Immunoglobulins (Antibodies): Structure, Characteristics and Functions

In the previous post, we have discussed the introductory features of antibodies. There we have also discussed the reason for calling Antibodies (Ab) as Immunoglobulins (Ig).  In this lesson, we will see the detailed account on Immunoglobulin Structure and Function.

What are Immunoglobulins (Ig)?

As we discussed earlier, the Antibodies or Immunoglobulins are globular proteins present in the serum and tissue fluids. They are produced by the plasma cells (B-cells) and are used in the immune system of the body to neutralize pathogenic microbes or other toxic foreign components.

Antibodies play a very crucial role in the immune system of an organism. Antibodies bind to definite molecules of microbes called antigens with high affinity and specificity. This enables our immune system to detect foreign organisms such as invading pathogens, of its products and initiate the mechanism to eliminate these foreign particles. The production of antibodies by the plasma cells is also stimulated by the antigens.  

How Immunoglobulins (Ig) are classified?

The immunoglobulins constitute about 20 – 25% of the total serum proteins. Based on the Physiochemical and Antigenic differences, the immunoglobulins are classified into FIVE categories. These immunoglobulins variants are called as Isotypes. The five isotypes or classes of the immunoglobulins are given below.

(1).   Immunoglobulin-G (IgG)

(2).   Immunoglobulin-M (IgM)

(3).   Immunoglobulin-A (IgA)

(4).   Immunoglobulin-E (IgE)

(5).   Immunoglobulin D (IgD)

Structure of Immunoglobulins

Ø  The basic unit of a single immunoglobulin consists of four linear polypeptide chains.

Ø  These peptide chains are named as two identical Heavy Chains and two identical Light Chains.

Ø  The heavy chains are long and heavy with a molecular weight of 50 – 70 kDa.

Ø  The light chains are smaller and lighter in weight with a molecular weight of 25 kDa.

Ø  The heavy chains are designated as ‘H’ and the light chains are designated as ‘L’

Ø  Since an immunoglobulin contain two heavy (H) chains and two light (L) chains, they are together represented as H₂L₂.

Ø  H₂L₂ is the basic structural unit of any class (isotypes) of immunoglobulins.

Ø  Both H chains and L chains are connected through disulfide bonds.

Ø  Some antibodies are very complex as in Immunoglobulin M (IgM) which is a pentamer. In such case, the basic structural units will be H₂L₂ and they are multiplied in ‘n’ times (H₂L₂)_n.

Ø  The H chain and L chains are inter-connected and oriented in such ways that the individual immunoglobulin unit (H2L2) appears in the shape of ‘Y’ or ‘T‘.

Heavy Chain of Immunoglobulins

Ø  As we mentioned above, each immunoglobulin is with two Heavy (H) chains.

Ø  Each heavy chain is 420 – 440 amino acids long with free ‘N‘ and ‘C‘ terminals.

Ø  The two heavy chains are covalently connected to each other through 1 to 5 disulfide (S-S) bonds.

Ø  Each heavy chain is bound to a light (L) chain with a single disulfide bond and many non-covalent interactions such as salt bridges, hydrogen bonds and hydrophobic interactions.

Ø  The binding of heavy chains with the light chain creates a heterodimer (HL).

Ø  The interaction between two such heterodimers through disulfide bonds, hydrogen bonds and hydrophobic interaction create a tetramer (HL)2 or H2L2.

Ø  The H2L2 is thus the basic structure of an immunoglobulin.

Ø  The heavy chains are structurally distinct for each class (isotypes) of immunoglobulins.

Ø  They differ in their size, amino acid sequence, antigenicity and the carbohydrate content.

Ø  For example, the heavy chains of Immunoglobulin M (IgM) contains mu (µ), IgG contains gamma (γ), IgA contains alpha (α), IgE contains epsilon (ε) and IgD contains delta (δ) chains.

Light Chain of Immunoglobulins

Ø  As the name suggests, the light (L) chains are lighter than the heavy chains in their molecular weight.

Ø  Each light chain consists of 220 to 240 amino acids with free ‘N’ and ‘C’ terminals.

Ø  Each light chain is attached to the heavy chain by a single disulfide bond and many non-covalent interactions.

Ø  The light chains are structurally and chemically similar in all classes of immunoglobulins.

Ø  There are two types of light chains, named as kappa (κ) and lambda (λ) chains.

Ø  The kappa and lambda chain differ in their amino acid sequence in their constant regions (see below for constant and variable region of the immunoglobulin).

Ø  Each immunoglobulin is with either two kappa chains or with two lambda chains, never both.

Ø  In humans, 60% of the light chains are kappa and 40% is lambda.

Variable and Constant regions fof Heavy and Light Chains

Ø  Each polypeptide chain (H and L chain) of an immunoglobulin possesses two terminal (end) regions designated as N-terminal (amino terminal) and C-terminal (carboxy terminal).

Ø  In each chain, the amino terminal region is called Variable region (V region) and the carboxy terminal is called Constant region (C region).

Ø  Both the ‘H’ chain and ‘L’ chains contain V and C regions.

Ø  The V and C region consists of repeating units of structural units called Domains.

Ø  The variable region of the heavy chain is called ‘VH’ region and the constant region of heavy chain is called ‘CH’ region

Ø  Similarly, the variable and constant regions of light chains are designated as ‘VL’ and ‘CL’ respectively.

Ø  The sequencing studies of the variable region of the heavy chains revealed the existence of five different categories of immunoglobulins designated as µ, δ, γ, ε and α.

Ø  Each of these five different heavy chains is called an isotype.

Ø  The sequence of the heavy chain molecule determines the class of that antibody –  IgG (γ), IgM (µ), IgA (α), IgE (ε) and IgD (δ).

Ø  Each heavy chain contains one variable region (VH) and three or four constant (CH) regions.

Ø  IgG and IgA have three constant regions in the heavy chains, designated as CH1, CH2 and CH3.

Ø  The heavy chains IgM and IgE have four constant regions designated as CH1, CH2, CH3 and CH4.

Ø  Each light chain consists of one variable (VL) and one constant (CL) region.

Ø  The variable region is different in each class of immunoglobulins.

Ø  The variable region of both H chain and L chains consist of 100 – 110 amino acids.

Ø  The variable regions of both heavy and light chains consist of three highly variable regions called Hyper Variable Regions.

Ø  The antigen binding site of immunoglobulin (called Fab) is located in this hyper-variable region.

Ø  The ‘Fab’ region consists of a short stretch of only 5 – 10 amino acids.

Ø  The hyper variable region is responsible for the high specificity of immunoglobulins towards specific antigens.

Ø  All immunoglobulins are glycoproteins (proteins conjugated with oligosaccharide units).

Ø  The site of attachment of carbohydrates in antibodies is restricted to the constant regions.

Ø  The exact role of carbohydrate regions in the immunoglobulins is not fully known. They may be helping in increasing the solubility of antibodies in the body fluids.

Ø  The constant (C) region consists of two basic amino acid sequences.

Ø  The ‘Fc’ fragment is located in the constant region of the heavy chain.

Ø  The Fc region crystallizes under low ionic conditions.

Details of Fab and Fc regions will be discussed in the next post

Ø  The constant regions of the heavy chains have much biological significance such as activation of complement, binding of cell surface receptors and placental transfer.

Ø  The constant region of the light chain has no biological functions.

Hinge Region of Immunoglobulin

Ø  The hinge region is a highly flexible amino acid stretch present in between the CH1 and CH2 region of heavy chains of some immunoglobulin isotypes (IgG, IgA and IgD).

Ø  The hinge region is absent in IgM and IgE.

Ø  The heavy chains of IgM and IgE possess additional hinge-like domain with approximately 110 amino acids.

Ø  The hinge region connects the two heavy chains with many disulfide bonds.

Ø  The hinge region is rich in cysteine and proline amino acids.

Ø  The number of disulfide bonds in the hinge region varies with different immunoglobulin isotypes.

References

(1). Goldsby, R.A., Kindt, T.J., Osborne, B.A. and Kuby, J., Immunology. 2003. NY, USA.

(2). Parija, S.C., Textbook of Microbiology & Immunology. 2014. Elsevier Health Sciences.

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