Unit 2

• Structural Architecture of the Antigen-Binding Site (Paratope):
• Immunoglobulin Anatomy:
• An antibody monomer consists of two identical heavy (H) chains and two identical light (L) chains.
• The antigen-combining paratope is formed by the pairing of the variable domain of the heavy chain ($V_H$) and the variable domain of the light chain ($V_L$), located at the N-terminal ends of both heavy and light chains (Fab fragment).
• terminal regions form the constant Fc effector domain.
• Conclusion: N terminal side of heavy and light chain (Option 3).

Correct option: (3) N terminal side of heavy and light chain

• Post-Translational Phosphorylation Site on Caseins:
• Phosphoserine Residues:
• Caseins ($\alpha_{s1}, \alpha_{s2}, \beta$-casein) are phosphoproteins where orthophosphate groups are monoesterified to the hydroxyl group of serine residues (as O-phosphoserine).
• Phosphorylation occurs within the recognition sequence motif $\text{Ser-X-Glu/SerP}$.
• These phosphoserine clusters bind calcium ions, forming nanoclusters of colloidal calcium phosphate that stabilize casein micelles.
• Conclusion: Serine (Option 3).

Correct option: (3) Serine

• Role of Potassium Sulphate in Kjeldahl Nitrogen Digestion:
• Boiling Point Elevation:
• Concentrated sulfuric acid boils at $338^\circ\text{C}$, which is insufficient for rapid organic matter destruction.
• Adding potassium sulphate ($\text{K}_2\text{SO}_4$) raises the boiling point of the digestion mixture to $370 - 380^\circ\text{C}$, accelerating oxidative digestion without causing nitrogen volatilization losses.
• Conclusion: Increasing the boiling point of sulphuric acid (Option 1).

Correct option: (1) Increasing the boiling point of sulphuric acid