Amino acid chart overview
An amino acid chart summarizes the 20 standard (proteinogenic) amino acids used to build proteins, listing the one-letter and three-letter codes alongside key chemical properties of the side chain (R group). Side-chain polarity, ionization, and functional groups govern protein folding, enzyme active-site chemistry, substrate binding, and stability.
In aqueous solution near physiological pH (about 7.4), most free amino acids exist predominantly as zwitterions with both a positively charged amino group and a negatively charged carboxyl group: \( \mathrm{H_3N^{+}\!-\!CH(R)\!-\!COO^{-}} \). Side chains that contain acidic or basic groups can add an extra negative or positive charge depending on pH and pKa.
Visualization of structure and classification
Ionization and charge tendencies
Amino-acid charge depends on whether ionizable groups are protonated at a given pH. The Henderson–Hasselbalch relation connects pH and pKa for a conjugate acid–base pair: \[ \mathrm{pH = p}K_a + \log\!\left(\frac{[\mathrm{A^-}]}{[\mathrm{HA}]}\right). \] Side-chain pKa values vary with local environment in a folded protein, yet typical aqueous values provide a reliable first approximation for charge at pH ≈ 7.4.
| Amino acid | Code | Ionizable side chain | Typical side-chain pKa (approx.) | Charge tendency at pH ≈ 7.4 |
|---|---|---|---|---|
| Aspartate | Asp (D) | Carboxyl | 3.9 | Mostly deprotonated → negative |
| Glutamate | Glu (E) | Carboxyl | 4.2 | Mostly deprotonated → negative |
| Histidine | His (H) | Imidazole | 6.0 | Partially protonated → can buffer near neutral pH |
| Cysteine | Cys (C) | Thiol | 8.3 | Mostly neutral; deprotonates in some active sites |
| Tyrosine | Tyr (Y) | Phenolic OH | 10.1 | Neutral at physiological pH; deprotonates at high pH |
| Lysine | Lys (K) | ε-amino | 10.5 | Mostly protonated → positive |
| Arginine | Arg (R) | Guanidinium | 12.5 | Strongly protonated → positive |
Complete amino acid chart (codes, classes, essentiality)
Essentiality reflects the human diet: “essential” amino acids cannot be synthesized in adequate amounts under typical physiological conditions; “conditionally essential” amino acids may require dietary supply during growth, illness, or limited precursor availability.
| Amino acid | 3-letter | 1-letter | Side-chain class | Polarity / charge tendency at pH ≈ 7.4 | Essentiality (humans) |
|---|---|---|---|---|---|
| Glycine | Gly | G | Nonpolar (small) | Neutral; flexible backbone contribution | Nonessential |
| Alanine | Ala | A | Nonpolar | Neutral; hydrophobic preference | Nonessential |
| Valine | Val | V | Nonpolar | Neutral; hydrophobic | Essential |
| Leucine | Leu | L | Nonpolar | Neutral; hydrophobic | Essential |
| Isoleucine | Ile | I | Nonpolar | Neutral; hydrophobic | Essential |
| Methionine | Met | M | Nonpolar (sulfur) | Neutral; hydrophobic; sulfur thioether | Essential |
| Proline | Pro | P | Nonpolar (cyclic) | Neutral; conformational rigidity in proteins | Nonessential |
| Phenylalanine | Phe | F | Aromatic | Mostly nonpolar; hydrophobic aromatic ring | Essential |
| Tyrosine | Tyr | Y | Aromatic | Polar aromatic; usually neutral; can ionize at high pH | Conditionally essential |
| Tryptophan | Trp | W | Aromatic | Mostly hydrophobic; indole can H-bond | Essential |
| Serine | Ser | S | Polar uncharged | Neutral; hydroxyl supports H-bonding | Nonessential |
| Threonine | Thr | T | Polar uncharged | Neutral; hydroxyl supports H-bonding | Essential |
| Asparagine | Asn | N | Polar uncharged | Neutral; amide supports H-bonding | Nonessential |
| Glutamine | Gln | Q | Polar uncharged | Neutral; amide supports H-bonding | Conditionally essential |
| Cysteine | Cys | C | Polar uncharged (sulfur) | Usually neutral; thiol can ionize; forms disulfide bonds | Conditionally essential |
| Aspartate | Asp | D | Acidic | Negative; carboxylate at physiological pH | Nonessential |
| Glutamate | Glu | E | Acidic | Negative; carboxylate at physiological pH | Nonessential |
| Lysine | Lys | K | Basic | Positive; ε-amino typically protonated | Essential |
| Arginine | Arg | R | Basic | Positive; guanidinium strongly protonated | Conditionally essential |
| Histidine | His | H | Basic (buffering) | Partially positive near neutral pH; catalytic roles common | Essential |
Interpretation for proteins and enzymes
- Hydrophobic side chains (nonpolar and many aromatic residues) favor burial inside folded proteins, stabilizing structure through the hydrophobic effect.
- Polar uncharged side chains contribute hydrogen bonds and specificity in binding pockets and at protein–protein interfaces.
- Acidic and basic side chains form salt bridges, tune local electrostatics, and participate in acid–base catalysis in enzyme active sites.
- Histidine commonly functions as a near-neutral pH proton donor/acceptor because its side-chain pKa is close to physiological pH.
- Cysteine can form disulfide bonds (oxidizing environments) and can act as a nucleophile in some catalytic mechanisms when deprotonated.
Common pitfalls
- Charge assignments treated as fixed labels rather than pH-dependent tendencies tied to pKa and microenvironment.
- “Aromatic” treated as a synonym for “nonpolar,” despite tyrosine having a polar phenolic group and tryptophan having an H-bond-capable indole.
- Essentiality treated as a universal property across species, despite being diet- and physiology-dependent.
A compact approximation for the isoelectric point of an amino acid with only two ionizable backbone groups (no ionizable side chain) is \( pI \approx \frac{pK_{a,\alpha\text{-COOH}} + pK_{a,\alpha\text{-NH}_3^+}}{2} \); amino acids with ionizable side chains require including the relevant side-chain pKa in the dominant neutral-species bracket.