Antibody structure- Structure of Antibody, H and L chains, Domain formation, Hinge region, Disulphide bond formation etc are explained.

1. STRUCTURE
OF
ANTIBODY (Ab)
Dr C R Meera, M Sc; Ph D,
Assistant Professor & HOD,
Department of Microbiology,
St. Mary’s College,
Thrissur-20, Kerala, India.

2. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
What are Antibodies (Abs)?
 Proteins that recognize and neutralize
microbial toxins and microbes
 B Lymphocyes- Only cells that produce
Abs
 Abs- Exist in two forms
1. Membrane Immunoglobulin on B cell
surface
2. Pathogen capturers in blood and
connective tissue GIF Image Courtesy: MakeAGIF.com

3. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
5 classes of Antibodies
They differ in
 Molecular size
 Structure
 Charge
 Aminoacid
composition
 Carbohydrate content

4. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Immunoglobulin structure studies
Alfred Nisonoff
Nobel Prize (1972)

5. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
• Papain cleave Ab
into 2 Fab and 1 Fc
portion
• Pepsin cleave Ab
into F(ab’) 2 and
fragmented Fc

6. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Structure of Antibody
 Glycoproteins with 4 polypeptide
chains
 A pair of Light (L) Chains
(25,000 MW)
 A pair of Heavy (H) Chains
(50,000 MW)
 L chain attached to H chain by a
disulphide bond
 H chains held together by 1-5
disulphide bonds
 Disulphide bonds are formed
between Cysteine residues
Light Chain
Heavy Chain
Disulphide bonds

7. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
H Chains
 Structurally & antigenically distinct
Immunoglobulin Class H Chain
IgG γ chain
IgA α chain
IgM µ chain
IgD δ chain
IgE ε chain

8. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
L Chains
 Similar in all classes of Ig
 2 Types of L chains
 Kappa (k) chain - Described by Korngold
 Lamda (λ) chain - Described by Lapari
 k & λ occur at a ratio 2:1 in human sera
Light
Chain
Heavy
Chains

9. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
• Ig has an Amino terminus & a
Carboxy terminus
• Amino terminus is the Antigen (An)
binding site
• Both L & H chains have a Constant
(C) region and a Variable (V) region
• Antigen binding site = V region of H
chains + V region of L chains
• Also called Fab Portion

10. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Constant (C) region and Variable (V) region
Light Chain
214 amino acid residues
107 in carboxy terminus 107 in amino terminus
Constant sequence Highly variable
Called “Constant region” (CL) Called “Variable region” (VL)
Determine type of chain Determine immunological
specificity

11. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Constant region and Variable region
Heavy Chain
• Variable (VH) & Constant (CH)
regions are not equal
• VH - 110 amino acid residues in
amino terminus (1/5 th of H chain)
• Antigen binding site: VH + VL
• Carbohydrate moieties are
attached to CH region

12. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Hypervariable Regions or Hot Spots
Image courtesy: Slideplayer.com
• Highly variable amino acid
sequences in variable regions
• Responsible for variations in
paratopes
• 3 each in H and L chains
• Makes actual contact with epitope
• Hence called “ Complementarity
Determining Regions” or CDRs
• Invariable regions in between
Hyper variable regions are called
“Frame work Regions (FRs)”

13. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
FC Portion
• Carboxy terminal with only constant region of
H Chains
• Determines biological properties of Ig
• Complement Fixation , Placental Transfer, Skin Fixation &
Catabolic Rate
• Portion of H chain in Fab fragment – Fd piece
• Carbohydrate moiety in H chain of FC Portion
• Proline in H chain – site of action of proteolytic
enzymes
Carbohydrate moiety

14. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Interchain & Intrachain disulphide bonds
Image Courtesy: Slideshare.net
Interchain
disulphide bond
Intrachain
disulphide bond
• Disulphide bonds form between
Cysteine residues
• Between chains- Inter chain
• With in chains- Intra chain
• Intra chain bonds produce loops –
“Globular domains”
• Each domain consist of 60 amino acids
lying in the homologous units (110
aminoacids) of H & L chains

15. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Globular domains
Image Courtesy: Slideshare.net
Interchain
disulphide bond
Intrachain
disulphide bond
• L Chain- 2 Domains – VL & CL
• H Chain- 4 to 5 Domains - CH1, CH2, CH3
& CH4
• Domains form compact structures called
“Immunoglobulin fold”
• Domains are anti-parallel β pleated
sheets
• Each domain has separate function
• CDRs are located with in domains

16. Structure of Antibody; Dr C R Meera, St Mary’s College, TCR, Kerala, India
Hinge Region
Hinge Region
• Area of H chain in the constant region between
CH1, CH2 domains
• More flexible
• More exposed to enzymes and chemicals
• Rich in Proline residues making vulnerable to
enzymes
• Cysteine residues of Hinge region – Inter chain S-
S bond between H chains
• Present in only γ, α, δ chains
• IgM & IgE – Lack hinge region, but have
additional domain