Q.92 If a heavy chain of an antibody molecule weighs 65,000 Daltons (ππ) and a light chain weighs
ππ, πππππ, the approximate calculated weight of an IgM antibody in Da will be
(A) 90,000
(B) 180,000
(C) 360,000
(D) 900,000
Calculate the exact molecular weight of IgM antibodies using heavy and light chain values for exam success.
IgM antibodies feature a pentameric structure that multiplies basic monomer weights. Given heavy chain at 65,000 Da and light chain at 25,000 Da, the calculation yields 900,000 Da.β
Antibody Monomer Basics
Standard antibodies contain two heavy chains and two light chains. One IgM monomer weighs (2 Γ 65,000) + (2 Γ 25,000) = 130,000 + 50,000 = 180,000 Da.β
IgM exists primarily as a secreted pentamer with five such monomers linked by disulfide bonds and a J-chain. This yields 5 Γ 180,000 = 900,000 Da, matching biological data around 900-970 kDa.β
Option Analysis
Examine each choice against IgMβs pentameric formula:
| Option | Weight (Da) | Explanation |
|---|---|---|
| (A) 90,000 | Too low | Matches one heavy + one light chain (65k+25k); ignores full monomer or pentamer.β |
| (B) 180,000 | Monomer only | 2H + 2L = 180kDa; correct for single unit but not secreted IgM pentamer.β |
| (C) 360,000 | Partial multimer | Suggests dimer (2Γ180k); IgM forms pentamers, not dimers.β |
| (D) 900,000 | Correct | 5 monomers (5Γ180k=900kDa); matches pentameric IgM structure.β |
Why IgM Pentamer Matters
IgMβs large size enables complement activation and high avidity despite lower affinity per site. Membrane-bound IgM on B cells is monomeric, but the question targets calculated secretory form.β
This structure suits NEET biology prep on immunoglobulins.β
