20 Common Amino Acids

Question 1

Alanine

Answer 1

3-Letter Code: Ala
1-Letter Code: A
Classification: Nonpolar, aliphatic
Key Point: Has a simple methyl side chain

Question 2

Arginine

Answer 2

3-Letter Code: Arg
1-Letter Code: R
Classification: Basic (positively charged at physiological pH)
Key Point: Contains a guanidinium group in the side chain

Question 3

Asparagine

Answer 3

3-Letter Code: Asn
1-Letter Code: N
Classification: Polar, uncharged
Key Point: Amide-containing side chain

Question 4

Aspartic Acid (Aspartate)

Answer 4

3-Letter Code: Asp
1-Letter Code: D
Classification: Acidic (negatively charged at physiological pH)
Key Point: Carboxyl group in side chain

Question 5

Cysteine

Answer 5

3-Letter Code: Cys
1-Letter Code: C
Classification: Polar, uncharged (can form disulfide bonds)
Key Point: Thiol (–SH) group

Question 6

Glutamic Acid (Glutamate)

Answer 6

3-Letter Code: Glu
1-Letter Code: E
Classification: Acidic (negatively charged at physiological pH)
Key Point: Similar to aspartate but with one extra methylene (–CH₂–)

Question 7

Glutamine

Answer 7

3-Letter Code: Gln
1-Letter Code: Q
Classification: Polar, uncharged
Key Point: Amide form of glutamate side chain

Question 8

Glycine

Answer 8

3-Letter Code: Gly
1-Letter Code: G
Classification: Nonpolar, aliphatic (though often considered its own category due to minimal side chain)
Key Point: Smallest amino acid; side chain is just H

Question 9

Histidine

Answer 9

3-Letter Code: His
1-Letter Code: H
Classification: Basic (positively charged sometimes, pKa near physiological pH)
Key Point: Imidazole ring in side chain; important in enzyme active sites

Question 10

Isoleucine

Answer 10

3-Letter Code: Ile
1-Letter Code: I
Classification: Nonpolar, aliphatic, branched
Key Point: One of the branched-chain amino acids (with Val, Leu)

Question 11

Leucine

Answer 11

3-Letter Code: Leu
1-Letter Code: L
Classification: Nonpolar, aliphatic, branched
Key Point: Another branched-chain amino acid

Question 12

Lysine

Answer 12

3-Letter Code: Lys
1-Letter Code: K
Classification: Basic (positively charged at physiological pH)
Key Point: Terminal amino group in side chain

Question 13

Methionine

Answer 13

3-Letter Code: Met
1-Letter Code: M
Classification: Nonpolar, aliphatic
Key Point: Contains a thioether group; often the start codon in mRNA (AUG)

Question 14

Phenylalanine

Answer 14

3-Letter Code: Phe
1-Letter Code: F
Classification: Nonpolar, aromatic
Key Point: Benzyl side chain

Question 15

Proline

Answer 15

3-Letter Code: Pro
1-Letter Code: P
Classification: Nonpolar, aliphatic (cyclic)
Key Point: Unique ring structure linking side chain to amino group; often disrupts secondary structures (like α-helices)

Question 16

Serine

Answer 16

3-Letter Code: Ser
1-Letter Code: S
Classification: Polar, uncharged
Key Point: Hydroxyl group in side chain

Question 17

Threonine

Answer 17

3-Letter Code: Thr
1-Letter Code: T
Classification: Polar, uncharged
Key Point: Hydroxyl group; branched at the β-carbon

Question 18

Tryptophan

Answer 18

3-Letter Code: Trp
1-Letter Code: W
Classification: Nonpolar, aromatic
Key Point: Largest amino acid; indole ring in side chain

Question 19

Tyrosine

Answer 19

3-Letter Code: Tyr
1-Letter Code: Y
Classification: Polar, aromatic (hydroxyl group on phenyl ring)
Key Point: Can be phosphorylated on the hydroxyl group

Question 20

Valine

Answer 20

3-Letter Code: Val
1-Letter Code: V
Classification: Nonpolar, aliphatic, branched
Key Point: One of the branched-chain amino acids