Question 1

Suppose a protein is treated with trypsin.Which of the following techniques could be used to determine its identity by peptide mass fingerprinting?

Accepted Answer

MALDI-TOF mass spectrometry is a commonly used technique for peptide mass fingerprinting as it can accurately determine the mass-to-charge ratio of peptides in a sample. This allows for identification of the protein through comparison with a known protein database. NMR is not typically used for peptide mass fingerprinting, HPLC is used for peptide separation but not mass determination, and gel electrophoresis separates proteins based on size and charge, but does not provide information on the specific peptide sequences present in the sample.

Question 2

Which of the following terms is used to characterize proteins destined for an extracellular location?

Accepted Answer

Glycoproteins are characterized by the addition of carbohydrates, such as sugars, to the protein structure. These modifications are important for a variety of cellular processes, including protein folding, transport, and signaling. Proteins destined for an extracellular location, such as those secreted by cells or found in the extracellular matrix, are frequently glycosylated. This modification can help to protect the protein from degradation, ensure proper folding, and facilitate interactions with other molecules in the extracellular environment.

Question 3

Which of the following best describes the use of the Edman degradation procedure?

Accepted Answer

The Edman degradation procedure is primarily used for determining the N-terminal amino acid sequence of a protein or peptide. It involves selectively removing and identifying one amino acid residue at a time from the N-terminus of a protein or peptide using a cyclic series of reactions.

Question 4

What is the overall net charge on the peptide Lys-Lys-Ser-Glu at pH 7.0?

Accepted Answer

At pH 7.0, Lysine (pKa = 10.8) and glutamic acid (pKa = 4.1) will be mostly in their positive and negative forms, respectively. Serine has a neutral R group. Thus, Lys-Lys will have a net charge of +2, Ser will have a net charge of 0, and Glu will have a net charge of -1. Summing up the charges, we get +2 + 0 - 1 = +1, which is the overall net charge of the peptide at pH 7.0. Therefore, the best choice is B.

Question 5

Suppose that the peptide Ala-Met-Lys-Ser is reacted with phenylisothiocyanate at pH 8.0,followed by mild acidification,which is the first cycle of Edman method? Which of the following would be released?

Accepted Answer

In the first cycle of the Edman method, only one amino acid is released, and it is the N-terminal amino acid, which reacts with phenylisothiocyanate to form a phenylthiocarbamoyl derivative. In this case, the N-terminal amino acid is Ala, so it will react with phenylisothiocyanate to form PTH-Ala, which will be released upon mild acidification. The remaining peptide is then subjected to a second cycle of Edman degradation to release the next N-terminal amino acid. Therefore, only PTH-Ala and the peptide Met-Lys-Ser will be released in the first cycle.

Question 6

Insulin is a polypeptide hormone that contains 2 short polypeptide chains linked by 2 interstrand disulfide bonds.Which of the following treatments would NOT be useful for determining its primary structure?

Accepted Answer

Reduction of the peptides with SDS would break the disulfide bonds that hold the short polypeptide chains together, making it impossible to determine the primary structure of insulin. Sequencing the peptides with Edman chemistry, separation of the peptides by chromatography techniques, and alkylation of the peptides with iodoacetate are all useful methods for determining insulin's primary structure.

Question 7

Which of the following molecules specifically hydrolyzes only those peptide bonds in which the carboxyl function is contributed by an arginine or a lysine residue?

Accepted Answer

Trypsin specifically hydrolyzes peptide bonds where the carboxyl group is contributed by an arginine or lysine residue, making it highly specific for these basic amino acids.

Question 8

Which of the following does NOT apply to the amino acid sequence of proteins?

Accepted Answer

The amino acid sequence of proteins is not constant for proteins with the same function from different organisms. While some amino acid sequences may be conserved across different species, there can be variations in the sequence due to evolutionary divergence.

Question 9

Various mutations can occur in the hemoglobin molecule resulting in the generation of very diverse mutants.What does the generation of this diversity indicate?

Accepted Answer

The generation of diverse mutants indicates that a specific amino acid changed in the sequence can drastically affect the function of the protein. Different mutations can cause different effects, such as altering the ability of hemoglobin to bind to oxygen or causing it to form abnormal structures. It does not necessarily mean that any amino acid change is relatively important or lethal, and the effects of mutations can vary widely.

Question 10

Which of the following treatments is NOT a useful method for dissociating a heteromultimeric protein during sequencing?

Accepted Answer

Treatment with 6 N HCl at 110°C for 24 hours is a very harsh method that can result in hydrolysis of the peptide bonds and destruction of the protein. It is not a useful method for dissociating a heteromultimeric protein during sequencing. The other listed methods, high salt concentrations (A), 8 M urea (C), and 6 M guanidinium chloride (D), are more commonly used and gentler methods that can effectively dissociate protein complexes.

Question 11

Which of the following characteristics about the peptide Val-Asp-Trp-Asn-Ser is correct?

Accepted Answer

The peptide contains tryptophan, which absorbs strongly at 280 nm.

Answer: B 
 The peptide contains the amino acid sequence Asp-Trp, which is a known cleavage site for clostripain. Therefore, reaction with clostripain would yield two peptides.

Answer: C 
 In the second round of Edman degradation, the N-terminal residue would be Asn, which would be converted to PTH-Asn upon reaction with PITC.

Answer: D 
 The question does not provide any information to suggest that the peptide is resistant to trypsin, so this statement is incorrect.

Question 12

Which of the following is a characteristic of homologous proteins from different organisms?

Accepted Answer

Homologous proteins from different organisms have the same or similar functions and structures, as they evolved from a common ancestor. Therefore, they often have similar lengths and primary sequences, despite some variations that may have accrued over time due to natural selection and genetic drift. Choice A is incorrect because homologous proteins often have significant sequence similarity with other proteins with similar functions. Choice C is incorrect because homologous proteins have significant sequence similarity, which is a key indicator of their homology. Choice D is incorrect because sequence similarity is directly proportional to the relatedness of the species from which homologous proteins were derived.

Question 13

When sequencing a protein,which of the following is an advantage of treating separate samples of the protein with 2 or more enzymes?

Accepted Answer

Treating separate samples of the protein with 2 or more enzymes generates fragments with different sizes and overlapping sequences. This allows for the generation of a longer contiguous sequence by piecing together the overlapping fragments. This process is known as shotgun sequencing and is commonly used in high-throughput sequencing methods.

Question 14

Which of the following tproteins do not perform any obvious chemical transformation,but control the ability of other proteins to carry out their physiological functions?

Accepted Answer

Regulatory proteins do not directly perform any chemical transformations, but rather control or regulate the activity of other proteins, allowing them to carry out their physiological functions. Transport, storage, and structural proteins are involved in transporting, storing, and providing structural support to cells and tissues, respectively.

Question 15

Which of the following proteases can be used to identify the C-terminal residue of a polypeptide?

Accepted Answer

Carboxypeptidase is a protease that cleaves the C-terminal amino acid of a polypeptide.

Question 16

Which of the following proteins has a significant degree of sequence similarity and tertiary structure with hen egg white lysozyme?

Accepted Answer

α-lactalbumin has a significant degree of sequence similarity and tertiary structure with hen egg white lysozyme. The other options do not have significant similarity in sequence or tertiary structure to lysozyme.

Question 17

Which of the following products are formed from the acid hydrolysis of a simple amide?

Accepted Answer

Acid hydrolysis of a simple amide results in the formation of an acid and an amine. This is because the amide functional group has an -NH2 group attached to a carbonyl group, which can be cleaved by acid to form the corresponding carboxylic acid and amine. Option A is incorrect because aldehydes are not formed from the hydrolysis of amides. Option C is incorrect because the ester functional group has an -OR group and not an -NH2 group, so it cannot be formed from the hydrolysis of an amide. Option D is incorrect because aldehydes cannot be formed from the hydrolysis of amides.

Question 18

Which of the following is determined by amino acid analysis of a protein?

Accepted Answer

The answer of Which of the following is determined by...

Question 19

Which of the following characteristics about the peptide Lys-Met-Cys-Phe-Ala-Trp is correct?

Accepted Answer

Trypsin is an enzyme that cleaves peptide chains after the amino acids lysine and arginine. Thus, in the peptide Lys-Met-Cys-Phe-Ala-Trp, tryptic digestion would result in a pentapeptide with Met on the amino end, in addition to an uncharacterized amino acid. This matches with option C. Options A, B, and D are incorrect because they describe properties that do not match with the given peptide sequence.

Question 20

Which of the following is now the most commonly used technique for finding the sequence of proteins?

Accepted Answer

The answer of Which of the following is now the...

Quiz 5: Proteins: Their Primary Structure and Biological Functions