Question 1

Which amino acid is represented by the three-letter code 'His' and what is its side chain pKa value?

Accepted Answer

Histidine, pKa $$\approx 6.0$$

Question 2

Given the structure at the bottom of the page, which vitamin does it most closely resemble?

Accepted Answer

Vitamin K

Question 3

If the pH of a solution is 5.7, which amino acid from the list in the blue box would be closest to its isoelectric point at this pH?

Accepted Answer

Histidine

Question 4

What is the Henderson-Hasselbalch equation used to calculate the pH of a buffer solution?

Accepted Answer

$$pH = pK_a + \log_{10} \left( \frac{[A^-]}{[HA]} \right)$$

Question 5

A solution contains 0.05 M acetic acid and 0.05 M acetate. If the $$pK_a of $$acetic acid is 4.76, what is the pH of the solution?

Accepted Answer

$$pH = 4.76$$

Question 6

Which amino acid has a side chain that can act as a buffer near physiological pH?

Accepted Answer

Histidine

Question 7

If you titrate glycine with NaOH, which two $$pK_a$$ values will you observe?

Accepted Answer

$$pK_a \approx 2.3$$ and $$pK_a \approx 9.6$$

Question 8

A patient has a blood pH of 7.1. Which amino acid from the blue box would be most likely to accept a proton at this pH?

Accepted Answer

Lysine

Question 9

Which of the following best describes the role of buffers in biological systems?

Accepted Answer

Buffers resist changes in pH when small amounts of acid or base are added.

Question 10

If you mix 0.1 M of a weak acid (HA) with 0.1 M of its conjugate base (A-) and the $$pK_a is 5.0$$, what is the resulting pH?

Accepted Answer

$$pH = 5.0$$

Question 11

Which amino acid would be most likely to be found in the active site of an enzyme that catalyzes acid-base reactions near neutral pH?

Accepted Answer

Histidine

Question 12

A solution contains 0.05 M phosphate buffer at pH 7.4. If you add a small amount of HCl, what will happen to the pH?

Accepted Answer

The pH will decrease slightly but remain close to 7.4.

Question 13

Which equation would you use to calculate the isoelectric point ($$pI) of an $$amino acid with two ionizable groups?

Accepted Answer

$$pI = \frac{pK_{a1} + pK_{a2}}{2}$$

Question 14

Which amino acid from the blue box is aromatic and can absorb UV light at 280 nm?

Accepted Answer

Tyrosine

Question 15

If a protein contains a high proportion of glutamic acid residues, what property will it likely exhibit at physiological pH?

Accepted Answer

It will be negatively charged.

Question 16

Which of the following is the correct structure for the side chain of lysine?

Accepted Answer

$$-(CH_2$)_4-NH_2$

Question 17

A solution of histidine is titrated and the following $$pK_a$$ values are observed: 1.8, 6.0, and 9.2. Which group corresponds to the $$pK_a of 6.0$$?

Accepted Answer

Imidazole side chain

Question 18

Which amino acid is most likely to be phosphorylated in a signaling pathway?

Accepted Answer

Serine

Question 19

If the pH of a solution is below the $$pI of an $$amino acid, what is the net charge on the amino acid?

Accepted Answer

Positive

Question 20

Which amino acid from the blue box contains a sulfur atom in its side chain?

Accepted Answer

Methionine

Amino Acids: Properties, pKa Values, and Structure

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