Structure Of Immunoglobulin
Introduction
- Constitute the gamma globulin portion of blood proteins
- Are soluble proteins secreted by activated B cells and plasma cells in response to an antigen
- Are capable of binding specifically with that antigen
Function
- Bind its antigen
- Direct it towards other components of the Immune System so that it can be destroyed.
Immunoglobulin molecule:
- Represents hetero-tetrameric quarternary structure between 4 polypeptide (2 heavy + 2 light) chains
- Y-shaped
- VL = Variable domain of light chain.
- CL = Constant domain of light chain.
- VH = Variable domain of a heavy chain
- CH = Constant domain of a heavy chain
- S-S = Disulphide bond.
CONSTANT REGION/CARBOXY TERMINUS
- Constant unit
- Fc fragment of Ig
Determines the biological properties of Ig:
- Complement fixation
- Placental transfer
- Skin fixation
- Catabolic rate
Amino terminus/Variable unit/FAB/ANTIGEN BINDING SITE
- Antigen binding site
- Composed of both L and H chains
Highly variable zones
- Hypervariable regions or Hotspot
- Involved in the formation of antigen-binding sites.
- Sites on the hypervariable regions which make actual contact with the epitope
- Are called complementarity determining regions or CDR’s
Heavy chain
- Have high molecular weight i.e 50,000-77,000d
- Composed of 450 amino acid
- Class specific antigenic determinants of an immunoglobulin is associated with H-chain
- H chain consist of 1 variable (VH) and three domains in constant region (CH1, CH2,CH3)
- CH2 region in IgG & IgM binds C1q in the classical complement sequence
- CH3 domain mediates monocyte adherence
|
Immunoglobulin class |
Heavy chain |
|
IgG |
γ (gamma) |
|
IgA |
α (alpha) |
|
IgM |
μ(mu) |
|
IgD |
δ(delta) |
|
IgE |
ε (epsilon) |
Light chain
- Molecular weight 25000
- Composed of 212 amino acid
- Light chain further divides into either kappa or lambda.
- Immunoglobulin may contain either kappa or lambda but not both
- L chain consists of 1 variable VL and 1 constant domain C1
Disulfide bond
- Light and heavy chain are joined to each other by disulfide bond
Hinge region
- The area of the H chain in the C region between the first and second C region domains (CHI and CH2).
- Papain acts in the hinge region
- Splits each Ig molecule into 3 parts: 1 Fc and 2 Fab
- Pepsin digests Fc portion, resulting in a 5S portion – 2 Fab fragments held together: F(ab)2
Exam Question
STRUCTURE OF ANTIBODIES:
Immunoglobulin molecule:
- Represents hetero-tetrameric quarternary structure between 4 polypeptide (2 heavy + 2 light) chains
CONSTANT REGION/CARBOXY TERMINUS
- Fc fragment of Ig
Determines the biological properties of Ig
- Complement fixation
- Placental transfer
- Skin fixation
- Catabolic rate
Amino terminus/Variable unit/FAB/ANTIGEN BINDING SITE
- Antigen binding site
Heavy chain
- have high molecular weight i.e 50,000-77,000d
- Composed of 450 amino acid.
- Class specific antigenic determinants of an immunoglobulin is associated with H-chain
- H chain consist of 1 variable (VH) and three domains in constant region (CH1, CH2,
CH3
Highly variable zones
- 3 in L and 4 in H chain
- Involved in the formation of antigen-binding sites.
- Sites on the hypervariable regions which make actual contact with the epitope
- Are called complementarity determining regions or CDR’s
- CH2 region in IgG & IgM binds C1q in the classical complement sequence
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