Serine proteases Named because they use a serine residue to cut peptide bonds Possess a catalytic triad His 57 Asp 102 Ser 195 Use 2 different types of.

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Serine proteases Named because they use a serine residue to cut peptide bonds Possess a catalytic triad His 57 Asp 102 Ser 195 Use 2 different types of catalysis

Mechanism of serine proteases Step 1: Peptide enters the enzyme’s active site What happens: Lone pair of electron on His side chain form bond with H of nearby Ser 195 Ser 195 side chain O performs nucleophilic attack on peptide carbonyl C

Step 2: Tetrahedral intermediate is formed What happens: Bond formed between N from peptide and H from Ser 195 Peptide carbonyl group reforms C-N peptide bond is broken

Step 3: Covalent intermediate is formed What happens: N-terminal portion of cleaved peptide is still bonded to Ser 195

Step 4: Water molecule enters active site What happens: Lone pair of electrons on His 57 side chain attacks H on water Bond forms between water O and peptide carbonyl C

Step 5: Second tetrahedral intermediate is formed What happens: Ser 195 side chain O forms bond with water H, breaking bond to peptide C Bond between His 57 side chain and H breaks Peptide carbonyl forms

Step 6: Enzyme is regenerated What happens: Cleaved peptide has a new C-terminus Catalytic triad is reset and ready for another round of catalysis Both acid-base catalysis and covalent catalysis occurred