Amino Acids & Peptides

• Proteins are naturally occurring, unbranched polymers where the monomer units are amino acids
• Amino acids are the building blocks of proteins, and their chemical properties determine the biological property of the protein
• Amino acids are alpha amino acids, proteinogenic, chiral, and amphoteric
• The structure of alpha amino acids involves the amino group and the carboxyl group attached to the ⍺-carbon atom
• Proline is an exception with a structural feature not found in other standard amino acids, having a propyl group bonded to both the ⍺-carbon atom and the amino nitrogen atom, giving a cyclic side chain
• Amino acids can be classified by R Group into non-polar, polar, acidic, and basic categories
• Essential amino acids are not synthesized by the body and must be obtained through the diet, while non-essential amino acids can be synthesized by the body
• Amino acids can be classified by metabolic fate into glucogenic, ketogenic, or both categories
• Zwitter ions in amino acids have both an acidic group (-COOH) and a basic group (-NH2) present on the same carbon
• In neutral solution, carboxyl groups in amino acids tend to lose protons (H+), producing a negatively charged species, while amino groups tend to accept protons (H+), producing a positively charged species
• Isoelectric point (iPH, pI) is the pH at which an amino acid solution has no net charge because an equal number of positive and negative charges are present
• Classification of peptides:

  • Dipeptide: compound containing two amino acids
  • Tripeptide: three amino acids joined together in a chain
  • Oligopeptide: peptides with 10 to 20 amino acid residues
  • Polypeptide: peptide containing many amino acid residues with a molecular weight of less than about 5000
• Peptide Bonds:

  • Bonds that link amino acids together in a peptide chain
  • Covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid
• Nature of Peptide Bonds:

  • Reaction: carboxylic acid and an amine react to produce an amide
  • Products: molecule of water and a molecule containing the two amino acids linked by an amide bond
• Parts of a Peptide Chain:

  • C-terminal end: the end with the free COO- group
  • N-terminal end: end with the free H3N+ group
  • Amino acid residue: individual amino acids within a peptide chain
• 4. Peptide Antioxidant: Glutathione

  • Glu–Cys–Gly (tripeptide)
  • Regulator of oxidation–reduction reactions
  • Protects cellular contents from oxidizing agents like peroxides and superoxides
• Biologically Important Small Peptides:

  1. Peptide Hormone: Vasopressin

  • Nona peptide also known as antidiuretic hormone
  • Decreases urine production
  • Increases blood volume when in excess
• 2. Peptide Hormone: Oxytocin

  • Nona peptide that enhances contraction of smooth muscle cells in the uterus
  • Stimulates milk ejection from mammary glands
  • Used to induce labor or control hemorrhage after giving birth
• 3. Peptide Neurotransmitter: Enkephalins

  • Pentapeptide that reduces pain
  • Body's natural painkillers
  • Role in memory, learning, body temperature control, sexual activity, and mental illness
  • Types: Met-enkephalin and Leu-enkephalin