Question 1

Which of the following statements best describes the primary structure of a protein?

Accepted Answer

It is the sequence of amino acids in the protein, read from the N-terminus to the C-terminus.

Question 2

A peptide bond is formed between which two functional groups of adjacent amino acids?

Accepted Answer

The carboxyl group of one amino acid and the amino group of the next amino acid.

Question 3

Which property of the peptide bond restricts rotation and leads to the formation of the amide plane?

Accepted Answer

Partial double-bond character due to resonance.

Question 4

If a protein has a pI (isoelectric point) of 5.5, what will be its net charge at pH 8.0?

Accepted Answer

Net negative charge

Question 5

Which of the following best explains why the theoretical diversity of polypeptide sequences is not realized in living cells?

Accepted Answer

Protein synthesis is limited by efficiency and the ability of polypeptides to fold into functional structures.

Question 6

During protein purification, which method would you use to separate proteins based on their size?

Accepted Answer

Gel filtration chromatography

Question 7

A researcher wants to purify a protein that binds specifically to a small molecule ligand. Which purification method should be used?

Accepted Answer

Affinity chromatography

Question 8

Which of the following statements about SDS-PAGE is correct?

Accepted Answer

Proteins are separated based on their molecular mass, with smaller proteins migrating further.

Question 9

What is the main reason for using a reducing agent such as DTT or β-mercaptoethanol during protein sequencing?

Accepted Answer

To cleave disulfide bonds and prevent their reformation.

Question 10

Which enzyme would you use to cleave a polypeptide specifically at the C-terminal side of lysine and arginine residues?

Accepted Answer

Trypsin

Question 11

A protein sample is placed in a dialysis bag and immersed in a beaker of buffer. What is the main purpose of this procedure?

Accepted Answer

To exchange buffer and remove small solutes such as salts.

Question 12

Which of the following best describes the outcome of cyanogen bromide (CNBr) treatment of a polypeptide?

Accepted Answer

Cleavage at methionine residues, producing peptides with C-terminal homoserine lactone.

Question 13

Why is it necessary to use more than one fragmentation method when sequencing a long polypeptide?

Accepted Answer

To generate overlapping fragments that allow reconstruction of the full sequence.

Question 14

Which of the following is a key advantage of mass spectrometry over Edman degradation for protein sequencing?

Accepted Answer

It can rapidly identify peptide sequences from small sample amounts.

Question 15

A protein is least soluble at its isoelectric point (pI) because:

Accepted Answer

It has no net charge, reducing electrostatic repulsion and promoting aggregation.

Question 16

Which of the following best describes the repeating sequence in the peptide backbone?

Accepted Answer

$$-N-C_\alpha-CO-$$

Question 17

In ion exchange chromatography, how are proteins typically eluted from the column?

Accepted Answer

By increasing the salt concentration in the elution buffer.

Question 18

Which of the following is a correct statement about the directionality of protein synthesis and sequence notation?

Accepted Answer

Proteins are synthesized and written from the N-terminus to the C-terminus.

Question 19

Which of the following is NOT a property that can be used to separate proteins during purification?

Accepted Answer

Genetic code

Question 20

A sample contains a mixture of proteins. Which method would you use to separate proteins based on their specific binding to a known ligand?

Accepted Answer

Affinity chromatography

Proteins: Primary Structure, Purification, and Sequencing

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