Question

Which of the following domain of antibodies act as antigen binding site?

• A. N-terminal domain of H-Chain alone
• B. C-terminal domain of L-Chain alone
• C. N-terminal domain of H and L-Chains combined
• D. C-terminal domain of H and L-Chains combined

Hint:

The "N-terminus" of the antibody is the tip of the "Y" arms.
Both heavy and light chains must contribute their variable domains to form the functional binding pocket.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
An antibody monomer is a Y-shaped molecule composed of two identical heavy chains and two identical light chains.
The functional regions of the antibody are divided into the antigen-binding fragments (Fab) and the crystallizable fragment (Fc).

Step 2: Detailed Explanation:
Each light and heavy chain has an amino-terminal (N-terminal) variable region and a carboxyl-terminal (C-terminal) constant region.
The N-terminal regions of both the heavy ($V_{H}$) and light ($V_{L}$) chains show high variability in their amino acid sequences.
Within these variable domains are hypervariable loops known as complementarity-determining regions (CDRs).
When the heavy and light chains pair, their N-terminal variable domains fold together to form the antigen-binding pocket (the paratope).
This combined pocket interacts with the specific epitope of an antigen.
The C-terminal domains of the light and heavy chains form the constant framework and the Fc stem, which mediates effector functions like complement activation but does not bind antigens.

Step 3: Final Answer:
The antigen-binding site is formed by the combination of the N-terminal variable domains of both the heavy and light chains.