Antibody – Structure Overview

Antibody – Structure Overview

In this post we will discuss the structure of antibody. We will try and understand all the terms related to antibody structure and why are they named so. Let’s begin 🙂

Antibody = is a globular protein which is synthesized by plasma B-cells in respond to foreign antigen.

Because antibody is globular protein with immune function, it is also known as Immunoglobulin.

So when we talk about the structure of antibody, this Y looking figure (figure a) comes in our mind. This figure (a) is the typical structure of an antibody. Now we will see each of the labels one by one and see why are they named so.

Figure a – Structure of Antibody

1. Heavy chain and Light chain – antibody is made up of two heavy chains and two light chains.Why are they named so – because of their molecular weight. That means heavy chain is more in molecular weight than light chains. Each heavy chain weighs 50 kD (kilodalton) whereas each light chain weighs 25 kD (kilodalton).Therefore, molecular weight of one single molecule of antibody = 150 kD (50 kD + 50 kD + 25 kD + 25 kD, because as mentioned before each antibody is made up of two heavy chains and two light chains). This is for one single molecule of antibody which means if you talk about IgA or IgM, molecular weight will be more because IgA is a dimer and IgM is pentamer.Types of heavy chains are µ, γ, α, δ and ε and light chains are κ and λ. Any of these heavy and light chains will be present in different types of antibody. The class of antibody is determined based on type of heavy chain present in it.That means heavy chain µ gives IgM, heavy chain γ gives IgG, α gives IgA, δ gives IgD and ε gives IgE. Let me tell you guys how to easily remember which heavy chain gives what antibody. Just look at the first letter of each heavy chain and there is the answer.Therefore, µ – Mu –IgM,γ – Gamma – IgG,α – Alpha – IgA,δ – Delta – IgD,ε – Epsilon – IgE
2. Variable and Constant region –
   As you can see in figure (a) the top most subunits in heavy as well light chains are labeled as V_H and V_L respectively.
   Why are they named so – because of the high variability of amino acid sequence present in these regions. Therefore, V_H is variable heavy chain and V_L is variable light chain. Similarly, other subunits are labeled as C_H and C_L meaning constant heavy chain and constant light chain because there is no much change in the amino acid sequence.
3. Antigen Binding site –
   It is present in the variable region. It is the specific amino acid sequence present in the variable region which can recognize the antigen and are called Complementarity Determining Region (CDR). CDR is also called Paratope. You must have read in immunology books that paratope of antibody binds to an epitope of antigen (epitope is part of antigen which can be recognized by paratope).
4. Glycosylation site –
   There is no clear reason known for sites of glycosylation in antibody but one of the possible reasons says that as we know antibody is protein compound and it might contain hydrophobic amino acids which decreases its solubility in blood plasma. And we know that glycosylation increases the protein solubility so antibody solubility increases in blood plasma.
5. Fab and Fc region
   Why are they named so – when the antibody structure was studied, it was digested by different enzymes which digested the antibody near hinge region giving two subunits. Upper subunit was named as Fab – Fragment antigen binding because this was the portion of antibody which binds to an antigen. Lower subunit was named as Fc – Fragment crystalizable and it is named so because when this fragment was stored it started crystalizing.
6. Hinge region
   It gives the flexibility to antibody so that it can bind with an antigen in the best way possible.

So, these are the points we should know when we talk about the structure of antibody.

I hope this blog was helpful 🙂

 
Watch a video on this topic here.