Quiz 2

Question 1

List the amino acids with non-polar, aliphatic R groups

Answer 1

Question 2

List the amino acids with aromatic R groups

Answer 2

Question 3

What two amino acids absorb light around 280nm?

Answer 3

Tryptophan and Tyrosine

Question 4

List the amino acids with polar, uncharged R groups

Answer 4

Question 5

List the amino acids with positively charged R groups

Answer 5

The side-chain of basic amino acids can take up an extra proton to become positively charged. Histidine is only weakly basic at neutral pH (pH 7), as only some of the side-chains have taken up a proton (about 10%).

Question 6

List the amino acids with negatively charged side groups

Answer 6

The side-chain of acidic amino acids can give up a proton (H+) to become negatively charged.

Question 7

What is the distinction between peptides and proteins proper?

Answer 7

Amino acid polymers < 10,000 Da in molecular weight (MW) are considered peptides; those above 10,000 Da are proteins.

Question 8

Polymers composed of two amino acids are called

Answer 8

dipeptides

Question 9

Polymers composed of three amino acids are called?

Answer 9

tripeptides

Question 10

Polymers composed of more than three amino acids are called?

Answer 10

polypeptides

Question 11

How do amino acids polymerise?

Answer 11

Through condensation (loss of H₂O)

Question 12

What is the accepted pK_a for C-termini

Answer 12

2.2

Question 13

What is the accepted pK_a for N-termini

Answer 13

9.5

Question 14

Which amino acids act as the primary acids and bases?

Answer 14

Those with polar, charged side chains:

Asp pK_a ~3.7

Glu pK_a ~4.2

His pK_a ~6.0

Lys pK_a ~10.5

Arg pK_a ~12.5

Additionally, Cys (pK_a ~8.4) and Tyr (pK_a ~10.5) are both weakly acidic

Question 15

What effect does the local environment have on the pK_a of an amino acid?

Answer 15

Hydrophobic environments may favor non-charged residues (e.g. protonated carboxylic acid groups or unprotonated basic groups)

Hydrophilic conditions/electrostatic interactions may favor charged residues (e.g. unprotonated carboxylic acid groups or protonated basic groups)

Additionally, stabilisation by nearby residues may affect acidity and basicity