Dairy enzyme specificity: Chymosin (rennin) specifically cleaves κ-casein at which peptide bond during cheese making?

Introduction:
Cheese manufacturing relies on precise proteolysis to destabilize casein micelles. Chymosin, historically called rennin, is highly specific for a single bond in κ-casein that triggers curd formation. This question evaluates recognition of that signature cleavage site.

Given Data / Assumptions:

• Substrate: κ-casein on the micelle surface.
• Outcome: micelle destabilization and curd formation.
• Need: exact peptide bond identity.

Concept / Approach:
Chymosin cleaves the peptide bond between Phe105 and Met106 in κ-casein, generating para-κ-casein and the glycomacropeptide. This removes steric and electrostatic stabilization, allowing casein aggregation in the presence of calcium ions.

Step-by-Step Solution:
Identify the protein and site: κ-casein Phe105–Met106.Describe the result: release of glycomacropeptide into whey.Explain the functional consequence: curd formation from aggregated casein micelles.

Verification / Alternative check:
Cheese process analytics show glycomacropeptide in whey as a marker of chymosin action, validating the specific cleavage site.

Why Other Options Are Wrong:
Alanine–Glycine, Glutamic acid–Alanine, Alanine–Phenylalanine: these are not the known κ-casein cleavage by chymosin and would not reproduce the characteristic micelle destabilization kinetics.

Common Pitfalls:

• Confusing general chymotrypsin aromatic preferences with the unique κ-casein site of chymosin.
• Assuming multiple bonds are equivalently cleaved during renneting.

Final Answer:
Phenylalanine–Methionine (Phe105–Met106) in κ-casein