Question 1

Which of the following is true when comparing an uncatalyzed reaction to the same reaction with a catalyst?
A)The catalyzed reaction will be slower.
B)The catalyzed reaction will have the same ∆G.
C)The catalyzed reaction will have higher activation energy.
D)The catalyzed reaction will consume all of the catalyst.

Accepted Answer

A catalyst lowers the activation energy required for a reaction to occur, which allows the reaction to proceed at a faster rate. This means that the catalyzed reaction will have the same ∆G (free energy change) as the uncatalyzed reaction, but the reaction will occur faster with the catalyst. Therefore, option B is the correct answer. Options A, C, and D are incorrect because they suggest that the catalyzed reaction behaves differently than what is generally observed in catalyzed reactions.

Question 2

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

Secondary structure is formed by hydrogen bonds between sections of the polypeptide backbone. These bonds occur between the carbonyl group of one amino acid and the amino group of another amino acid, forming an alpha helix or beta sheet. Peptide bonds between adjacent or non-adjacent amino acids are important for forming the primary structure of the protein, but not the secondary structure. Hydrogen bonds between side chains of amino acids are important for stabilizing the tertiary and quaternary structure of the protein, but not the secondary structure.

Question 3

How does primary protein structure affect the function of protein enzymes?
A)Substrates interact with R-groups at the enzyme's active site.
B)Substrates interact with R-groups at the enzyme's external surface.
C)Substrates interact with hydrophobic R-groups at any region of the enzyme.
D)Substrates permanently bind to R-groups at the enzyme's active site.

Accepted Answer

The primary protein structure determines the sequence and arrangement of amino acids in the enzyme, which ultimately determines the shape of the enzyme's active site. The R-groups of amino acids at the enzyme's active site interact with substrates to catalyze specific chemical reactions.

Question 4

Aquaporins are proteins that control the passage of water molecules across the cell membrane.The protein forms a pore,or opening,in the membrane.You isolate what you think are two different molecules of aquaporin,and determine that one of the proteins has a larger pore diameter than the second.Which of the following do you conclude?
A)These two forms of aquaporin will have identical sequences of amino acids.
B)These two forms of aquaporin will have different sequences of amino acids.
C)You will have to sequence the proteins to compare their primary structure,because it should have no effect on pore diameter.
D)These molecules both can't have aquaporin because all proteins that do the same type of job (such as catalyze a reaction)have the exact same 3-D structure.

Accepted Answer

The diameter of the pore in the cell membrane is determined by the 3-D structure of the protein, which is in turn determined by its amino acid sequence. Therefore, if the two forms of aquaporin have different pore diameters, they must have different sequences of amino acids that result in different protein structures.

Question 5

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 6

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 as it is required for the proper oxygen binding and delivery by hemoglobin.

Question 7

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 indicates that the enzyme has quaternary structure, meaning it is composed of multiple polypeptide subunits that come together to form a functional molecule with multiple active sites. This is only possible in quaternary structured proteins, and not in proteins with a primary, secondary, or tertiary structure alone. Therefore, the correct answer is D.

Question 8

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 their activity must be regulated to ensure that the right amount of product is produced at the right time. If the product is not required, the enzyme may need to be inhibited, and if more of the product is needed, the enzyme needs to be activated. Therefore, the activity of enzymes is often regulated by other molecules to ensure that the right amount of product is produced at the right time.

Question 9

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 these cofactors are missing, enzymes cannot catalyze important reactions in cells, leading to disease. Vitamin deficiencies cause disease because many vitamins act as enzyme cofactors, and a lack of these cofactors leads to enzyme dysfunction.

Question 10

A peptide bond _____.
A)forms between the functional groups of different amino acids
B)forms between the central carbon and the amino group of a single amino acid
C)forms the primary structure of proteins
D)does not play a role in maintaining the tertiary structure of proteins

Accepted Answer

The answer of A peptide bond _____.
A)forms between the functional...

Question 11

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 the enzyme can catalyze two different reactions, it must have two distinct active sites for each reaction or the reactants involved in both reactions are similar in size and shape. This is because enzymes typically catalyze reactions based on their specific active sites, which will differ based on the reactants involved. Therefore, option D is the most likely explanation. Options A and B do not necessarily follow from the discovery of a versatile enzyme, and option C suggests that the enzyme can be both activated and inhibited by different molecules, which is less likely than the enzyme having two active sites or similar reactants.

Question 12

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 HIV protease are highly hydrophobic, the drug molecule designed to bind to the active site and jam it should also be hydrophobic in nature. This is because hydrophobic molecules tend to interact favorably with other hydrophobic molecules, and hence a hydrophobic drug would be able to bind more tightly to the active site of the enzyme. Therefore, Option A is the best choice to design a drug that would bind to the active site and jam it.

Question 13

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 presence of two binding sites suggests that this protein functions through allosteric regulation, where binding of a regulatory molecule to the small site causes a change in shape of the protein and affects its activity towards the large substrate at the large binding site. This is a common mechanism for enzymes.

Question 14

The lock-and-key analogy for enzymes applies to the _____.
A)specificity of enzyme primary,secondary,and tertiary structure
B)specificity of enzyme tertiary subunits joining to form a quaternary structure
C)specificity of enzymes binding to their substrate
D)specificity of enzymes interacting with water
E)specificity of enzymes interacting with ions

Accepted Answer

The lock-and-key analogy refers to the specificity of enzyme-substrate binding, where the enzyme's active site is the "lock" and the substrate is the "key" that fits perfectly into the active site. This analogy does not refer to the other choices given.

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 often form disulfide bridges, which play a critical role in stabilizing the tertiary structure of many proteins. The regular occurrence of cysteine residues suggests that these disulfide bridges may be important for the function of the protein.

Question 16

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 proteins, which have lost their three-dimensional structure, also lose their function, indicating a strong correlation between protein structure and function. This observation provides the strongest argument in favor of the hypothesis that protein structure and function are correlated.

Question 17

You determine the amino acid sequence of a protein and find it contains a long sequence of methionine,followed by a long sequence of proline,followed by a long sequence of valine.Using these data you predict the sequence of this protein's secondary structure will be _____.
A)beta sheets,then a region of no secondary structure,then beta sheets
B)alpha helices,then a region of no secondary structure,then alpha helices
C)beta sheets,then a region of no secondary structure,then alpha helices
D)alpha helices,then a region of no secondary structure,then beta sheets

Accepted Answer

Methionine is known to promote alpha helix formation, and proline promotes a bend in the protein chain, often leading to the formation of beta turns. Valine tends to stabilize beta sheets. Therefore, the sequence of methionine-proline-valine is suggestive of a pattern where alpha helices are followed by bends, leading into beta sheets. This pattern appears in answer choice D.

Question 18

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 primary structure of a protein is the linear sequence of amino acids. It is the first level of protein structure and is complete once polymerization is finished, even if the protein has not yet folded into more complex structures.

Question 19

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 tend to avoid water and will cluster together in a protein's interior, away from the aqueous environment. This is stabilized by van der Waals interactions between the nonpolar side chains. Disulfide bonds and hydrogen bonds are more involved in stabilizing the protein's tertiary or secondary structure, respectively. Quaternary structure bonds involve interactions between multiple protein subunits.

Question 20

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

The strategy described in the passage involves blocking the active site of the HIV enzyme protease, which is an example of competitive inhibition. The drugs being developed would compete with the enzyme's natural substrate for access to the active site, preventing it from functioning properly and thus inhibiting the virus's ability to replicate. Vaccination involves triggering an immune response to a virus or other pathogen, while allosteric regulation involves the binding of a molecule to a site on an enzyme other than the active site to either activate or inhibit its function. Poisoning is not relevant to the context of the passage.

Quiz 3: Protein Structure and Function

Which of the following is true when comparing an uncatalyzed reaction to the same reaction with a catalyst?

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

How does primary protein structure affect the function of protein enzymes?

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

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

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?

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

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

A peptide bond _____.

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

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?

The lock-and-key analogy for enzymes applies to the _____.

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?

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

You determine the amino acid sequence of a protein and find it contains a long sequence of methionine,followed by a long sequence of proline,followed by a long sequence of valine.Using these data you predict the sequence of this protein's secondary structure will be _____.

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