Every amino acid shares a core structure:

• A central α-carbon bonded to:
  • An amino group (–NH₂, becomes –NH₃⁺ at body pH)
  • A carboxyl group (–COOH, becomes –COO⁻ at body pH)
  • A hydrogen atom (H)
  • A unique R-group (side chain)
    • Example:
      • Glycine (simplest): R-group = H
      • Alanine: R-group = –CH₃
        The R-group dictates the amino acid’s chemical behavior (e.g., polarity, charge).

Peptide bonds form via dehydration synthesis:

1. The –COOH group of one amino acid reacts with the –NH₂ group of another.
2. A water molecule (H₂O) is released.
3. The resulting covalent C–N bond is called a peptide bond.

Outcome: Dipeptides (e.g., Gly-Ala) → Polypeptides → Proteins. Key Note: This reaction is reversible (hydrolysis breaks peptide bonds).

At near-neutral pH, amino acids bear both positive (–NH₃⁺) and negative (–COO⁻) charges. For instance, leucine and serine exist in zwitterion form at pH 7. This dual charge improves solubility and impacts protein interactions.

Side chains vary in polarity, size, and charge, influencing folding through hydrogen bonds or ionic attractions. For example, polar serine can form hydrogen bonds, while hydrophobic valine promotes core packing. Thus, side chains help define final protein structures.

Proline’s cyclic side chain links back to its amino group, restricting flexibility. For example, proline often bends polypeptide chains or disrupts alpha-helices. This property makes it crucial in turns and loops within proteins.

Twenty-two amino acids are proteinogenic, and nearly all are L-chiral. For example, L-lysine fits enzyme active sites, while D-lysine usually does not. Therefore, chirality ensures correct protein assembly.

Essential amino acids must come from diet because the body cannot make them. For example, lysine and tryptophanare essential for protein synthesis and neurotransmitter production.