Chymotrypsin Mechanism: Correct Steps for Enzymatic Catalysis

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SUMMARY

The mechanism of chymotrypsin involves a series of well-defined steps crucial for enzymatic catalysis. The process begins with His 57 catalyzing the removal of a proton from Ser 195, enabling Ser 195's nucleophilic oxygen to attack the carbonyl carbon of the substrate. Following the formation of a tetrahedral intermediate, His 57 donates a proton to the nitrogen of the scissile peptide bond, leading to the release of the C-terminal portion of the substrate. The mechanism concludes with water facilitating the regeneration of Ser 195 and the release of the N-terminal portion of the substrate.

PREREQUISITES
  • Understanding of enzymatic catalysis mechanisms
  • Familiarity with amino acid structures, particularly Serine and Histidine
  • Knowledge of tetrahedral intermediates in biochemical reactions
  • Basic principles of peptide bond formation and cleavage
NEXT STEPS
  • Study the detailed mechanism of serine proteases, focusing on chymotrypsin
  • Learn about the role of histidine in enzyme catalysis
  • Explore the concept of nucleophilicity in organic chemistry
  • Investigate the structural biology of chymotrypsin using tools like PyMOL
USEFUL FOR

Biochemists, molecular biologists, and students studying enzymatic mechanisms will benefit from this discussion, particularly those focusing on proteolytic enzymes and their catalytic processes.

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what are the correct steps for the mechanism of chymotrypsin?

Below is my answer but I don't know if it is right.


a) His 57 catalyzes removal of H from Ser 195 hydroxyl.
b. Ser 195's nucleophilic O attacks carbonyl C of substrate.
c. His 57 donates H to N of sissile peptide bond, tetrahedral intermediate decomposes.
d. C- terminal portion of oribinal substrate with its new amino acid terminus diffuses away.
e. water donates H to His 57.
f. Resulting OH attacks carbonyl of remaining substrate.
g. His 57 donates H to Ser 195 O, leading to collapse of tetrahedral intermediate.
h.N-terminal portion of original substrate with its new carboxy terminus diffuses away.

My answer is b,a,c,e,f,g,h,d. Is this right? Thank you! :)
 
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Two comments/questions:

Why do you believe that the O of Ser195 would be nucleophilic before its proton was partially abstracted by His57?

Why do you believe that the substrate would remain in the reaction center if it is no longer involved in the reaction after step c?