Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
Which one of the combinations of the following statements is true about antibody structure?
P. Limited proteolysis of rabbit IgG with the enzyme pepsin generates two antigen-binding
regions (Fab) and an Fc fragment
Q. Limited proteolysis of rabbit IgG with the enzyme papain generates a single bivalent
antigen-binding region F(ab)2 and peptide fragments
R. The Fc fragment of IgG can self-associate and crystallize into a lattice
S. The F(ab)2 fragment of IgG is composed of both light and heavy chains
Options:
(A) P and Q only
(B) P and R only
(C) R and S only
(D) Q and S only
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.42)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Immunology (Antibody Proteolysis & Structural Organization)
**2. Detailed Step-by-Step Analysis**:
- Let us evaluate each statement concerning IgG immunoglobulin structure and enzymatic cleavage:
- **Statement P**: *Limited proteolysis of rabbit IgG with the enzyme pepsin generates two antigen-binding regions (Fab) and an Fc fragment.*
- **FALSE**: Cleavage with **papain** (above the hinge disulfides) produces two separate univalent **Fab** fragments and one **Fc** fragment. Cleavage with **pepsin** (below the hinge disulfides) produces a single bivalent **$F(ab')_2$** fragment and degrades the Fc region into smaller peptides ($pFc'$). Statement P confuses pepsin with papain.
- **Statement Q**: *Limited proteolysis of rabbit IgG with the enzyme papain generates a single bivalent antigen-binding region $F(ab')_2$ and peptide fragments.*
- **FALSE**: As noted above, this describes **pepsin** digestion, not papain digestion.
- **Statement R**: *The Fc fragment of IgG can self-associate and crystallize into a lattice.*
- **TRUE**: In fact, the name **Fc** historically stands for **Fragment crystallizable** because Rodney Porter discovered that this homogeneous fragment readily crystallized upon cold storage.
- **Statement S**: *The $F(ab')_2$ fragment of IgG is composed of both light and heavy chains.*
- **TRUE**: $F(ab')_2$ contains two intact light chains ($V_L + C_L$) joined by interchain disulfide bonds to two heavy chain fragments ($V_H + C_H1$ plus the hinge region holding them together).
- Therefore, statements **R and S only** are true.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Statements P and Q invert the definitions of papain and pepsin cleavage.
- **Option (B) [INCORRECT]**: P is false.
- **Option (C) [CORRECT]**: R (Fc fragment readily crystallizes) and S ($F(ab')_2$ contains both light and heavy chains) are completely true.
- **Option (D) [INCORRECT]**: Q is false.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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