Enzyme Mechanisms - Serine Proteases
Part VI
Part VI. Mechanism of Catalysis of Serine Proteases.
OK- now you know which AA side chains are involved in catalysis. Make a hypothesis how these chemical groups of the enzyme can assist the reaction to happen more rapidly. Shown below are the steps in the proposed mechanism of catalysis of serine proteases. Only the three key amino acid side chains are illustrated in these graphics - the key 3 are His-57, Asp-102 and Ser-195.
The hypothesis is divided into 3 parts:
1) the substrate is bond to form the 'ES' complex like a standard Michaelis-Menten enzyme and then the activated Ser-195 (where the hydrogen on its -OH group has been pulled off by His-57 with the assistance of Asp-102) attacks the susceptible carbonyl carbon of the substrate to form what is called a tetrahedral intermediate. This is the first of two intermediates of this type to form in the proposed mechanism of catalysis by serine proteases. This first part of the mechanism is shown in Fig. 16. You might want to look back at Lecture 15A where I described the mechanism for the organic reaction where an ester is hydrolyzed and see the tetrahedral intermediate that was formed in that proposed mechanism.
Figure 15. First part of the Mechanism of Catalysis by Serine Proteases.
2) The next step is where the tetrahedral intermediate decomposes with bond rearrangements take place just like in a proposed organic chemical mechanism. This leads to the formation of the first product, which can then leave the active site and return to solution. It also leads to the formation of a covalent bond between the other half of the substrate and the enzyme's Ser-195, which is called the Acyl-enzyme since this formed between the acyl or carbonyl group and the oxygen of Ser-195. This is an important part of the mechanism because the data collected with the simple amide and ester substrates showed that a partially stable (metastable) covalent intermediate formed between the substrate and the enzyme. So the mechanism of catalysis for serine proteases has bond rearrangements in it as well as the important step where the enzyme reacts with the substrate. It should be noted that most enzyme do not form covalent intermediates with their substrates but many catalyze bond rearrangements like those hypothesized here, which will be illustrated in the next lecture.
Figure 16. Second part of the Mechanism of Catalysis by Serine Proteases.
3) The last steps in the mechanism are the activation of water by His-57 with the assistance of Asp-102 and the attack of the activated water on the Acyl-Enzyme intermediate which leads to the formation of the second tetrahedral intermediate. This tetrahedral intermediate decomposes then with bond rearrangements and leads to the formation of the second product and returns the enzyme to its starting state so that it can catalyze another round of the reaction of water with the protein or peptide where the amide bond is hydrolyzed.
Figure 17. Third and final part of the Mechanism of Catalysis by Serine Proteases.
©Wilbur H. Campbell, 1995; wcampbel@mtu.edu