W2: Proteases and Esterases

1. Creation of a nucleophile through a catalytic triad

1. Nucleophilic attack leading to a tetrahedral intermediate

   • stabilization of negative charge by oxyanion hole

2. Cleavage of the peptide bond

   • serine is bound to C-terminus

3. Cleavage of the peptide from the serine (hydrolysis)

   • tetrahedral intermediate stabilized by oxanion hole

Chymotrypsin (peptide bond cleavage C-terminal to aromatic amino acids)

• catalytic triad, H-bond between Asp and His might be LBHB

Lysozyme cleaves bond between NAM (N-acetylmuramic acid) and NAG (N-acetylglucosamine)

• threonine proteases

  • proteasome

• serine proteases

  • chymotrypsin

• cysteine proteases

  • papain

• aspartic proteases

  • HIV protease

• metalloproteases

  • carboxypeptidase

Example: papain (cysteine protease)

Example: Carboxypeptidase A (metalloprotease)

Both esterases and (serine) proteases have serine residues that act as nucleophiles.

But: Esters are more reactive, therefore in esterases, no catalytic triade for serine activation is needed.

(mesomeric stabilization of the peptide bond)

Inhibitors

Cationic compounds

• mimic ammonium part of acetylcholine

• noncovalent inhibition

Carbamates

• covalently modify active site serine

• reactivation of the enzyme with strong nucleophile

Organophosphates

• covalently modify active site serine

• reactivation with strong nucleophile