GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Protein Modification

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 Modification

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

Possible Answers:

Acetylation of lysine residues

Myristoylation

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 #2 : Help With Protein Modification

An isomerase __________.

Possible Answers:

catalyzes a hydrolytic cleavage reaction

catalyzes the addition of a phosphate group

catalyzes the rearrangement of bonds in a single molecule

catalyzes a polymerization reaction

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 #3 : Help With Protein Modification

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

Possible Answers:

Myristoylation

Palmitoylation

Glycosylation

Ubiquitination

Isoprenylation

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 #4 : Help With Protein Modification

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:

Histidine

Tyrosine

Palmitate

Phosphate

Acetyl 

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 #5 : Help With Protein Modification

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

Possible Answers:

Nicotinamide adenine dinucleotide 

Nicotinamide adenine dinucleotide phosphate

Adenosine triphosphate 

Guanine nucleotide exchange factor

Flavin 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 #6 : 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 #7 : Help With Protein Modification

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

Possible Answers:

Phosphatase

Catalase

Hydrolase

Kinase

Dehydrogenase

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.

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

1 Diagnostic Test 201 Practice Tests Question of the Day Flashcards Learn by Concept
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