Amino Acids Found in Proteins
High-Yield Summary
- Every standard amino acid is an alpha amino acid: a central alpha carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), an H atom, and a variable R group that gives it its identity.
- 20 proteinogenic amino acids split into 5 side-chain categories: nonpolar/nonaromatic, aromatic, polar uncharged, acidic (negative), basic (positive).
- Glycine is the only achiral amino acid (side chain = single H, so its alpha carbon has two identical substituents).
- Cysteine is the only amino acid with R (not S) absolute configuration — its sulfur side chain outranks –COOH under CIP rules, even though it's still biologically an L-amino acid.
- Selenocysteine is a 21st amino acid, recoded from a UGA stop codon via an mRNA SECIS element — recognize it, not required for recall.
Amino Acids by Side-Chain Category
- Nonpolar, nonaromatic (hydrophobic)
- Glycine (G), Alanine (A), Valine (V), Leucine (L), Isoleucine (I), Proline (P, rigid ring — disrupts helices), Methionine (M, sulfur-containing, first residue in eukaryotic translation).
- Aromatic
- Phenylalanine (F), Tyrosine (Y, has –OH, mildly polar), Tryptophan (W, bulky double ring). Ring pi electrons absorb UV — basis for spectrophotometric protein detection.
- Polar, uncharged
- Serine (S), Threonine (T), Asparagine (N), Glutamine (Q), Cysteine (C, thiol — two Cys can form a disulfide bond).
- Acidic (negative at physiological pH)
- Aspartate (D) and Glutamate (E) — deprotonated conjugate bases of aspartic/glutamic acid; strongly hydrophilic.
- Basic (positive at physiological pH)
- Lysine (K, charged amino group), Arginine (R, resonance-stabilized guanidinium — one of the strongest bases of the 20), Histidine (H, imidazole ring, pKa near physiological pH, common in active sites).
Common MCAT Trap
- Glycine breaks the "all amino acids are chiral" rule — its side chain is just H, so no chiral center.
- Cysteine breaks the "all amino acids are S configuration" rule — sulfur's CIP priority flips it to R, despite still being an L-amino acid biologically. Don't confuse D/L (biological series) with R/S (CIP designation).
- Aspartic acid/aspartate and glutamic acid/glutamate are the same residue at different protonation states, not different amino acids.
Quick Recall
What four groups attach to the alpha carbon of a standard amino acid?
Which amino acid is achiral, and why?
Which amino acid has R (not S) configuration, and why?
How is selenocysteine incorporated into a protein?