Antibodies are proteins with around 150 kDa molecular weight. They have a similar basic structure comprising of four polypeptide chains held together by disulfide bonds. These four polypeptide chains form a symmetrical molecular structure. There is a hinge in the center between heavy chains to allow flexibility to the protein. There are:
- Two light chains – Containing around 220 amino acids
- Two heavy chains – Containing around 440 amino acids.
Light and heavy chains
There are two types of light chain among all classes of immunoglobulin, a lambda chain and a kappa chain. Both are similar in function. Each type of immunoglobulin has a different type of heavy chain. Depending on the heavy chains they are classified into five classes.
Other components of antibodies
Apart from amino acids there are sugar molecules as well. Thus antibodies are glycoproteins rather than proteins alone. The immunoglobulins exists as monomers (e.g. only one Ig unit) or as dimmers (two molecules. E.g. IgA) or as tetramers (four molecules e.g. teleost fish IgM) or exists as pentamers (five molecules e.g. in mammalian IgM)
Fragments of Immunoglobulin
Immunoglobulin fragments produced by proteolytic digestion by enzymes are the basis of study of structure/function relationships.
When Ig are broken down with papain it breaks at the hinge region before the H-H inter-chain disulfide bond; this results in the formation of two identical fragments that contain the light chain and the VH (Variable Heavy chain) and CH1 (Constant heavy chain) domains of the heavy chain. These fragments are then called Fab since they contained the antigen binding sites of the antibody. Each Fab fragment is monovalent.
Digestion with papain also leads to formation of a fragment that contains rest of the two heavy chains each containing a CH2 and CH3 domain. This fragment is easily crystallized.
When digested with pepsin, the Igs are cleaved at the heavy chain after the H-H inter-chain disulfide bonds. The resulting fragments contain both antigen binding sites. This fragment was called F(ab')2 because it is divalent. This can bind to the antigens but does not lead to effector functions.
Domains of the immunoglobulin
The Ig monomer is a "Y"-shaped molecule. It has four polypeptide chains - two identical ''heavy chains'' and two identical ''light chains''. There are five types of mammalian Ig heavy chain denoted by the Greek letters: α, δ, ε, γ, and μ. These form respectively IgA, IgD, IgE, IgG and IgM.
The Ig has a paratope at the amino terminal end of the antibody monomer. This exists at the variable domains from the heavy and light chains. The variable domain is the FV region and is the most important region for binding to antigens. At this region are variable loops of β-strands. On the light chain are three loops - VL and on the heavy chain are three loops VH. These are responsible for binding to the antigen. These loops are referred to as the complementarity determining regions (CDRs). These CDRs are also called idiotypes.
The base of the Y modulates the immune cell activity. This region is called the ''Fc (Fragment, crystallizable) region''. In this area are two heavy chains that contribute two or three constant domains depending on the class of the antibody.