Q.60 Given below are two statements :
Statement I : Chymotrypsin hydrolyzes peptide bond through the formation of a transient covalent acyl-
enzyme intermediate.
Statement II : This type of mechanism in enzyme catalysis is operative in all the enzymes in biology.
In the light of the above statements, choose the correct answer from the options given below :
1. Both Statement I and Statement II are true
2. Both Statement I and Statement II are false
3. Statement I is true but Statement II is false
4. Statement I is false but Statement II is true
Chymotrypsin uses a covalent catalysis mechanism involving an acyl-enzyme intermediate to hydrolyze peptide bonds, but this is not universal across all enzymes.
Correct Answer
Option 3: Statement I is true but Statement II is false
Statement I Explanation
Chymotrypsin, a serine protease, catalyzes peptide bond hydrolysis via a two-step process. In the acylation phase, the active site’s serine nucleophilically attacks the substrate’s carbonyl carbon, forming a transient covalent acyl-enzyme intermediate (an ester bond) while releasing the amine product. Deacylation follows, where water hydrolyzes this intermediate to regenerate the enzyme and release the carboxylate product.
Statement II Explanation
Covalent catalysis with acyl-enzyme intermediates occurs in specific enzyme classes like serine proteases (e.g., chymotrypsin, trypsin) and some hydrolases, but not all enzymes. Many enzymes use alternative mechanisms, such as acid-base catalysis (e.g., ribonuclease), metal ion catalysis (e.g., carbonic anhydrase), proximity/orientation effects, or direct group transfer without covalent intermediates (e.g., choline acetyltransferase forms a ternary complex).
Options Analysis
| Option | Description | Correctness |
|---|---|---|
| 1. Both true | Assumes acyl-enzyme mechanism is universal | Incorrect – Statement II false |
| 2. Both false | Denies chymotrypsin’s known mechanism | Incorrect – Statement I true |
| 3. I true, II false | Matches chymotrypsin’s mechanism but rejects universality | Correct |
| 4. I false, II true | Reverses factual roles | Incorrect |
