Amino acids and peptide bonds
Twenty building blocks.
Amino acid structure. Central carbon (α-carbon) bonded to:
- Amino group (−NH₂).
- Carboxyl group (−COOH).
- Hydrogen (−H).
- R group (side chain) — varies between the 20 amino acids; determines polarity, charge and chemistry.
R group categories:
- Non-polar (hydrophobic) — glycine, alanine, leucine, etc.
- Polar uncharged — serine, threonine, cysteine (contains −SH; forms disulfide bridges).
- Acidic (negatively charged) — glutamate, aspartate.
- Basic (positively charged) — lysine, arginine, histidine.
Peptide bond formation. Condensation reaction between the carboxyl of one amino acid and the amino group of the next; water released. The bond (−CO−NH−) is rigid and planar.
A dipeptide = 2 amino acids; tripeptide = 3; polypeptide = many (>10).
- Central C + NH₂ + COOH + H + R group.
- Peptide bond: condensation; rigid, planar.
- R groups determine chemistry → folding.