Compare and contrast serine proteases and aspartic proteases. Not all answers will be used. Serine protease Both Aspartic protease two aspartic acids in active site catalytic triad of serine, histidine, and aspartic acid tetrahedral intermediate direct nucleophilic attack by water Answer Bank covalent intermediate trigonal planar intermediate catalytic triad of serine, leucine, and valine four aspartic acids in active site direct electrophilic attack by water
The Correct Answer and Explanation is:
Correct Matching:
- Serine protease:
- Catalytic triad of serine, histidine, and aspartic acid
- Tetrahedral intermediate
- Covalent intermediate
- Both:
- Direct nucleophilic attack by water
- Aspartic protease:
- Two aspartic acids in active site
- Electrophilic attack by water
Explanation (300+ words):
Serine proteases and aspartic proteases are both classes of proteolytic enzymes that hydrolyze peptide bonds, but they do so using distinct mechanisms and structural features in their active sites.
Serine proteases (e.g., trypsin, chymotrypsin) use a catalytic triad composed of serine, histidine, and aspartic acid. In these enzymes, the serine residue acts as a nucleophile, the histidine acts as a general base to activate the serine, and the aspartic acid helps stabilize the positively charged histidine during catalysis. The reaction proceeds through the formation of a tetrahedral intermediate, a high-energy structure where the peptide bond being hydrolyzed is temporarily converted into a tetrahedral geometry. This is followed by the formation of a covalent intermediate between the enzyme and the substrate before hydrolysis completes and the enzyme is regenerated.
Aspartic proteases (e.g., pepsin, HIV protease) have a different mechanism. They contain two aspartic acid residues in the active site that are essential for catalysis. These residues work together to activate a water molecule, which directly attacks the carbonyl carbon of the peptide bond. This attack by water is often described as electrophilic, as the oxygen of water becomes more nucleophilic due to proton sharing with the aspartic acids. Aspartic proteases do not form a covalent intermediate, and the mechanism does not involve a tetrahedral intermediate stabilized by an enzyme residue like in serine proteases.
Common to both enzyme types, however, is the fact that they ultimately involve nucleophilic attack by water to break the peptide bond, though the specifics of how this attack is achieved differ.
In conclusion, while both classes cleave peptide bonds and share the role of water in hydrolysis, their mechanistic pathways, active site residues, and intermediate structures differ significantly.