4. When the molecular weight of human immunoglobulin light chain is 24 kDa, the total molecular weight of human IgG is _____ kDa.
Molecular Weight of Human IgG
Introduction
Immunoglobulins, commonly known as antibodies, are specialized glycoproteins produced by plasma cells in response to foreign antigens. They are the principal effector molecules of humoral immunity and play an essential role in neutralizing pathogens, activating the complement system, promoting opsonization, and preventing infections. Among the five major classes of immunoglobulins—IgG, IgM, IgA, IgD, and IgE—IgG is the most abundant antibody in human serum, accounting for nearly 75–80% of circulating immunoglobulins.
Every IgG molecule has a characteristic Y-shaped structure composed of two identical heavy (H) chains and two identical light (L) chains connected through disulfide bonds.
Correct Answer: 150 kDa
Step-by-Step Solution
An IgG molecule consists of:
- Two identical heavy chains (H chains)
- Two identical light chains (L chains)
The molecular weight of one light chain is given as:
Light chain = 24 kDa
The molecular weight of one heavy chain in human IgG is approximately:
Heavy chain = 51 kDa
Therefore, the total molecular weight of IgG is:
Total Molecular Weight = (2 × Heavy Chain) + (2 × Light Chain)
= (2 × 51) + (2 × 24)
= 102 + 48
= 150 kDa
Thus, the molecular weight of a complete human IgG molecule is 150 kDa.
Detailed Explanation
Human IgG is a tetrameric glycoprotein composed of four polypeptide chains held together by interchain disulfide bonds. Two heavy (γ) chains form the central framework of the antibody, while two light chains are attached laterally. Each light chain pairs with one heavy chain to form an antigen-binding arm.
The heavy chains contain one variable domain and three constant domains, whereas each light chain contains one variable domain and one constant domain. Together, these domains create two identical antigen-binding sites that allow IgG to recognize specific antigens with high affinity.
The molecular weight of an IgG molecule is approximately 150 kDa, making it the smallest antibody capable of crossing the placenta and providing passive immunity to the developing fetus.
Calculation Formula
Total Molecular Weight of IgG = (2 × Heavy Chain) + (2 × Light Chain)
Given:
Heavy Chain = 51 kDa
Light Chain = 24 kDa
Total = 2 × 51 + 2 × 24 = 150 kDa
Structure of Human IgG
| Component | Number Present | Molecular Weight | Total Contribution |
|---|---|---|---|
| Heavy Chains | 2 | 51 kDa each | 102 kDa |
| Light Chains | 2 | 24 kDa each | 48 kDa |
| Total IgG | 4 Chains | — | 150 kDa |
Explanation of the Components of IgG
Heavy Chains
The heavy chains determine the class of the immunoglobulin. In IgG, the heavy chains are gamma (γ) chains. They contain the Fc region responsible for complement activation, binding to Fc receptors, placental transfer, and interaction with immune cells.
Light Chains
Each IgG molecule contains two identical light chains, which may be either kappa (κ) or lambda (λ). Light chains contribute to antigen recognition by forming the variable antigen-binding region together with the heavy chains.
Antigen-Binding Region
The variable domains of one heavy chain and one light chain combine to form one antigen-binding site. Since IgG possesses two heavy chains and two light chains, each molecule contains two identical antigen-binding sites.
Major Functions of IgG
| Function | Biological Role |
|---|---|
| Neutralization | Blocks toxins and viruses |
| Opsonization | Enhances phagocytosis by macrophages and neutrophils |
| Complement Activation | Activates the classical complement pathway |
| Placental Transfer | Provides passive immunity to the fetus |
| ADCC | Facilitates antibody-dependent cellular cytotoxicity |
Comparison of Human Immunoglobulin Classes
| Immunoglobulin | Approximate Molecular Weight | Main Function |
|---|---|---|
| IgG | 150 kDa | Secondary immune response and placental immunity |
| IgA | 160 kDa (monomer) | Mucosal immunity |
| IgM | 900 kDa | Primary immune response |
| IgD | 180 kDa | B-cell receptor |
| IgE | 190 kDa | Allergic reactions and parasite defense |
Biological Significance of IgG
IgG is the most abundant antibody in human blood and extracellular fluids. It provides long-lasting protection against bacterial, viral, and fungal infections. Its ability to cross the placenta makes it the only immunoglobulin class capable of providing natural passive immunity to the fetus. IgG also plays central roles in neutralizing toxins, activating complement, promoting phagocytosis, and mediating antibody-dependent cellular cytotoxicity. Because of these diverse functions, IgG is widely used in immunotherapy, monoclonal antibody production, vaccine evaluation, and clinical diagnostics.
Final Answer
Correct Answer: 150 kDa
Human IgG contains two heavy chains (approximately 51 kDa each) and two light chains (24 kDa each). Therefore, the total molecular weight of IgG is:
(2 × 51) + (2 × 24) = 102 + 48 = 150 kDa.
Hence, the molecular weight of human IgG is 150 kDa.
