Q.47 When the molecular weight of human immunoglobulin light chain is 24 kDa, the total molecular
weight of human IgG is ___ kDa.Human IgG consists of two heavy chains and two light chains, with a total molecular weight of 148 kDa when each light chain is 24 kDa. This calculation accounts for standard heavy chain weights of approximately 50 kDa each. The structure forms a Y-shaped tetramer essential for immune function.
IgG Structure Basics
Immunoglobulin G (IgG) is the most abundant antibody in human serum, forming a tetrameric protein with two identical heavy chains (γ chains) and two identical light chains (κ or λ). Each heavy chain weighs about 50 kDa, while light chains are typically 23-25 kDa; here, they are specified as 24 kDa each. Disulfide bonds link the chains into a Y-shaped molecule that binds antigens effectively. The total mass is calculated as (2 × 50 kDa) + (2 × 24 kDa) = 100 + 48 = 148 kDa.
Correct Answer Explanation
The total molecular weight of human IgG is 148 kDa. This derives from standard biochemistry: heavy chains at 50 kDa each contribute 100 kDa, and the given light chains at 24 kDa each add 48 kDa. Sources often approximate to 150 kDa using 25 kDa light chains (50+50+25+25=150), but with 24 kDa, it adjusts precisely to 148 kDa. This value aligns with IgG1, the primary human subclass.
Common MCQ Options Breakdown
Typical multiple-choice options for this question test understanding of IgG stoichiometry:
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148 kDa (Correct): Matches 2H (100 kDa) + 2L (48 kDa), as per the query.
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150 kDa (Common distractor): Uses rounded 25 kDa light chains; close but inaccurate here.
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100 kDa (Incorrect): Only heavy chains; ignores light chains essential for structure.
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196 kDa (Incorrect): Assumes ~60 kDa heavy chains (as in some IgM subunits); wrong for IgG.
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72 kDa (Incorrect): Only light chains multiplied erroneously; ignores tetrameric form.
Biological Relevance
IgG’s 148-150 kDa size enables serum circulation and placental transfer, crucial for immunity. Variations occur across isotypes (e.g., IgG3 at 170 kDa due to extended hinge), but standard human IgG1 fits this calculation. For researchers in molecular biology, SDS-PAGE confirms these masses under reducing conditions.
