20 Standard Amino Acids
Introduction
Section titled “Introduction”Amino acids are the fundamental building blocks of all peptides and proteins. Each amino acid consists of a central carbon atom (alpha carbon) bonded to four groups: an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a unique side chain (R group). The side chain determines the chemical properties of each amino acid.
Hydrophobic Amino Acids
Section titled “Hydrophobic Amino Acids”Hydrophobic amino acids have nonpolar side chains that avoid water. They are typically found in the interior of proteins, contributing to protein stability through hydrophobic interactions.
| Amino Acid | 3-Letter | 1-Letter | Side Chain Properties |
|---|---|---|---|
| Glycine | Gly | G | Smallest, most flexible, achiral |
| Alanine | Ala | A | Simple methyl group |
| Valine | Val | V | Branched-chain, isopropyl group |
| Leucine | Leu | L | Branched-chain, isobutyl group |
| Isoleucine | Ile | I | Branched-chain, sec-butyl group |
| Proline | Pro | P | Cyclic, rigid, unique backbone |
| Phenylalanine | Phe | F | Aromatic benzyl group |
| Tryptophan | Trp | W | Indole ring, largest amino acid |
| Methionine | Met | M | Thioether, contains sulfur |
Mnemonic: “GAVLIMP FW” (Gave Limp Fw)
Polar Uncharged Amino Acids
Section titled “Polar Uncharged Amino Acids”Polar amino acids have hydrophilic side chains that can form hydrogen bonds with water. They are often found on protein surfaces or in active sites.
| Amino Acid | 3-Letter | 1-Letter | Side Chain Properties |
|---|---|---|---|
| Serine | Ser | S | Hydroxyl group, polar |
| Threonine | Thr | T | Hydroxyl group, branched |
| Cysteine | Cys | C | Thiol group, forms disulfide bonds |
| Tyrosine | Tyr | Y | Phenol group, amphipathic |
| Asparagine | Asn | N | Amide group, polar |
| Glutamine | Gln | Q | Amide group, polar |
Positively Charged Amino Acids
Section titled “Positively Charged Amino Acids”These amino acids carry a positive charge at physiological pH (7.4). They often participate in ionic interactions and are found on protein surfaces.
| Amino Acid | 3-Letter | 1-Letter | Side Chain Properties |
|---|---|---|---|
| Lysine | Lys | K | Primary amine, basic |
| Arginine | Arg | R | Guanidinium group, most basic |
| Histidine | His | H | Imidazole group, pKa ~6.0 |
Negatively Charged Amino Acids
Section titled “Negatively Charged Amino Acids”These amino acids carry a negative charge at physiological pH. They are often involved in salt bridges and catalytic mechanisms.
| Amino Acid | 3-Letter | 1-Letter | Side Chain Properties |
|---|---|---|---|
| Aspartate | Asp | D | Carboxyl group, acidic |
| Glutamate | Glu | E | Carboxyl group, acidic |
Essential vs Non-Essential
Section titled “Essential vs Non-Essential”Essential amino acids (must be obtained from diet): Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.
Non-essential amino acids (can be synthesized by the body): Alanine, Arginine, Asparagine, Aspartic Acid, Cysteine, Glutamic Acid, Glutamine, Glycine, Proline, Serine, Tyrosine.
One-Letter Code Memory Aid
Section titled “One-Letter Code Memory Aid”- F for Phenylalanine (F resembles the benzene ring)
- W for Tryptophan (W resembles the indole structure)
- R for Arginine
- K for Lysine (K is next to L in the alphabet, but L was taken by Leucine)
- D for Aspartate (D is “aspartate” minus the A)
- E for Glutamate (E is “glutamate” minus the G)