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Test: Proteins - MCAT MCQ


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10 Questions MCQ Test Organic Chemistry for MCAT - Test: Proteins

Test: Proteins for MCAT 2024 is part of Organic Chemistry for MCAT preparation. The Test: Proteins questions and answers have been prepared according to the MCAT exam syllabus.The Test: Proteins MCQs are made for MCAT 2024 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Proteins below.
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Test: Proteins - Question 1
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A new drug is developed which selectively cleaves covalent bonds between two sulfur atoms of non-adjacent amino acids in a polypeptide chain. Which level of protein structure in affected molecules would be most directly affected by the drug?

Detailed Solution for Test: Proteins - Question 1
  • Primary structure is defined by the peptide bonds between amino acids. These bonds occur between the amino and carboxy termini of two respective amino acids.
  • Secondary structure is largely defined by characteristic folding patterns such as the alpha helix and B-sheet. These are stabilized by hydrogen bonds between main chain carbonyl oxygens and amide hydrogens.
  • Tertiary structure is the 3D shape of the amino acid, which is stabilized by various interactions and forces within the folded polypeptide chain. Disulfide bonds are unique to the tertiary structure, which provide a strong, stable bond between the sulfur atoms of two cysteines in a protein.
  • This drug would mostly affect the tertiary structure of proteins.
Test: Proteins - Question 2
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A group of researchers isolate ‘Protein X’ from the wall of a human stomach with the intent of learning how to synthesize stomach tissue in the lab. Subsequently, they determine the exact sequence of amino acids of the protein in its unfolded state, and create a functional mRNA template to translate Protein X in vitro. They manage to translate an exact copy of the polypeptide chain in the lab, but then realize that it takes several days for the protein to fold into its final tertiary structure. In vivo, they observe that several thousand copies of Protein X are folded from polypeptide chains every minute. What is NOT a plausible explanation for this difference in folding times?

Detailed Solution for Test: Proteins - Question 2
  • Some proteins may not fold correctly in vitro if folding is performed under noncellular conditions, or in the absence of enzymes which facilitate folding.
  • Proteins are known to denature, or ‘unfold’, under certain conditions. They will not fold in conditions where the temperature or pH is either too high or too low.
  • In this question, the polypeptide chains have already been successfully translated. tRNA would no longer be needed.
  • Lack of a tRNA molecule is not a plausible explanation.
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Test: Proteins - Question 3
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Albumin is a large protein which circulates freely in human plasma. Another type of protein, called aquaporins, act as a channel for water to enter and exit a cell, and are located within the lipid bilayer of a cell’s plasma membrane. Based on their physiological locations, how would the tertiary structure of these two proteins mostly likely compare?

Detailed Solution for Test: Proteins - Question 3
  • Water is a polar solvent, well known for interacting with polar molecules. The polarity of a given molecule helps determine its solubility in water.
  • Molecules which are water insoluble, or those which cannot form hydrogen bonds, tend to cluster together tightly to minimize their exposure to water. These molecules are typically hydrophobic.
  • Single molecules within the aqueous human plasma are water soluble, so molecules within it tend to be water soluble. Thus, albumin must have a more polar outer layer. Hint 4: Aquaporins allow water to flow through them by encouraging transient hydrogen bonds to form in the lumen of the channel.
  • Aquaporins allow water to flow through them by encouraging transient hydrogen bonds to form in the lumen of the channel.
  • Aquaporins are also embedded in the very hydrophobic lipid bilayer of the plasma membrane. Therefore, Aquaporins must have a hydrophobic, nonpolar outer layer, but a polar core.
  • Albumin has a more nonpolar core with a polar outer layer, while aquaporins have a more polar core with a nonpolar outer layer.
Test: Proteins - Question 4
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What relative change in protein concentrations would most be the most direct result of a disease which halts the function of the rough endoplasmic reticulum?
Detailed Solution for Test: Proteins - Question 4
  • mRNA is translated to form polypeptide chains on ribosomes in the cell.
  • Ribosomes exist as two forms-- either free floating in the cytoplasm, or bound to the rough ER.
  • Proteins translated freely in the cytoplasm tend to remain within the cell as cytoplasmic proteins.
  • Proteins translated on the rough ER are further modified as they enter the ER, bud from it through a vesicle, and move progressively through the Golgi to the plasma membrane.
  • Ribosomes on the RER synthesize membrane and secretory proteins.
  • The most direct result of a disease which halts the function of the RER would be decreased concentrations of extracellular proteins.
Test: Proteins - Question 5
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The major protein component of human hair is α-keratin. Hair ‘straighteners’ are commonly used tools which use heat to iron hair into temporarily lying flat and straight. What is the best biological explanation for this phenomenon?
Detailed Solution for Test: Proteins - Question 5
  • Disulfide bonds are covalent bonds within proteins. Breaking these bonds requires the use of a chemical agent.
  • Ionic bonds are seen between ions of opposite charge. These occur in salts or transient associations of charged molecules in solution, and would dissociate in aqueous conditions. Our hair changes shape, but does not separate or dissolve when we shower.
  • When using a straightener on hair, the heat causes water to evaporate. In the absence of water, polar groups on amino acids are freed from hydrogen bonds with the water molecules.
  • These polar groups are free to be rearranged to form new bonds. The iron presses the proteins into a new configuration, which is disrupted by water or humidity.
  • Hair straighteners disrupt hydrogen bonds in α-keratin.
Test: Proteins - Question 6
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Creutzfeldt-Jakob Disease (CJD) is a degenerative neurological disorder. Biopsy of the the brains of affected individuals reveals aggregations of a “prion protein”―a protein which is capable of folding into multiple, structurally distinct forms. In CJD, two versions of the offending protein have been identified. The normal form is predominantly composed of alpha helix, while the variant has significant beta structure. The beta structure presumably has more solvent accessible hydrophobic groups, lacking a polar outer layer. Knowing only this, how might you differentiate between the normal and variant conformations given a sample of each in the lab?
Detailed Solution for Test: Proteins - Question 6
  • The first key fact to note is that with a prion protein, even though the folded structures differ, the polypeptide itself is identical.
  • Therefore, sequencing the polypeptides, determining the mass of either, or NMR analysis would show identical results.
  • One protein has a more polar surface, therefore it would be water soluble. The variant form is nonpolar, has a more nonpolar surface. Because of this, this form tends to form aggregations in the brain which destroy tissue.
  • Attempt to dissolve both samples in water, knowing that if it dissolves it is the normal protein.
Test: Proteins - Question 7
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A certain genetic disorder prevents the production of signalling molecules which direct the intracellular trafficking of proteins. If the disease exclusively affects the signal which directs proteins to the proteasome, what primary effect would most likely be observed?
Detailed Solution for Test: Proteins - Question 7
  • Proteins in the golgi are chemically modified which affects their activity and location. Intuitively, these proteins have already been synthesized, and a loss of this function would not directly affect their synthesis.
  • The proteasome is an organelle responsible for breaking down proteins for recycling or for metabolism of their components.
  • If proteins are not tagged to enter the proteasome, they will accumulate within the cell.
  • The most likely primary effect would be decreased degradation of proteins.
Test: Proteins - Question 8
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If the Golgi apparatus were damaged by an ingested toxin, what aspect of protein synthesis would be most affected?
Detailed Solution for Test: Proteins - Question 8
  • Proteins are translated on ribosomes in the cytoplasm or on ribosomes attached to the ER, not the golgi.
  • Alpha helix and Beta sheet formation occurs spontaneously due to intramolecular forces. Loss of the golgi would not affect this process.
  • Proteins in the golgi undergo extensive modifications including glycosylation.
  • If the golgi apparatus were damaged, protein glycosylation would be most affected.
Test: Proteins - Question 9
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Certain bacteria synthesize toxic proteins which are responsible for many of the problems they cause in humans. If you were to develop a drug designed to inhibit bacterial protein synthesis without interfering with normal human protein synthesis, what might be a logical target for the drug?
Detailed Solution for Test: Proteins - Question 9
  • Recall that there are several important differences in how bacteria and humans synthesize proteins.
  • Bacteria do not typically have a golgi apparatus.
  • A drug targeting DNA or RNA would ultimately have undesired effects on human nucleic acids.
  • Humans and bacteria use different types of ribosomes.
  • The most logical target for the drug would be the bacterial ribosome.
Test: Proteins - Question 10
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A nonsense mutation has occurred in the DNA segment which codes for “protein A”. After it has been transcribed and translated, what is the most likely product formed from this mutated region?
Detailed Solution for Test: Proteins - Question 10
  • A missense mutation is a point mutation which replaces a single base pair, causing the codon to code for a different amino acid. This would generate a protein of the same size +/- changes in function or shape.
  • A silent mutation is a point mutation which replaces a single base pair, but which does not change the amino acid for which the codon codes.
  • A nonsense mutation is a point mutation which replaces a single base pair, causing the codon to code as a “STOP” codon. This prematurely terminates translation.
  • A nonsense mutation would yield a small, truncated, nonfunctional “protein A”.
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