Q.28 Which one of the following proteases does NOT cleave on the carboxyl side of any Arginine
residue in a protein?
(A) Trypsin (B) Proteinase K
(C) Thrombin (D) Chymotrypsin
Answer: (B) Proteinase K
Proteinase K is a broad-specificity serine protease that cleaves peptide bonds indiscriminately, without preference for arginine residues on the carboxyl side. Unlike trypsin, thrombin, and chymotrypsin, which have defined specificities including arginine, Proteinase K lacks such selectivity and hydrolyzes proteins non-specifically.
Option Analysis
(A) Trypsin: Incorrect—cleaves specifically C-terminal to arginine (and lysine), except when followed by proline.
(B) Proteinase K: Correct—non-specific endopeptidase; degrades proteins regardless of residue type, including no arginine preference.
(C) Thrombin: Incorrect—cleaves C-terminal to arginine in fibrinogen (e.g., Arg-Gly bond in prothrombin activation).
(D) Chymotrypsin: Incorrect—primarily cleaves after aromatic residues but exhibits secondary activity after arginine/lysine.
Introduction: Proteases Cleavage Specificity Arginine
Proteases cleaving on the carboxyl side of arginine residues are critical in protein sequencing and biotech applications. This guide analyzes which protease—trypsin, Proteinase K, thrombin, or chymotrypsin—lacks arginine specificity, essential for molecular biology exams.
Specificity Profiles Table
| Protease | Primary Cleavage Sites | Arginine Cleavage? | Mechanism |
|---|---|---|---|
| Trypsin | Arg↓X, Lys↓X (X≠Pro) | Yes (primary) | S1 pocket fits Arg/Lys |
| Proteinase K | Non-specific (all peptide bonds) | No preference | Broad substrate access |
| Thrombin | Arg↓Gly (fibrinogen-specific) | Yes | Extended specificity |
| Chymotrypsin | Phe/Tyr/Trp↓X; secondary Arg/Lys↓X | Yes (secondary) | Hydrophobic S1 pocket |
Biochemical Basis
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Trypsin family: Deep aspartate-lined S1 pocket accommodates protonated arginine guanidino group.
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Proteinase K: Shallow active site accepts diverse substrates; used for total protein digestion in DNA prep.
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Salt bridges and hydrogen bonding dictate arginine recognition in specific proteases.
This distinction appears frequently in biochemical engineering and microbiology curricula for protease applications in bioprocessing.
