MCAT Biology : Proteins
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Example Questions
Example Question #61 : Proteins
A supercoiled helix is described by which level of peptide structure?
Quartenary
Secondary
There is not enough information to tell
Tertiary
Primary
Secondary
Secondary peptide structure refers to alpha-helices and beta-sheets, which are formed by hydrogen bonding. A supercoiled helix is due to the secondary structure of a peptide. Primary structure is the sequence of amino acids, tertiary structure is the three-dimensional arrangement of the protein, and quatenary structure arises when more than one peptide subunit interacts.
Example Question #62 : Proteins
Disulfide linkages are connections made between polypeptide chains to increase the cohesion of a protein. These bonds fall into the category of __________.
hydrogen bonds
metallic bonds
covalent bonds
ionic bonds
covalent bonds
Disulfide bonds are covalent attachments created between two sulfur atoms. Cysteine is usually the amino acid that creates these connections.
Example Question #63 : Proteins
Pepsin is an enzyme found within the stomach. As a physiologist you are setting up an experiment to study the properties of pepsin. You place pepsin enzymes into a solution and notice that the pH of the solution is 4.
Which of the following would you add in order to maximize the enzyme's ability to function normally?
Equal portions of HCl and NaOH
Neither HCl, nor NaOH would have an effect on pepsin's ability to operate normally
NaOH
HCl
HCl
Pepsin is used to break down protein in the stomach by hydrolyzing some peptide bonds. Pepsin can operate best at a pH of 2 (same pH as the stomach). Adding HCl to the solution will bring the solution's pH closer to this optimal pH.
Example Question #61 : Proteins
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 lysosome
The plasma membrane
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 #231 : Organic Chemistry, Biochemistry, And Metabolism
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 eIF2
Dephosphorylation of transcription factors
Phosphorylation of transcription factors
Phosphorylation of the unfolded proteins
Dephosphorylation of eIF2
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 #67 : Proteins
Which of the following statements about enzymes is false?
Enzymes speed up the rate of reaction in DNA synthesis
The Keq of a reaction remains unchanged in the presence of an enzyme
Harsh, acidic conditions can completely denature an enzyme
An enzyme is completely converted to product during metabolism
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 #61 : Proteins
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 straight line with a positive slope
An exponential curve
A bell shaped curve
A straight line with a negative 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 #62 : Proteins
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 climb quickly, then will start to even off before reaching a plateau
The graph will be exponentially increasing curve
The graph will be a line with a positive slope
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 #70 : Proteins
What would you predict would happen to pancreatic enzymes if they were introduced to the stomach?
Their function would increase due to decreased pH
Their function would decrease due to decreased pH
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
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.
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