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The 20 Amino Acids

The 20 Amino Acids

6 min read Updated Apr 18, 2026

You need to know all 20 by structure, name, three-letter code, one-letter code, and category. On test day you may see any of these formats in a passage and have to reason about it instantly. The table below is worth memorizing cold.

The twenty amino acids, sorted by what the side chain does

Reference
Ask three questions of any side chain: does it carry charge at pH 7, is it polar, and is it bulky. Nonpolar buried in the core Gly G not chiral Ala A Alanine Val V Valine Leu L Leucine Ile I Isoleucine Pro P kinks the chain Met M starts translation Aromatic absorb at 280 nm Phe F Phenylalanine Trp W absorbs most Tyr Y can be phosphorylated Polar on the surface Ser S phosphorylated Thr T phosphorylated Cys C makes disulfides Asn N Asparagine Gln Q Glutamine Acidic negative at pH 7 Asp D pKa ≈ 3.9 Glu E pKa ≈ 4.3 Basic positive at pH 7 Lys K pKa ≈ 10.5 Arg R pKa ≈ 12.5 His H pKa ≈ 6.0 Essential your body cannot build these, so they have to be eaten His · Ile · Leu · Lys · Met · Phe · Thr · Trp · Val Glycine
Side chain is a single hydrogen, so it has no chiral centre and fits into turns nothing else can.
 Proline
Its side chain loops back onto its own nitrogen. No backbone N-H to donate, so it breaks α-helices.
 Cysteine
Two of them oxidize into a covalent disulfide bridge, the only side-chain-to-side-chain bond.
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Scroll sideways to see the whole map.

Acidic: deprotonated and negative at pH 7 Basic: protonated and positive at pH 7 Polar but uncharged Aromatic

Why the grouping is the answerNonpolar side chains end up buried in the core of a folded protein and facing outward in a membrane protein. Polar and charged ones end up on the surface, in the active site, or facing the aqueous channel. Predicting where a residue sits is predicting what its side chain wants.

The five that carry chargeOnly Asp, Glu, Lys, Arg, and His have an ionizable side chain, and only they move the isoelectric point away from neutral. Histidine is the one that matters most: its pKa near 6 means it is the only residue that can gain or lose a proton at physiological pH, which is why it does the acid-base work in so many active sites.

The odditiesGlycine has no chiral centre and fits where nothing else will. Proline has its side chain bonded back to its own nitrogen, so it cannot donate a backbone hydrogen bond and it breaks helices. Cysteine is the only one that makes a covalent bond with another residue.

You are not asked to draw these; you are asked to predict behavior from them. Read a residue and ask three questions: does it carry charge at pH 7, is it polar, and is it big. Those three answers cover almost every amino acid question the exam sets.

The Master Table

| Name | 3-letter | 1-letter | Category | Side chain feature | pKa of R group (if any) |
|------|----------|----------|----------|--------------------|-------------------------|
| Glycine | Gly | G | Special (nonpolar) | -H (achiral) | - |
| Alanine | Ala | A | Nonpolar aliphatic | -CH3 | - |
| Valine | Val | V | Nonpolar aliphatic (branched) | -CH(CH3)2 | - |
| Leucine | Leu | L | Nonpolar aliphatic (branched) | -CH2CH(CH3)2 | - |
| Isoleucine | Ile | I | Nonpolar aliphatic (branched) | -CH(CH3)CH2CH3 | - |
| Methionine | Met | M | Nonpolar, sulfur | -CH2CH2SCH3 | - |
| Proline | Pro | P | Special (nonpolar) | Cyclic, bonds back to N | - |
| Phenylalanine | Phe | F | Aromatic (nonpolar) | -CH2-C6H5 | - |
| Tryptophan | Trp | W | Aromatic (nonpolar) | Indole ring | - |
| Tyrosine | Tyr | Y | Aromatic (polar) | Phenol ring (-OH) | ~10 |
| Serine | Ser | S | Polar uncharged | -CH2OH | - |
| Threonine | Thr | T | Polar uncharged | -CH(OH)CH3 | - |
| Cysteine | Cys | C | Special (polar) | -CH2SH | ~8.3 |
| Asparagine | Asn | N | Polar uncharged | -CH2CONH2 | - |
| Glutamine | Gln | Q | Polar uncharged | -CH2CH2CONH2 | - |
| Aspartate | Asp | D | Acidic (negative) | -CH2COO- | ~3.9 |
| Glutamate | Glu | E | Acidic (negative) | -CH2CH2COO- | ~4.1 |
| Lysine | Lys | K | Basic (positive) | -(CH2)4NH3+ | ~10.5 |
| Arginine | Arg | R | Basic (positive) | Guanidinium | ~12.5 |
| Histidine | His | H | Basic (partial +) | Imidazole | ~6.0 |

The alpha-amino group of any free amino acid has a pKa around 9 to 10. The alpha-carboxyl group has a pKa around 2. Memorize these two values - they appear in every titration question.

Essential vs. Nonessential

Humans can synthesize 11 of the 20 amino acids from scratch. The other 9 must come from diet - these are the essential amino acids.

Amino Acids with Ionizable Side Chains

Seven amino acids have R groups with ionizable protons. These are the only ones whose charge state depends on pH, and they are the ones you will see on titration and isoelectric-point questions.

| Amino acid | R-group pKa | Charge below pKa | Charge above pKa |
|------------|-------------|------------------|------------------|
| Asp (D) | 3.9 | 0 | -1 |
| Glu (E) | 4.1 | 0 | -1 |
| His (H) | 6.0 | +1 | 0 |
| Cys (C) | 8.3 | 0 | -1 |
| Tyr (Y) | 10.1 | 0 | -1 |
| Lys (K) | 10.5 | +1 | 0 |
| Arg (R) | 12.5 | +1 | 0 |

How to Study This Table

Do not try to memorize everything at once. Start with categories (previous section). Then memorize the three-letter codes by writing them out with structures five times. Then add the one-letter codes. Then pKa values for the seven ionizable ones. This layered approach beats rote flashcards.

Which seven amino acids have ionizable side chains?
Click to reveal answer
Aspartate (D, pKa ~3.9), Glutamate (E, pKa ~4.1), Histidine (H, pKa ~6.0), Cysteine (C, pKa ~8.3), Tyrosine (Y, pKa ~10.1), Lysine (K, pKa ~10.5), Arginine (R, pKa ~12.5). These are the amino acids whose charge state depends on pH and that appear in titration questions.
What are the one-letter codes for the 9 essential amino acids?
Click to reveal answer
F (Phe), V (Val), T (Thr), W (Trp), I (Ile), M (Met), H (His), K (Lys), L (Leu). Mnemonic “PVT TIM HALL” covers them (plus Arginine as conditionally essential).
Which amino acid has a pKa near 6 and is commonly found in enzyme active sites as a proton shuttle?
Click to reveal answer
Histidine. Its imidazole side chain has a pKa of about 6, which means at physiological pH (7.4) both the protonated and deprotonated forms coexist. This lets histidine both donate and accept protons during catalysis - hence its starring role in enzymes like chymotrypsin and carbonic anhydrase.
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