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Biochemistry · Amino acids and proteins

Amino acid quiz: structures, one-letter codes, and side-chain classes

All 20 amino acids share the same backbone and differ only in their side chains. The big idea: if you can describe a side chain in words, you can classify it, and the classification tells you where it sits in a protein. Learn the nine odd one-letter codes and the rest of this college biochemistry topic falls into place.

Already know the basics? Jump to the 8 practice questions or the worked examples.

All 20 amino acids: codes and side-chain classes

Every amino acid shares the same backbone: an amino group, a carboxyl group, and a hydrogen on the alpha carbon. Only the side chain (R group) changes, and the side chain decides how the amino acid behaves. The table uses the five groups from OpenStax Biology 2e. Other books group them a little differently, for example listing tyrosine as polar or histidine as aromatic, so learn the reasons, not just the bins.

The 20 standard amino acids, grouped as in OpenStax Biology 2e
Amino acidThree-letterOne-letterSide-chain classSide chain in words
GlycineGlyGNonpolarA single hydrogen atom. The only standard amino acid with no chiral center.
AlanineAlaANonpolarA methyl group (–CH₃).
ValineValVNonpolarA branched three-carbon group (isopropyl).
LeucineLeuLNonpolarA branched four-carbon group (isobutyl).
IsoleucineIleINonpolarA four-carbon group branched at the first carbon (sec-butyl); has a second chiral center.
MethionineMetMNonpolarA chain with a sulfur inside it, –CH₂CH₂–S–CH₃ (a thioether).
ProlineProPNonpolarA three-carbon chain that loops back and bonds to the backbone nitrogen, making a ring.
SerineSerSPolar uncharged–CH₂OH, a hydroxyl group.
ThreonineThrTPolar uncharged–CH(OH)CH₃, a hydroxyl on a branched carbon; has a second chiral center.
CysteineCysCPolar uncharged–CH₂SH, a thiol (sulfhydryl). Two can join as a disulfide bond.
AsparagineAsnNPolar uncharged–CH₂–C(=O)NH₂, an amide.
GlutamineGlnQPolar uncharged–CH₂CH₂–C(=O)NH₂, an amide one carbon longer than asparagine's.
Aspartic acid (aspartate)AspDAcidic (negatively charged)–CH₂–COO⁻, a carboxylate.
Glutamic acid (glutamate)GluEAcidic (negatively charged)–CH₂CH₂–COO⁻, a carboxylate one carbon longer than aspartate's.
LysineLysKBasic (positively charged)A four-carbon chain ending in an amine, –(CH₂)₄NH₃⁺.
ArginineArgRBasic (positively charged)A three-carbon chain ending in a guanidinium group.
HistidineHisHBasic (positively charged)–CH₂– attached to an imidazole ring (a five-membered ring with two nitrogens).
PhenylalaninePheFAromatic–CH₂– attached to a benzene ring (a benzyl group).
TyrosineTyrYAromaticLike phenylalanine, plus an –OH on the far end of the ring (a phenol).
TryptophanTrpWAromatic–CH₂– attached to an indole, a two-ring system with a nitrogen.

Memory tips for the tricky one-letter codes

Eleven codes are just the first letter: G, A, V, L, I, M, P, S, T, C, H. The other nine needed a different letter because the first was taken. These are the ones exams love.

  1. F, phenylalanine: Say it out loud: "Fenylalanine." P was already taken by proline.
  2. Y, tyrosine: tYrosine. T went to threonine, so tyrosine got its second letter.
  3. W, tryptophan: Think "tWyptophan." Its big two-ring indole is also the bulkiest side chain, and W is the widest letter.
  4. R, arginine: Say it: "aRginine." A was taken by alanine.
  5. K, lysine: L went to leucine. K sits right before L in the alphabet.
  6. D, aspartate: "AsparDic." D comes before E, and aspartate is the shorter of the two acidic side chains.
  7. E, glutamate: "GlutEmate," right after D. Glutamate is one carbon longer than aspartate.
  8. N, asparagine: asparagiNe. The side chain is an amide, so it has an extra N.
  9. Q, glutamine: "Q-tamine" (sounds like "cute-amine"). It is the amide partner of glutamate.

How to sort side chains fast

  1. Only carbon and hydrogen (plus Met's buried sulfur)? Nonpolar. These cluster inside folded proteins, away from water.
  2. OH, SH, or an amide (C=O with NH₂)? Polar uncharged: Ser, Thr, Cys, Asn, Gln.
  3. An extra COOH? Acidic. Asp and Glu are negatively charged at pH 7.4.
  4. An extra nitrogen group that can pick up H⁺? Basic: Lys, Arg, His. Lys and Arg are positive at pH 7.4. Histidine is filed here too, but at pH 7.4 most histidine side chains are actually neutral.
  5. A flat ring? Aromatic: Phe, Tyr, Trp.

Watch the look-alikes. Asn and Gln are amides, not acids, even though their names sound like aspartate and glutamate. Met has sulfur but cannot form disulfides; only Cys has the free –SH.

Worked examples

Example 1: Decode a short peptide

Write out the peptide WYKED using full names, and say which residues are charged at pH 7.4.

  1. W = tryptophan, Y = tyrosine, K = lysine, E = glutamate, D = aspartate.
  2. Lysine (K) is basic and carries + at pH 7.4.
  3. Glutamate (E) and aspartate (D) are acidic and carry − at pH 7.4.
  4. Tryptophan and tyrosine are aromatic and uncharged.

Answer: Trp-Tyr-Lys-Glu-Asp. One + (Lys) and two − (Glu, Asp).

Example 2: Classify from a description

A side chain is –CH₂CH₂–C(=O)NH₂. Name the amino acid and classify it.

  1. The end group is C=O with NH₂: that is an amide, not a carboxylic acid.
  2. Amide side chains belong to asparagine (one CH₂) and glutamine (two CH₂).
  3. There are two CH₂ groups, so it is glutamine.
  4. Amides do not gain or lose a proton at physiological pH, so it is polar uncharged.

Answer: glutamine (Gln, Q), polar uncharged.

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One-letter code flashcards

You get a random amino acid name. Type its one-letter code and press Enter or Check. Then press Next for a new one. Your score is kept until you reload the page.

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Practice questions

Pick an answer, then check it. A wrong answer tells you what went wrong, so you can try again. Every question also has a full solution.

Question 1What is the one-letter code for tryptophan?

Show solution

T was taken by threonine, so tryptophan is W. The three aromatic amino acids are F (Phe), Y (Tyr), and W (Trp).

Question 2Which amino acid has the one-letter code K?

Show solution

K = lysine (Lys). Leucine kept L, and K is next to it in the alphabet.

Question 3Which amino acid has a negatively charged side chain at pH 7.4?

Show solution

The acidic amino acids are aspartate (D) and glutamate (E). Their side-chain carboxyl groups are deprotonated at pH 7.4, so they carry a negative charge.

Question 4Which amino acid can form a disulfide bond with another residue of the same kind?

Show solution

Cysteine (Cys, C) has a thiol side chain, –CH₂SH. Two cysteines can be oxidized to form a covalent disulfide bond, S–S, inside or between protein chains.

Question 5Which standard amino acid has no chiral center?

Show solution

A chiral carbon needs four different groups. Glycine's alpha carbon carries two hydrogens, so it is achiral. All the other 19 are chiral.

Question 6Whose side chain bonds back to the backbone nitrogen, forming a ring?

Show solution

Proline is the only standard amino acid whose side chain is bonded to the backbone nitrogen. That ring makes proline rigid, and it often shows up at bends in protein chains.

Question 7Which three are grouped as the aromatic amino acids in OpenStax Biology 2e?

Show solution

F, Y, and W are the aromatic amino acids. Some textbooks also call histidine aromatic, but in this classification it is listed as basic.

Question 8Which pair of amino acids both have amide side chains?

Show solution

Asparagine (N) and glutamine (Q) have amide side chains. They are polar but uncharged, unlike their acidic partners aspartate (D) and glutamate (E).

Answer key

  1. B, W
  2. D, lysine
  3. B, glutamate
  4. C, cysteine
  5. A, glycine
  6. D, proline
  7. B, Phe, Tyr, Trp
  8. C, Asn and Gln

Frequently asked questions

Do I need to know the amino acids for the MCAT?

Yes. The AAMC content outline includes amino acids, including their structure, classification by side chain, and acid-base behavior. Many students memorize all 20 structures along with the one- and three-letter codes, since passages often show sequences in one-letter form.

What is the fastest way to learn the one-letter codes?

Learn the 11 that use their first letter, then focus on the nine exceptions: F, Y, W, R, K, D, E, N, and Q. Short, frequent flashcard sessions like the one on this page work better than one long cram.

Why do textbooks classify some amino acids differently?

The categories are conveniences, not strict rules. Tyrosine has both an aromatic ring and a polar OH, and histidine has an aromatic ring and a basic nitrogen, so different books put them in different groups. Check which scheme your course uses.

Is histidine charged at physiological pH?

Mostly not. Its side chain pKa is about 6, so at pH 7.4 only a small fraction is protonated. It is still classed as basic because it picks up a proton easily, which is why it shows up so often in enzyme active sites.

Sources

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