Structure Function Relationship of Antibodies

Immunology

1. Immunology Structure Function Relationship of Antibodies By Ahmed Riyadh Abdul Rahman Al-Noor University College 1

2. Structure Function Relationship of Antibodies Antibodies possess both an antigen binding capacity and a biological activity . This bifunctional nature of antibodies was initially demonstrated by proteolytic cleavage (digestive) of antibody. 2

3. 1-Digestion of antibodies with papain yields three fragments, two copies of a single antigen binding region, F( ab ), and one readily crystallizable fragment (Fc) which cannot bind antigen. Papain cleaves immunoglobulin G molecules behind the hinge region (disulfide bond) which results in the generation of three ~50kDa fragments; two Fab fragments (~100kDa) and an Fc fragment (~50kDa), (total molecular weight of IgG molecule ~150 kDa ). 

4. 4 2-Digestion of antibodies with pepsin Digestion of immunoglobulins by pepsin occurs in front of hinge region (disulfide bond i.e. below the hinge region) of the heavy chains, The resulting F( ab ')2 fragment is comprised of two Fab units joined by disulfide of their heavy chains at the hinge region. F( ab ')2 possessing two antigen binding sites, that can bind two antigen molecules. In pepsin digests, the Fc portion is proteolytically degraded so it loses its biological activity .

5. Antibody IgG structure and cleavage sites for fragmentation by papain & pepsin

6. 6 Immunoglobulin Classes and Subclasses Immunglobulin molecules are divided into distinct classes and subclasses in terms of the differences in amino acid sequence of constant region of heavy chain, i.e. γ,α ,ε , μ, and δ.

7. 7 Immunoglobulin Classes of Mammals IgG – Gamma (γ) heavy chains. IgM – Mu (μ) heavy chains. IgA – Alpha (α) heavy chains. IgD – Delta (δ) heavy chains . IgE – Epsilon (ε) heavy chains.

8. 8 Immunoglobulin G ( IgG ) Immunoglobulin G, or IgG , is the major immunoglobulin in human serum, accounting for 70-80% of the total immunoglobulin in serum. IgG mostly found in serum and extracellular fluid for example in pleural fluid, Synovial fluid, Peritoneal fluid,…etc. IgG is a monomer consisting of identical pairs of H and L chains linked by disulfide bond. Each IgG has two antigen binding sites ( divalent ).

9. 9 Sedimentation coefficient of IgG is ( 7s ), and molecular weight about~150-180 kDa . There are four human IgG isotypes ( IgG1 , IgG2 , IgG3 , and I gG4 ); based on amino acid sequence differences in the H chain constant region and on the number and location of disulfide bonds. IgG is the only immunoglobulin that can cross the placenta in humans and protect the infant during the first months of life. IgG has a half – life of approximately (21) days .

10. 10 IgG is the predominant (the major) antibody in secondary immune responses and constitutes an important defense against bacteria (Antibacterial), viruses (antiviral) and parasite ( antiparasite ). IgG plays a major role in ( i ) opsonization by phagocytes; (ii) antibody-dependent cell mediated cytotoxicity (ADCC) by natural killer cells; (iii) complement activation; and (iv) neutralization of viruses and toxins(ѵ) agglutinating and heamagglutinating activity . IgG the most effective Ig in immunity IgG affected by the proteolytic enzyme.

11. 11 IgG plays a major role in opsonization by phagocytes

12. 12 IgG an important defense against viruses (antiviral activity)

13. 13 IgG plays a major role in antibody-dependent cell mediated cytotoxicity (ADCC) by natural killer cells

14. 14 neutralization of viruses and toxins complement activation

15. 15

16. 16 ANY QUESTION