Question 5

Which of the following observations is the strongest argument in favor of the hypothesis that protein structure and function are correlated?
A)Proteins function best at certain temperatures.
B)Proteins have four distinct levels of structure and many functions.
C)Enzymes tend to be globular in shape.
D)Denatured (unfolded)proteins do not function normally.

Accepted Answer

Denatured (unfolded) proteins do not function normally is the strongest argument in favor of the hypothesis that protein structure and function are correlated. This observation suggests that the specific three-dimensional shape of a protein is directly related to its function. When proteins are denatured and lose their shape, they often lose their function as well.

Question 6

When polymerization of a protein is complete,but the protein is still completely linear,what is the highest level of structure that has been completed?
A)primary
B)secondary
C)tertiary
D)quaternary

Accepted Answer

The answer of When polymerization of a protein is complete,but...

Question 7

In cells,the activity of enzymes is often regulated by other molecules.Why is this necessary?
A)because all enzymes require some help from another molecule to function correctly
B)because other molecules are necessary to prevent enzymes from denaturing
C)because each enzyme has multiple functions
D)because it is unlikely that all reaction products are required all of the time

Accepted Answer

Enzymes catalyze specific reactions, and it is not necessary for all reaction products to be produced at all times. Therefore, the activity of enzymes must be regulated by other molecules to control when and where reactions occur. This allows cells to conserve energy and resources by producing only the necessary products when needed.

Question 8

A peptide bond is
A)an ionic bond,not a covalent one.
B)a triple covalent bond.
C)a particularly stable,planar covalent bond.
D)a particularly unstable covalent bond.

Accepted Answer

A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amine group of another amino acid. This bond is particularly stable and planar due to the resonance structure of the peptide bond.

Question 9

Which of the following would be an example of a cofactor?
A)an enzyme active site that contains an α-helix
B)the nonprotein heme group in a hemoglobin molecule
C)the disulfide bridge that forms between cysteine residues
D)a β-pleated sheet hidden on the inside of a protein's tertiary structure

Accepted Answer

A cofactor is a nonprotein molecule or ion that is required for the proper functioning of an enzyme. The nonprotein heme group in a hemoglobin molecule is an example of a cofactor.

Question 10

You are studying a protein that is shaped like a doughnut.The shape is a function of which level(s)of protein structure?
A)primary only
B)secondary only
C)tertiary only
D)secondary and tertiary only
E)primary,secondary,and tertiary

Accepted Answer

The answer of You are studying a protein that is...

Question 11

Recent technological advances have made it more feasible than ever to work out the three-dimensional structure of proteins.There is intense interest in this research field,called structural biology.Why?
A)Understanding structure should help us understand function.
B)Understanding structure can help in the design of drugs that alter the function of certain proteins.
C)Solving a protein's 3-D structure can lead to a better understanding of how the molecule works-for example,by identifying the active site and determining if there are regulatory sites.
D)All of the above apply.

Accepted Answer

All of the given options are valid reasons for the intense interest in structural biology. Understanding the protein structure provides insights into its function, which is crucial for developing drugs that alter the protein's activity. Additionally, knowledge of a protein's 3-D structure helps identify the active and regulatory sites, and thereby, provides a better understanding of how the molecule works. Thus, recent technological advances that make it feasible to determine protein structures have pushed the field of structural biology forward.

Question 12

What type of interaction is directly responsible for the formation of secondary structure?
A)peptide bonds between adjacent amino acids
B)peptide bonds between nonadjacent amino acids
C)hydrogen bonds between sections of the polypeptide backbone
D)hydrogen bonds between side chains of amino acids

Accepted Answer

Hydrogen bonds between sections of the polypeptide backbone are directly responsible for the formation of secondary structure. These bonds occur between the amino and carboxyl groups of nearby amino acids in the polypeptide backbone, stabilizing the folding of the protein into α-helices or β-sheets. Peptide bonds between adjacent or nonadjacent amino acids and hydrogen bonds between side chains of amino acids can contribute to tertiary and quaternary structure, but are not directly responsible for the formation of secondary structure.

Question 13

When,and by whom,was the lock-and-key model of enzyme specificity developed?
A)1894 by Fischer
B)1894 by Miller
C)1936 by Fischer
D)1956 by Fischer
E)1974 by Haldane and Oparin

Accepted Answer

The lock-and-key model of enzyme specificity was developed by Emil Fischer in 1894.

Question 14

Which of the following is not true when comparing an uncatalyzed reaction to the same reaction with a catalyst?
A)The catalyzed reaction will be faster.
B)The catalyzed reaction will have a different ∆G.
C)The catalyzed reaction will have lower activation energy.
D)The catalyzed reaction will not consume any of the catalyst.

Accepted Answer

The presence of a catalyst does not affect the thermodynamics of a reaction, so the ∆G of the catalyzed reaction will be the same as the uncatalyzed reaction. However, a catalyst can lower the activation energy required for the reaction, making it faster, and will not be consumed in the reaction, so options A, C, and D are true.

Question 15

You've just sequenced a new protein found in mice and observe that sulfur-containing cysteine residues occur at regular intervals.What is the significance of this finding?
A)Cysteine residues are required for the formation of α-helices and β-pleated sheets.
B)It will be important to include cysteine in the diet of the mice.
C)Cysteine residues are involved in disulfide bridges that help form tertiary structure.
D)Cysteine causes bends,or angles,to occur in the tertiary structure of proteins.

Accepted Answer

Cysteine residues are involved in the formation of disulfide bridges that help stabilize the tertiary structure of proteins. This is important for the proper folding and function of the protein. Regularly spaced cysteine residues can indicate specific structural motifs or domains within the protein.

Question 16

You have isolated a previously unstudied protein,identified its complete structure in detail,and determined that it catalyzes the breakdown of a large substrate.You notice it has two binding sites.One of these is large,apparently the bonding site for the large substrate;the other is small,possibly a binding site for a regulatory molecule.What do these findings tell you about the mechanism of this protein?
A)It is probably a structural protein that is involved in cell-to-cell adhesion.
B)It is probably an enzyme that works through allosteric regulation.
C)It is probably an enzyme that works through competitive inhibition.
D)It is probably a cell membrane transport protein,like an ion channel.
E)It is probably a structural protein found in cartilage or skeletal tissue.

Accepted Answer

The fact that the protein has two binding sites suggests that it is an enzyme that works through allosteric regulation, where binding of a regulatory molecule to the small site can either activate or inhibit the enzyme's activity towards the large substrate at the other site.

Question 17

You've discovered an enzyme that can catalyze two different chemical reactions.Which of the following is most likely to be correct?
A)The enzyme contains both α-helices and β-pleated sheets.
B)The enzyme is subject to both competitive inhibition and allosteric regulation.
C)Two types of allosteric regulation occur-the binding of one molecule activates the enzyme,while the binding of a different molecule inhibits it.
D)Either the enzyme has two distinct active sites,or the reactants involved in the two reactions are very similar in size and shape.

Accepted Answer

If one enzyme can catalyze two different reactions, it is likely that either the enzyme has two distinct active sites to catalyze the two different reactions or that the reactants involved in the two reactions are very similar in size and shape, and thus can fit into the same active site. Options A, B, and C are not necessarily true in all cases of enzymes with two different catalytic activities.

Question 18

A series of hydrophobic side chains will congregate together as a protein folds in an aqueous solution and be stabilized by
A)disulfide bonds.
B)van der Waals interaction.
C)hydrogen bonds.
D)quaternary structure bonds.

Accepted Answer

Hydrophobic side chains will avoid contact with water and will therefore be attracted to each other through weak van der Waals forces, leading to the formation of a hydrophobic core in the folded protein. Disulfide bonds, hydrogen bonds, and quaternary structure bonds primarily stabilize other parts of the protein structure.

Question 19

Which of the following best describes primary structure in proteins?
A)It is the number of amino acids present in the complete protein.
B)It is the number of peptide bonds in the complete protein.
C)It is the sequence of amino acids in the complete protein.
D)It is the number of α-helices and β-pleated sheets in the complete protein.

Accepted Answer

Primary structure refers to the specific sequence of amino acids that make up a protein, which is determined by the genetic code. This sequence ultimately determines the three-dimensional structure and function of the protein.

Question 20

Several of the molecules called vitamins act as enzyme cofactors.Vitamin deficiencies cause disease.What is the most direct explanation for this?
A)Vitamins combine with nonprotein molecules to delay the onset of disease.
B)If cofactors are missing,enzymes cannot function properly,and important reaction products will be absent from cells.
C)Normal regulation cannot occur in the absence of cofactors.As a result,all enzymes will function all of the time.
D)Cofactors inhibit enzymes found in disease-causing bacteria and viruses.When cofactors are absent,these disease-causing agents multiply.

Accepted Answer

Enzymes require cofactors to function properly. If a cofactor, such as a vitamin, is missing, the enzyme cannot function properly, and important reaction products will be absent from cells. This can lead to disease.

Quiz 3: Protein Structure and Function

Consider the HIV enzyme called protease.The amino acid residues at the active site are highly hydrophobic.In designing a drug that would bind to the active site and jam it,researchers should use which type of molecule?

An enzyme has a total of four active sites.When you denature the molecule and study its composition,you find that each active site occurs on a different polypeptide.Which of the following hypotheses does this observation support?

If the primary structure of a protein is incorrect

Which of the following observations is the strongest argument in favor of the hypothesis that protein structure and function are correlated?

When polymerization of a protein is complete,but the protein is still completely linear,what is the highest level of structure that has been completed?

In cells,the activity of enzymes is often regulated by other molecules.Why is this necessary?

A peptide bond is

Which of the following would be an example of a cofactor?

You are studying a protein that is shaped like a doughnut.The shape is a function of which level(s) of protein structure?

Recent technological advances have made it more feasible than ever to work out the three-dimensional structure of proteins.There is intense interest in this research field,called structural biology.Why?

What type of interaction is directly responsible for the formation of secondary structure?

When,and by whom,was the lock-and-key model of enzyme specificity developed?

Which of the following is not true when comparing an uncatalyzed reaction to the same reaction with a catalyst?

You've just sequenced a new protein found in mice and observe that sulfur-containing cysteine residues occur at regular intervals.What is the significance of this finding?

You've discovered an enzyme that can catalyze two different chemical reactions.Which of the following is most likely to be correct?

A series of hydrophobic side chains will congregate together as a protein folds in an aqueous solution and be stabilized by

Which of the following best describes primary structure in proteins?

Several of the molecules called vitamins act as enzyme cofactors.Vitamin deficiencies cause disease.What is the most direct explanation for this?