Last modified 30th Jun '95 © Birkbeck College 1995

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Page 1 (in Ch.7) Page 2 Tertiary Structure; Substrates Page 3 Active Site
Page 4 Diagrams Page 6 Crystal Structures Page 7 References

An example of an active site: the serine proteases

Reaction Mechanism

The model for the reaction mechanism is as follows:
(in the diagrams, Ser 195 is red, His 57 blue, substrate black and water pink)
Acylation step
Firstly, the nucleophilic OG of Ser 195 attacks the carbonyl carbon of the scissile bond, forming the tetrahedral transition state, as described in the section on the oxyanion hole. The proton donated from the OG atom to His 57 is then donated to the N atom of the scissile bond, cleaving the C-N peptide bond (or C-O ester bond) to produce the amine and the acyl-enzyme intermediate. The amine is that part of the substrate which follows the scissile bond in the sequence; the acyl-enzyme intermediate consists of the remaining fragment covalently bound to Ser 195.

Deacylation step
Secondly, the acylation step essentially occurs in reverse, but involving a water molecule instead of the amine (which diffuses away): the water loses a proton to His 57, and the resulting OH- nucleophile attacks the acyl-enzyme intermediate forming another tetrahedral transition state. The proton is then donated to the Ser OG, releasing the acid product.

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J. Walshaw