MCAT Biology : MCAT Biological Sciences
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Example Questions
Example Question #12 : Enzymes And Enzyme Inhibition
The functional properties of an enzyme are dependent on the pH of the body as well as temperature. Each protein has specific conditions at which it will function optimally. These conditions can help predict where a protein will be found in the body.
In what area of the cell would you expect to find an enzyme that functions best in acidic conditions?
The mitochondria
The nucleus
The plasma membrane
The lysosome
The lysosome
Lysosomes are responsible for the degradation of macromolecules, and typically have an internal pH of 5. They contain acid hydrolases: enzymes that function optimally in an acidic environment.
Example Question #12 : 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:
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.
The enzyme PERK is a kinase. Which of the following is not true of all kinases?
All kinases are proteins
All kinases preserve thermodynamic properties of reactions
All kinases add phosphate groups
All kinases lower activation energies of reactions
All kinases modify translation factors
All kinases modify translation factors
Kinases are protein enzymes that add phosphate groups to targets. These targets can be diverse, however, and are not always translation factors.
Example Question #61 : Proteins
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:
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?
Phosphorylation of the unfolded proteins
Phosphorylation of transcription factors
Phosphorylation of eIF2
Dephosphorylation of eIF2
Dephosphorylation of transcription factors
Phosphorylation of eIF2
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?
Enzymes speed up the rate of reaction in DNA synthesis
Harsh, acidic conditions can completely denature an enzyme
An enzyme is completely converted to product during metabolism
The Keq of a reaction remains unchanged in the presence of an enzyme
An enzyme is completely converted to product during metabolism
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 #21 : 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?
A bell shaped curve
A straight line with a negative slope
An exponential curve
A straight line with a positive slope
A bell shaped curve
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 #22 : 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?
The graph will be a line with a positive slope
The graph will climb quickly, then will start to even off before reaching a plateau
The graph will be exponentially increasing curve
The graph will be a bell shaped curve
The graph will climb quickly, then will start to even off before reaching a plateau
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 #23 : Enzymes And Enzyme Inhibition
What would you predict would happen to pancreatic enzymes if they were introduced to the stomach?
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
Their function would decrease due to decreased pH
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 #21 : Enzymes And Enzyme Inhibition
Which term is used to refer to an inactive enzyme precursor?
Apoenzyme
Inhibitor
Holoenzyme
Zymogen
Null enzyme
Zymogen
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 #22 : Enzymes And Enzyme Inhibition
Which of the following changes could lead to loss of enzymatic function?
Change in overall free energy of the reaction
Change in overall enthalpy of the reaction
Decrease in activation energy of the reaction
Increase in enzyme concentration
Increase in pH of the reaction
Increase in pH of the reaction
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 #241 : Organic Chemistry, Biochemistry, And Metabolism
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?
Signaling protein
Chemical receptor
Transmembrane protein
Ion channel
Fibrous protein
Transmembrane protein
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.
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