Amino Acids - Basic 10

Question 1

Which carbon in an amino acid is chiral (except in glycine)?
a) Carboxyl carbon
b) Side chain carbon
c) Alpha carbon (Cα)
d) Amino group nitrogen

Answer 1

Answer: c) Alpha carbon (Cα)

Explanation: The alpha carbon in amino acids is bonded to four different groups and is therefore chiral in all amino acids except glycine, which has two hydrogens attached.

Question 2

What does the pI (isoelectric point) of an amino acid represent?
a) The pH at which the amino acid is fully protonated
b) The pH at which net charge is zero
c) The pH at which it donates protons
d) The pH where it is most hydrophobic

Answer 2

Answer: b) The pH at which net charge is zero

Explanation: The isoelectric point is the pH at which the amino acid has no net electrical charge (zwitterion form dominates).

Question 3

Which amino acid has a nonpolar, aliphatic side chain?
a) Glutamate
b) Serine
c) Tyrosine
d) Valine

Answer 3

Answer: d) Valine

Explanation: Valine has a branched, nonpolar aliphatic side chain, making it hydrophobic.

Question 4

Which amino acid contains a thiol group and can form disulfide bonds?
a) Methionine
b) Cysteine
c) Serine
d) Threonine

Answer 4

Answer: b) Cysteine

Explanation: Cysteine contains a thiol (–SH) group that can form covalent disulfide bridges during protein folding.

Question 5

What is the typical pKa of the amino group in an amino acid?
a) ~2
b) ~4
c) ~7
d) ~9

Answer 5

Answer: d) ~9

Explanation: The amino group generally has a pKa around 9, meaning it is protonated (–NH₃⁺) under physiological pH.

Question 6

At physiological pH (~7.4), what is the net charge on glutamic acid?
a) +1
b) 0
c) –1
d) –2

Answer 6

Answer: c) –1

Explanation: At pH 7.4, glutamic acid has a deprotonated (–) side chain and a protonated (+) amino group, resulting in a net charge of –1.

Question 7

Which interaction is most likely to occur between the side chains of aspartate and lysine at physiological pH?
a) Hydrophobic
b) Disulfide bond
c) Hydrogen bond
d) Ionic bond

Answer 7

Answer: d) Ionic bond

Explanation: Aspartate is negatively charged, and lysine is positively charged at physiological pH, allowing them to form a salt bridge (ionic interaction).

Question 8

How do you estimate the pI of an amino acid with no ionizable side chain?
a) Average the pKa of amino and carboxyl groups
b) Average the two lowest pKa values
c) Average the pKa of the side chain and amino group
d) Use only the carboxyl group pKa

Answer 8

Answer: a) Average the pKa of amino and carboxyl groups

Explanation: For amino acids without ionizable side chains, pI is calculated as the average of the amino and carboxyl group pKa values.

Question 9

Which amino acid is achiral?
a) Glycine
b) Alanine
c) Phenylalanine
d) Serine

Answer 9

Answer: a) Glycine

Explanation: Glycine is the only standard amino acid without a chiral center, as its side chain is a hydrogen atom.

Question 10

Which amino acid has a side chain that can hydrogen bond and also acts as a nucleophile in catalysis?
a) Lysine
b) Histidine
c) Serine
d) Proline

Answer 10

Answer: c) Serine

Explanation: Serine has a hydroxyl group that can both hydrogen bond and act as a nucleophile in enzyme active sites.