Peptide Bond Formation and Hydrolysis Notes

Peptide Structure

  • Peptides consist of amino acid subunits, also known as residues.
    • Examples:
    • Dipeptides: Composed of 2 amino acids.
    • Tripeptides: Composed of 3 amino acids.
    • Oligopeptides: Small peptides, defined as having up to about 20 residues.
    • Polypeptides: Longer chains of amino acids.

Peptide Bond Formation

  • Peptide bonds are a specialized form of amide bonds.
    • Formation Process:
    • Involves the joining of the carboxyl (COO\text{^-}) group of one amino acid and the amino (NH\text{^+}) group of another.
    • Results in the functional group CONH\text{^-}.
  • Condensation Reaction:
    • Peptide bond formation is a condensation (or dehydration) reaction, resulting in the removal of a water molecule (H₂O).
  • Mechanism:
    • Viewed as an acyl substitution reaction involving carboxylic acid derivatives:
    • The electrophilic carboxyl carbon of one amino acid attacks the nucleophilic amino group of another amino acid.
    • This attack displaces the hydroxyl group from the carboxylic acid, forming a peptide amide bond.
  • Resonance and Rigidity:
    • Amide groups have delocalizable pi electrons, contributing to resonance:
    • The C-N bond in amides displays partial double bond character.
    • Due to this characteristic, rotation around C-N amide bonds is restricted, enhancing the rigidity of the protein.
    • Rotation around other bonds remains unrestricted (single sigma bonds).
  • Terminology:
    • First amino end = Amino Terminus (N terminus).
    • Free carboxyl end = Carboxyl Terminus (C terminus).
    • Convention: Peptides are drawn N terminus on the left and C terminus on the right.

Peptide Bond Hydrolysis

  • Stability in Solution:
    • Peptides need to be relatively stable to function properly and do not spontaneously dissociate.
  • Necessity for Digestion:
    • To digest proteins, they must be broken down into their constituent amino acids.
  • Hydrolysis in Organic Chemistry:
    • Amides can be hydrolyzed using acid or base catalysts.
  • Enzymatic Catalysis:
    • In living organisms, hydrolysis is catalyzed by hydrolytic enzymes like trypsin and chymotrypsin:
    • Specificity:
      • Trypsin: Cleaves at the carboxyl end of arginine and lysine.
      • Chymotrypsin: Cleaves at the carboxyl end of phenylalanine, tryptophan, and tyrosine.
  • Hydrolysis Mechanism:
    • Enzymes break apart the amide bond:
    • Adds a hydrogen atom to the amide nitrogen and an OH group to the carbonyl carbon.