Question 1

Which of the following statements is TRUE regarding the titration curve of glycine shown in the image, specifically at the point where $$pH = pK_1$?

Accepted Answer

The concentration of $$\mathrm{H_3N^+-CH_2$-COOH}$$ equals that of $$\mathrm{H_3N^+-CH_2$-COO^-}.$$

Question 2

A peptide has the sequence: Glu-His-Trp-Ser-Gln-Leu-Arg-Pro-Gly. Using the pKa values for side chains and terminal groups, which of the following groups is expected to be positively charged at pH 7?

Accepted Answer

The side chain of Arg

Question 3

Which of the following best describes the isoelectric point ($$pI) of $$glycine?

Accepted Answer

It is the pH at which the net charge of glycine is zero.

Question 4

Given the following amino acid structure: $$\mathrm{COO^-}$$, $$\mathrm{CH}$$, $$\mathrm{CH_2CH(CH_3$)_2}$$, $$\mathrm{NH_3$^+}$$, how many chiral centers does isoleucine have?

Accepted Answer

2

Question 5

A protein sample is purified through several steps. After gel filtration chromatography, the total activity is 800 units and the total protein is 200 mg. What is the specific activity after this step?

Accepted Answer

$$4\ \mathrm{units/mg}$$

Question 6

Which purification procedure from the table is LEAST effective in removing contaminating proteins, based on the fold purification value?

Accepted Answer

Crude extract

Question 7

A mixture of three polypeptides is separated by size-exclusion chromatography. Which peptide will elute first?

Accepted Answer

The largest peptide

Question 8

During electrophoresis at pH 8.6, which amino acid will migrate the fastest toward the anode?

Accepted Answer

Ala-Ala-Asp

Question 9

Which of the following statements about the peptide bond is CORRECT?

Accepted Answer

It is formed by a condensation reaction between the carboxyl group of one amino acid and the amino group of another.

Question 10

A protein has a molecular weight of 40,000 Da and consists of 400 amino acids. What is the average molecular weight of an amino acid in this protein?

Accepted Answer

Question 11

Which of the following amino acids is most likely to be found on the surface of a protein that is soluble in water?

Accepted Answer

Glutamate

Question 12

Which of the following best explains why histones are rich in lysine and arginine?

Accepted Answer

These residues are positively charged and interact with the negatively charged phosphate backbone of DNA.

Question 13

Which of the following amino acids has the highest pI value?

Accepted Answer

Lysine

Question 14

Which of the following is TRUE about the structure of alanine at its isoelectric point?

Accepted Answer

It exists predominantly as the zwitterion.

Question 15

Which of the following procedures would be most effective for separating a protein based on its net charge at pH 7?

Accepted Answer

Ion-exchange chromatography

Question 16

A peptide is treated with trypsin. Which of the following amino acids will be cleaved at the carboxyl side?

Accepted Answer

Lysine

Question 17

Which of the following is the correct one-letter code for the amino acid isoleucine?

Accepted Answer

I

Question 18

Which of the following amino acids contains a sulfur atom in its side chain?

Accepted Answer

Methionine

Question 19

Which of the following statements about the pKa values of amino acids is CORRECT?

Accepted Answer

The pKa of the amino group is typically around 9-10.

Question 20

A protein is analyzed by SDS-PAGE and found to have a single band at 60 kDa. When analyzed by gel filtration, it elutes at a volume corresponding to 120 kDa. What does this suggest about the protein's structure?

Accepted Answer

It is a dimer of 60 kDa subunits.

Amino Acids and Protein Purification: Study Notes for Biochemistry

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