Question

If an antibody against the malarial antigen is of IgM type. Its heavy chain has a molecular weight of 50,000 Da and light chain has a molecular weight of 25,000 Da. Then what would be the molecular weight of the whole antibody molecule?

• A. 75,000 Da
• B. 1,50,000 Da
• C. 3,00,000 Da
• D. 7,50,000 Da

Answer 1

The correct option is D 7,50,000 Da

IgM is a pentamer with 5 immunoglobulin (Ig) subunits.
Structure of a single subunit of $Ig\to H_2{L}_2$

MW of Ig = $2\times MWofHchain$ + $2\times MWofLchain$
$=2\times 50,000+2\times 25,000$
$=1,50,000Da$

MW of IgM = 5 $\times$ MW of Ig subunit
= 5 $\times$ 1,50,000 Da
= 7,50,000 Da