MCAT Biology : Enzymes and Enzyme Inhibition

Study concepts, example questions & explanations for MCAT Biology

varsity tutors app store varsity tutors android store

Example Questions

Example Question #21 : Enzymes And Enzyme Inhibition

In 2013, scientists linked a cellular response called the unfolded protein response (UPR) to a series of neurodegenerative diseases, including such major health issues as Parkinson’s and Alzheimer’s Disease. According to their work, the unfolded protein response is a reduction in translation as a result of a series of enzymes that modify a translation initiation factor, eIF2, as below:

Untitled

In the above sequence, the unfolded protein sensor binds to unfolded protein, such as the pathogenic amyloid-beta found in the brains of Alzheimer’s Disease patients. This sensor then phosphorylates PERK, or protein kinase RNA-like endoplasmic reticulum kinase. This leads to downstream effects on eIF2, inhibition of which represses translation. It is thought that symptoms of neurodegenerative disease may be a result of this reduced translation.

We do not know the exact action of eIF2 after it has been acted upon by PERK, and therefore cannot draw conclusions about the phosphorylation or dephosphorylation of transcription factors.

Which of the following is most likely the molecular event that causes repression of translation, based on the information in the passage?

Possible Answers:

Dephosphorylation of transcription factors

Phosphorylation of transcription factors

Phosphorylation of eIF2

Phosphorylation of the unfolded proteins

Dephosphorylation of eIF2

Correct answer:

Phosphorylation of eIF2

Explanation:

The diagram in the passage shows the kinase PERK, which must phosphorylate its substrate, acts on eIF2. Based on its kinase nature and the diagram, phosphorylation of eIF2 is the most likely answer that would lead to propagation of the signal shown.

Example Question #22 : Enzymes And Enzyme Inhibition

Which of the following statements about enzymes is false?

Possible Answers:

Harsh, acidic conditions can completely denature an enzyme

Enzymes speed up the rate of reaction in DNA synthesis

The Keq of a reaction remains unchanged in the presence of an enzyme

An enzyme is completely converted to product during metabolism

Correct answer:

An enzyme is completely converted to product during metabolism

Explanation:

While enzymes do not change the amount of product formed in a reaction (no change to Keq) they do speed up the rate of reaction. It is also true that under certain conditions pH and/or heat can denature an enzyme.

During a reaction, an enzyme does not get used up and is regenerated; enzymes are a type catalyst. Essentially, the enzyme is both a reactant and a product of the reaction it catalyzes.

Example Question #23 : Enzymes And Enzyme Inhibition

Enzymes are proteins that catalyze the biological reactions in the body. Every enzyme has a unique set of conditions in which it functions optimally. The function of an enzyme can be plotted on a graph, with the functionality of the enzyme on the y-axis, and the factor being manipulated on the x-axis.

What shape would you expect the graph for an enzyme to look like with temperature as the factor being manipulated?

Possible Answers:

A straight line with a positive slope

An exponential curve

A bell shaped curve

A straight line with a negative slope

Correct answer:

A bell shaped curve

Explanation:

Keep in mind that enzymes are proteins. They will increase in efficiency as temperature increases, but eventually too much heat will start to denature the protein. As a result, the graph will climb to maximum effeciency at a specific temperature. After that peak, it will decrease due to the denaturing of the enzyme.

Very low temperatures result in very low functionality. Mid-range temperatures result in maximum functionality. Very high temperatures result in very low functionality. As a result, the graph will be shaped like a bell-curve.

Example Question #24 : Enzymes And Enzyme Inhibition

Enzymes are proteins that catalyze the biological reactions in the body. Every enzyme has a unique set of conditions in which it functions optimally. The function of an enzyme can be plotted on a graph, with the functionality of the enzyme on the y-axis, and the factor being manipulated on the x-axis.

What will be the shape of a graph with enzyme reaction rate on the y-axis, and substrate concentration on the x-axis?

Possible Answers:

The graph will be exponentially increasing curve

The graph will climb quickly, then will start to even off before reaching a plateau

The graph will be a bell shaped curve

The graph will be a line with a positive slope

Correct answer:

The graph will climb quickly, then will start to even off before reaching a plateau

Explanation:

As substrate concentration is increased, the reaction rate will increase accordingly; however, let's think about the extreme case where there is an extremely large amount of substrate. Eventually, every binding site of every molecule of enzyme will be filled. Substrate molecules will have to wait in order to be catalyzed by the enzyme. When this happens, we say that the enzyme is saturated. At this point, the graph will begin to level off and look like a horizontal line.

In summary, the graph will rise quickly in the beginning, but will eventually level off as substrate concentration becomes excessive compared to the available enzyme in solution.

Example Question #61 : Proteins

What would you predict would happen to pancreatic enzymes if they were introduced to the stomach?

Possible Answers:

Their function would decrease due to increased pH

Their function would increase due to decreased proton concentration

Their function would decrease due to decreased pH

Their function would increase due to decreased pH

Correct answer:

Their function would decrease due to decreased pH

Explanation:

The efficiency of an enzyme is dependent on the pH (as well as other features) of the environment in which it acts. The pancreatic digestive enzymes are typically secreted into the small intestine, which has a pH of about 6. As a result, the acidic pH of the stomach (about 2) would significantly reduce the efficiency of the pancreatic enzymes.

Remember that, though the stomach contents is highly acidic, it is neutralized in the duodenum before continuing through the small intestine, thus allowing these enzymes to function.

Example Question #26 : Enzymes And Enzyme Inhibition

Which term is used to refer to an inactive enzyme precursor?

Possible Answers:

Holoenzyme

Inhibitor

Null enzyme

Apoenzyme

Zymogen

Correct answer:

Zymogen

Explanation:

Zymogen is the correct term for the inactive precursor of an enzyme. Zymogens are cleaved by other enzymes in order to become active. The zymogen form can help prevent improper action of the enzyme in different regions of the body. For example, trypsinogen is a zymogen released from the pancreas. It is transported to the small intestine before become active trypsin to prevent the trypsin from accidentally digesting and damaging the pancreatic cells.

Apoenzymes refer to enzymes without cofactors, while holoenzymes are enzymes bound to their cofactors. Inhibitors bind to enzymes to block their activity.

Example Question #27 : Enzymes And Enzyme Inhibition

Which of the following changes could lead to loss of enzymatic function?

Possible Answers:

Change in overall enthalpy of the reaction

Decrease in activation energy of the reaction

Change in overall free energy of the reaction

Increase in pH of the reaction

Increase in enzyme concentration

Correct answer:

Increase in pH of the reaction

Explanation:

Enzymes are pH and temperature sensitive., and only function in optimal ranges of these conditions. Certain enzymes will only function in acidic environments, while others require basic conditions.

The overall free energy and enthalpy of the reaction, activation energy, and enzyme concentration do not have a bearing on enzymatic activity.

Example Question #28 : Enzymes And Enzyme Inhibition

Duchenne Muscular Dystrophy is an X-linked recessive genetic disorder, resulting in the loss of the dystrophin protein. In healthy muscle, dystrophin localizes to the sarcolemma and helps anchor the muscle fiber to the basal lamina. The loss of this protein results in progressive muscle weakness, and eventually death.

In the muscle fibers, the effects of the disease can be exacerbated by auto-immune interference. Weakness of the sarcolemma leads to damage and tears in the membrane. The body’s immune system recognizes the damage and attempts to repair it; however, since the damage exists as a chronic condition, leukocytes begin to present the damaged protein fragments as antigens, stimulating a targeted attack on the damaged parts of the muscle fiber. The attack causes inflammation, fibrosis, and necrosis, further weakening the muscle.

Studies have shown that despite the severe pathology of the muscle fibers, the innervation of the muscle is unaffected.

Which of the following would best describe the dystrophin protein?

Possible Answers:

Ion channel

Chemical receptor

Transmembrane protein

Fibrous protein

Signaling protein

Correct answer:

Transmembrane protein

Explanation:

The passage tells us that "dystrophin localizes to the sarcolemma," so we know it is located at the membrane of the muscle fiber. We also know that its role is to structurally link the muscle fiber and the basal lamina. We can eliminate the choices for ion channel, signaling protein, and chemical receptor based on what we know about dystrophin's function. We are left with either fibrous protein or transmembrane protein. Though fibrous proteins also have structural roles, transmembrane protein is the best choice because we know that dystrophin is linking the muscle fiber to another structure, meaning that it must span the membrane.

Example Question #29 : Enzymes And Enzyme Inhibition

Which of the following is FALSE concerning enzymes?

Possible Answers:

Enzymes increase both the forward rate and reverse rate of a reaction

Substrates must bind the enzyme's active site in order to initiate its effects

Enzymes reduce reaction activation energy

Enzymes are not destroyed in a reaction and can be used in the same reaction countless times

Enzymes increase the amount of product created in a reaction

Correct answer:

Enzymes increase the amount of product created in a reaction

Explanation:

Enzymes will increase the rate of a chemical reaction, but will not alter the equilibrium of a reaction. As a result, the amount of product is not affected by enzymes. The same amount of product will be made; it will just be made at a faster rate.

Example Question #21 : Enzymes And Enzyme Inhibition

The end product of an enzymatic reaction inhibits formation of product in an earlier step. This type of enzymatic regulation is known as __________.

Possible Answers:

feedback inhibition

negative regulation

enzyme-substrate complex

allosteric regulation

metabolic pathway loop

Correct answer:

feedback inhibition

Explanation:

Feedback inhibition is a type of regulation in which an enzyme product blocks an earlier part of a metabolic reaction. This allows cells to regulate resources by signaling when enough product is made.

Learning Tools by Varsity Tutors