Amino Acid
Also known as: AA, Amino acids, AAs
Amino Acid is an organic molecule containing both an amino group (-NH2) and a carboxyl group (-COOH) that serves as the fundamental building block of peptides and proteins. There are 20 standard amino acids that combine in various sequences to form all peptides and proteins in living organisms.
Last updated: February 1, 2026
Amino Acid Structure
All amino acids share a common core structure consisting of five key components:
- Central carbon (alpha carbon) - The tetrahedral center connecting all groups
- Amino group (-NH2) - Defines the N-terminus of peptide chains
- Carboxyl group (-COOH) - Defines the C-terminus of peptide chains
- Hydrogen atom - Attached to the alpha carbon
- Side chain (R group) - Unique to each amino acid and determines its properties
The side chain is what gives each of the 20 standard amino acids their distinct chemical properties, from hydrophobic to charged to aromatic.
The 20 Standard Amino Acids
Non-polar (Hydrophobic)
| Amino Acid | Abbreviation | Properties |
|---|---|---|
| Glycine | Gly, G | Smallest, flexible |
| Alanine | Ala, A | Simple, common |
| Valine | Val, V | Branched-chain* |
| Leucine | Leu, L | Branched-chain* |
| Isoleucine | Ile, I | Branched-chain* |
| Proline | Pro, P | Rigid, kinks chains |
| Methionine | Met, M | Contains sulfur |
| Phenylalanine | Phe, F | Aromatic |
| Tryptophan | Trp, W | Largest, aromatic |
*BCAAs - Branched Chain Amino Acids
Polar (Uncharged)
| Amino Acid | Abbreviation | Properties |
|---|---|---|
| Serine | Ser, S | Hydroxyl group |
| Threonine | Thr, T | Hydroxyl group |
| Cysteine | Cys, C | Forms disulfide bonds |
| Tyrosine | Tyr, Y | Aromatic, hydroxyl |
| Asparagine | Asn, N | Amide group |
| Glutamine | Gln, Q | Amide group |
Charged
| Amino Acid | Abbreviation | Charge |
|---|---|---|
| Aspartic acid | Asp, D | Negative |
| Glutamic acid | Glu, E | Negative |
| Lysine | Lys, K | Positive |
| Arginine | Arg, R | Positive |
| Histidine | His, H | Positive (pH dependent) |
Essential vs Non-essential
Essential (must obtain from diet)
- Histidine, Isoleucine, Leucine
- Lysine, Methionine, Phenylalanine
- Threonine, Tryptophan, Valine
Non-essential (body can synthesize)
- Alanine, Asparagine, Aspartic acid
- Glutamic acid, Serine
Conditionally Essential
- Arginine, Cysteine, Glutamine
- Glycine, Proline, Tyrosine
Amino Acids in Peptide Design
When designing therapeutic peptides, amino acid properties determine drug behavior:
| Property | Examples | Effect |
|---|---|---|
| Hydrophobic | Val, Leu, Ile | Membrane interaction, stability |
| Charged | Lys, Glu | Solubility, receptor binding |
| Small | Gly, Ala | Flexibility, conformational freedom |
| Aromatic | Phe, Trp, Tyr | Receptor interactions, UV detection |
| Cysteine | Cys | Disulfide bonds, structural stability |
Frequently Asked Questions
Why are there only 20 amino acids?
The genetic code uses 64 codons (triplets of nucleotides) to specify 20 amino acids plus stop signals. This appears to be the result of evolutionary optimization, providing enough variety for complex proteins without excessive complexity.
What makes an amino acid “essential”?
Essential amino acids cannot be synthesized by the human body from other compounds, so they must come from diet. The specific essential amino acids can vary by species; what’s essential for humans may not be for other animals.
How do amino acid modifications affect peptide drugs?
Modifications can dramatically improve drug properties. Replacing L-amino acids with D-amino acids makes peptides resistant to digestive enzymes. Adding unnatural amino acids like Aib (in semaglutide) prevents enzymatic breakdown and extends half-life.
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Disclaimer: This glossary entry is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for medical questions.