Question

The catalytic center of which of the following protease families consists of a catalytic triad of aspartate, histidine and serine ?

• A. Cysteine proteases
• B. Aspartic proteases
• C. Serine proteases
• D. Metalloproteases

Hint:

The Asp-His-Ser catalytic triad in serine proteases is a classic example of convergent evolution. Two completely unrelated enzyme families, the chymotrypsin-like and subtilisin-like proteases, independently evolved the exact same catalytic triad configuration.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Proteases (peptidases) are enzymes that hydrolyze peptide bonds in proteins.
They are classified into distinct families based on the principal catalytic residue or mechanism at their active sites.

Step 2: Detailed Explanation:
The catalytic triad composed of Aspartate, Histidine, and Serine is the signature active-site motif of Serine proteases (such as chymotrypsin, trypsin, and elastase).
In this triad, the three amino acid residues are positioned close together in three-dimensional space, cooperating to perform catalysis:
1. Histidine acts as a base, polarizing and removing a proton from the hydroxyl group of Serine.
2. Aspartate forms a hydrogen bond with Histidine, stabilizing the positive charge that develops on the imidazole ring.
3. This charge-relay system turns the Serine oxygen into a highly nucleophilic alkoxide ion, which attacks the carbonyl carbon of the substrate's peptide bond, forming a covalent acyl-enzyme intermediate.
Let us review the active sites of the other protease families:
- Cysteine proteases use a Cysteine-Histidine-Aspartate/Asparagine catalytic triad where Cysteine acts as the nucleophile.
- Aspartic proteases use two highly conserved aspartate residues to activate a water molecule for nucleophilic attack.
- Metalloproteases utilize a coordinated metal ion (typically zinc) to activate water.
Therefore, only serine proteases contain the Asp-His-Ser catalytic triad.

Step 3: Final Answer:
The protease family whose catalytic center consists of the Asp-His-Ser triad is the Serine proteases, corresponding to option (C).