chymotrypsin

Chymotrypsin

Hydrolyzes dietary proteins into small peptides and amino acids for absorption by the small intestine

N terminal half

Carboxyl component

C terminal half

Amino component

Optimal activity of chymotrypsin

At pH 8

Specificity of chymotrypsin

Cleaves preferentially on the carboxylic terminal side of large hydrophobic side chains (Phe, Met, Ile, Trp, Tyr)

Specificity pocket of chymotrypsin

Hydrophobic therefore prefers hydrophobic side chains

Catalytic triad of chymotrypsin

Asp 102, His 57, Ser 195

Asp 102

Plays a roles in orienting His 57 side chain through hydrogen bonding and electrostatic effects (in its deprotonated form, -ve)

His 57

Acts as a general acid/base catalyst to initially withdraw a proton from the Ser 195 side chain to generate a nucleoside

See 195

Acts as a nucleotide to attach a substrate

Peptide hydrolysis part 1

Substrate binds to active site

Peptide hydrolysis part 2

Tetrahedral intermediate is stabilized by hydrogen bonds with amide group in the oxianion hole. His 57 acts as general base (give electrons)

Peptide hydrolysis part 3

Tetrahedral intermediate collapse breaking the peptide bond and N grabs H from His. C is covalently attached to Ser

Peptide hydrolysis part 4

First product is released - amino component. Enzyme is now acyl enzyme with carboxyl component still covalently attached to Ser

Peptide hydrolysis part 5

Second substrate (water) binds to acyl enzyme

Peptide hydrolysis part 6

His 57 deprotonates the water to generate a nucleophile, acting as a general base. Water attacks the electrophillic carbon forming the second tetrahedral intermediate that is stabalized by hydrogen bonds in the oxianion hole

Peptide hydrolysis part 7

Seconds tetrahedral intermediate collapses. His acts as general acid. Second product (carboxyl component) is released and catalytic triad is reformed.