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

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Example Question #1 : Protein Regulation

Which of the following does not require a post-translational modification of a protein to be properly targeted to its destination?

I. Nuclear localization

II. Targeting proteins to the lysosome

III. Targeting proteins to the proteasome

IV. Nuclear export

Possible Answers:

I, II, and IV

II and III

I only

I and IV

Correct answer:

I and IV

Explanation:

Nuclear import and export do not require post-translational modifications. The nuclear localization sequence or the nuclear export sequence is contained within the amino acid sequence itself (primary structure), and does not require and special modification.

For targeting to the proteasome, proteins must be ubiquinated. To target a protein to the lysosome the addition of a mannose-6-phosphate is commonly made. 

Example Question #1 : Help With Protein Transport

Which of the following organelles would be least important in a cell that creates and secretes proteins?

Possible Answers:

Ribosomes

Smooth endoplasmic reticulum

Golgi apparatus

Rough endoplasmic reticulum

Correct answer:

Smooth endoplasmic reticulum

Explanation:

When considering a protein creating/secreting cell, the organelles that are responsible for packaging and secreting the proteins will be most important. Starting at the beginning, proteins will be translated using the ribosomes found on the rough endoplasmic reticulum. Those will then be sent to the golgi apparatus for proper packaging. Those packages will then be released from the cell. The smooth endoplasmic reticulum will not be a vital organelle for protein creation and secretion.

Example Question #1 : Help With Protein Modification

Which of the following is a common post-translational modification used to target proteins to the lysosome?

Possible Answers:

Myristoylation

Acetylation of lysine residues

Ubiquination

Addition of a mannose-6-phosphate

Correct answer:

Addition of a mannose-6-phosphate

Explanation:

Mannose-6-phosphate is a post-translational modification found on proteins important to the functionality of the lysosome (such as acid hydrolases). Ubiquination is a signal for proteins to be brought to the proteosome and degraded. Myristoylation involves the addition of a fatty acid chain, and is often seen in proteins targeted to the plasma membrane. Acetylation is a common modification found on histones that can help make genes transcriptionally active. 

Example Question #1 : Help With Protein Modification

An isomerase __________.

Possible Answers:

catalyzes the rearrangement of bonds in a single molecule

catalyzes a hydrolytic cleavage reaction

catalyzes a polymerization reaction

catalyzes the addition of a phosphate group

Correct answer:

catalyzes the rearrangement of bonds in a single molecule

Explanation:

An isomerase is an enzyme that catalyzes the rearrangement of bonds in a single molecule. For example glucose-6-phosphate isomerase catalyzes the conversion of glucose-6-phosphate into fructose-6-phosphate during glycolysis.

A hydrolase catalyzes a hydrolytic cleavage reaction, a kinase catalyzes the addition of a phosphate group, and a polymerase catalyzes polymerization reactions.

Example Question #2 : Help With Protein Modification

Which of the following is a protein modification that can initiate the degradation of the modified protein? 

Possible Answers:

Glycosylation

Isoprenylation

Ubiquitination

Palmitoylation

Myristoylation

Correct answer:

Ubiquitination

Explanation:

The correct answer is ubiquitination. Ubiquitin is added to the substrate protein to target the protein for degradation by the proteasome, serving as an efficient mechansim to control cellular protein levels. Myristoylation, palmitoylation, isoprenylation, and glycosylation are all post-translational protein modifications that involve the addition of a 14-carbon saturated acid, a 16-carbon saturated acid, an isoprenoid group, and a glycosyl group, respectively. These modifications have diverse functions, however, do not initiate the degradation of the protein.  

Example Question #1 : Protein Regulation

Lipidation is a post-translational modification to a protein that often targets that protein to the plasma membrane. Knowing that lipidation involves covalent bonding of a fatty acid group to a protein, which of the following molecules would be most likely to be attached to a protein for anchorage to a membrane?

Possible Answers:

Palmitate

Tyrosine

Histidine

Acetyl 

Phosphate

Correct answer:

Palmitate

Explanation:

While each of these molecules could potentially be bound to a protein as a post-translational modification, the only one listed that is a fatty acid is palmitate. Thus, this is the correct answer. 

Example Question #1 : Protein Regulation

In order for kinases to modify their substrates, what small molecule is needed for this reversible post-translational modification? 

Possible Answers:

Guanine nucleotide exchange factor

Nicotinamide adenine dinucleotide phosphate

Adenosine triphosphate 

Flavin adenine dinucleotide 

Nicotinamide adenine dinucleotide 

Correct answer:

Adenosine triphosphate 

Explanation:

The correct answer is adenosine triphosphate (ATP). In order to phosphorylate a substrate, kinases catalyze the hydrolysis of ATP to adenosine diphosphate (ADP) and inorganic phosphate. This released phosphate by the hydrolysis reaction is covalently added to an amino acid residue on the substrate. Nicotinamide adenine dinucleotide phosphate, flavin adenine dinucleotide, and nicotinamide adenine dinucleotide are proton carriers. Guanine nucleotide exchange factor aids in exchanging guanine diphosphate for guanine triphosphate in a substrate. 

Example Question #1 : Help With Protein Modification

Ubiquitination of proteins is a form of post-translational modification on proteins. Which of the following cellular processes is protein ubiquitination not part of? 

Possible Answers:

Protein degredation

Protein recruitment to substrates

Immune response

Apoptosis 

All of the answers are cellular processes in which ubiquitination is involved

Correct answer:

All of the answers are cellular processes in which ubiquitination is involved

Explanation:

The correct answer is all of the answers are cellular processes in which ubiquitination is involved. Post-translational ubiquitination of proteins initiates many cellular processes by altering protein activity and the proteins that interact with the ubiquitinated protein. 

Example Question #1 : Help With Protein Modification

What type of enzyme adds a phosphate group to a protein?

Possible Answers:

Dehydrogenase

Hydrolase

Kinase

Catalase

Phosphatase

Correct answer:

Kinase

Explanation:

A kinase is an enzyme that adds a phosphate group. Do not get this confused with a phosphatase. A phosphatase is an enzyme that removes a phosphate group. The other enzymes listed do not deal with the addition or removal of a phosphate group from a protein.

Example Question #1 : Protein Regulation

Which of the following are means of controlling protein concentrations?

Possible Answers:

Gene silencing

Polyubiquination 

All of these are means of controlling protein concentration

RNAi

Correct answer:

All of these are means of controlling protein concentration

Explanation:

All of the given answers are ways that a cell may regulate protein concentrations. Polyubiquination is a signal for the protein to be degraded by a proteasome. Gene silencing will prevent transcription, which will lower the amount of mRNA template that can be translated into a protein. RNAi will degrade specific mRNAs or prevent the translation of specific mRNAs into proteins.

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