GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Protein Regulation

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Help With Protein Degradation

Proteins are brought to a __________ for degradation after they undergo __________

Possible Answers:

lysosome . . . glycosylation

proteasome . . . glycosylation

lysosome . . . polyubiquination

proteasome . . . polyubiquination

Correct answer:

proteasome . . . polyubiquination

Explanation:

Polyubiquination is a modification results from the binding of small ubiquitin residues to a protein. Polyubiquination of a protein signals damage or problems with functionality, and triggers the mechanisms that result in protein degradation. The polyubiquinated protein is then brought to a proteasome (a complex of proteins) that will degrade the protein.

Glycosylation involves the attachment of a carbohydrate complex to a protein. The identity of the carbohydrate is essential for determining the functional outcome of glycosylation, but generally results in signaling and transport labels for the protein. Glycosylation is not by itself a signal to be brought to either a proteasome or a lysosome.

Example Question #300 : Gre Subject Test: Biochemistry, Cell, And Molecular Biology

Which of the following answers best describes the sequence of events carried out by enzymes E1, E2, and E3 in the ubiquitin-proteasome pathway?

Possible Answers:

E1 transfers ATP to target proteins, E2 processes target proteins, and E3 degrades proteins through ATP-dependent complexes. 

E1 activates autophagosomes which fuse with proteins marked for degradation. E2 adds ubiquitin molecules to proteins within the autophagosome, and E3 activates acids to degrade the marked proteins. 

E1 senses cellular stress and activates E2, which phosphorylates E3 to conjugate ubiquitin to proteins to target them for post-translational modifications like acetylation. 

E1 activates ubiquitin molecules, E2 carries activated ubiquitin molecules to E3, and E3 covalently attaches activated ubiquitin molecules to a protein to mark it for degradation by a protease complex.

E1, E2, and E3 are all redundant in their activity within the protein destruction pathway; each covalently attach ubiquitin to target proteins to mark them for degradation by the proteasome. 

Correct answer:

E1 activates ubiquitin molecules, E2 carries activated ubiquitin molecules to E3, and E3 covalently attaches activated ubiquitin molecules to a protein to mark it for degradation by a protease complex.

Explanation:

E1, E2, and E3 all have unique activities that progress step-wise to activate ubiquitin and then attach those ubiquitins to mark a protein for degradation. Their functions are not redundant, nor do they activate acids, autophagosomes or ATP complexes over the course of their pathways. 

Example Question #93 : Cell Biology

Ubiquitination of a protein is one way to mediate protein degradation, however, ubiquitination is only a signal. What is ultimately responsible for ubiquitin-mediated degradation of a protein?

Possible Answers:

None of these

Peroxisome

Proteasome

Hydrolysis

Lysosome

Correct answer:

Proteasome

Explanation:

The correct answer is proteasome. Ubiquitination of a protein signals for the proteasome to degrade it and recycle the ubiquitin. Alternatively, the lysosome does degrade proteins, however, this process is independent of ubiquitin. The peroxisome is responsible for the degradation of fatty acids, certain amino acids, and reactive oxygen species. Hydrolysis simply refers to a chemical mechanism that splits apart a compound by the addition of water, it does not however, describe an organelle or cellular compartment that is reponsible for protein degradation. 

Example Question #12 : Protein Regulation

The ubiquitin-mediated protein degradation process targets proteins to which cellular structure for degradation? 

Possible Answers:

Golgi

Endoplasmic reticulum

Vesicle 

Lysosome 

Proteasome

Correct answer:

Proteasome

Explanation:

The correct answer is proteasome. There are two general protein degradation processes: the first involving the lysosome and the second involving the proteasome. Lysosomal protein degradation is non-selective and occurs during cell starvation. Degradation through the proteasome is dependent on ubiquitination of the target protein, and as such, ensures protein-specific degradation. 

Example Question #2 : Help With Protein Degradation

How does ubiquitination of a protein facilitate its degradation? 

Possible Answers:

Recognition of ubiquitin by the proteasome

None of the other answers 

Promotes exocytosis of the protein

Promotes reprocessing through the Golgi apparatus

Recognition of ubiquitin by the lysosome

Correct answer:

Recognition of ubiquitin by the proteasome

Explanation:

The correct answer is recognition of ubiquitin by the proteasome. Ubiquitin-mediated protein degradation by the proteasome is a well characterized method of specific protein degradation. The protein targeted for degradation is phosphorylated, then ubiquitinated. The proteasome recognizes these distinct ubiquitin chains and degrades the protein. Protein degradation can also occur through the lysosome, but this is independent of ubiquitination and is less specific. The golgi complex is involved in protein folding and modification of recently translated amino acid chains. 

Example Question #3 : Help With Protein Degradation

What is the difference between proteolysis and ubiquitin-mediated protein degradation? 

Possible Answers:

Proteolysis occurs in the lysosome but ubiquitin-mediated protein degradation is in the proteasome

Proteolysis occurs in only in the nucleus, but ubiquitin-mediated protein degradation occurs only in the cytoplasm

These two processes are synonymous 

Proteolysis is the degradation of organelles whereas proteins are degraded through ubiquitin-dependent mechanisms

Correct answer:

Proteolysis occurs in the lysosome but ubiquitin-mediated protein degradation is in the proteasome

Explanation:

The correct answer is that proteolysis occurs in the lysosome but ubiquitin-mediated protein degradation is in the proteasome. Proteolysis-lysosomal degradation is non-selective and is activated upon cellular starvation. ubiquitin-mediated protein degradation is highly specific and functions to promote a wide range of cellular processes. 

Example Question #3 : Help With Protein Degradation

Which of the following additions to a protein will signal the cell to degrade it?

Possible Answers:

Ubiquitination

Hydroxylation

Glycosylation

Glycation

Correct answer:

Ubiquitination

Explanation:

Older proteins in our bodies need to be degraded once they become damaged or no longer necessary. One way that the cell tags these proteins is by adding a ubiquitin tag, which can then be recognized by a proteasome, leading to the proteins' deconstruction.

Example Question #103 : Cell Biology

Which of the following protein coats would most likely be seen on a vesicle directed towards the plasma membrane?

Possible Answers:

COPI

None of the answers

COPII

Clathrin

Correct answer:

Clathrin

Explanation:

Clathrin coats are often seen trafficking vesicles from the Golgi apparatus to the plasma membrane. Clathrin protein is used to facilitate membrane invagination and vesicle formation, as well as direct vesicle release.

COPI coats are seen in vesicles headed from the Golgi apparatus back to the endoplasmic reticulum. COPII coats are seen in vesicles headed towards the Golgi apparatus from the endoplasmic reticulum.

Example Question #104 : Cell Biology

Which of the following portions of the cytoskeleton are used extensively for vesicular transport?

I. Actin

II. Intermediate filaments

III. Microtubules

Possible Answers:

I, II, and III

I and III

I and II

III only

Correct answer:

I and III

Explanation:

Actin and microtubules have similar chemical properties. They maintain a nucleotide gradient (ADP/ATP for actin and GDP/GTP for microtubules) across their structures and have two distinct polarized ends. Intermediate filaments do not have either of these characteristics. For that reason, motor proteins associate with actin and microtubules as opposed to intermediate filaments. The polarity of the microtubules and actin allow the motor proteins to become oriented, transporting cargo in a particular direction along the structure. These motor proteins (such as myosin, dynein, and kinesin) are essential for vesicular transport.

Example Question #34 : Cellular Processes

Which of the following proteins/structures are involved in the mechanism of vesicular transport?

I. Actin microfilament cytoskeleton

II. Intermediate filament cytoskeleton

III. Kinesin

IV. Microtubule cytoskeleton

Possible Answers:

I and II

I, III, and IV

III and IV

II only

Correct answer:

I, III, and IV

Explanation:

Both the actin microfilament cytoskeleton and the microtubule cytoskeleton serve important functions in vesicular transport. They serve as the structures upon which motor proteins move, essentially providing a directional tract for vesicular transport. Motor proteins, such as kinesin, associate with vesicles and bring them from one area of the cell to the other along the directional filaments.

The intermediate filament cytoskeleton lacks the polarity displayed by actin microfilaments and microtubules, making it not very useful for vesicular transport.

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