G. Mathew Srirangam
Antibodies are glycoproteins that bind antigens with high specificity and affinity (they hold on tightly). They are molecules, originally identified in the serum, which are also referred to as ‘immunoglobulins’; a term often used interchangeably with antibodies. In humans there are five chemically and physically distinct classes of antibodies – IgG, IgA, IgM, IgD, IgE). Immunoglobulins are synthesized by plasma cells and to some extent by lymphocytes also. All antibodies are immunoglobulins but all immunoglobulins may not be antibodies. Immunoglobulins constitute 20-25 percent of the total serum proteins.
Basic Structure of Immunoglobulin:
Immunoglobulins when digested by papain in the presence of cysteine, will split into two fractions – an insoluble fraction which crystallized in the cold called – Fc (for crystallizable) fragment, and a soluble fragment which is unable to precipitate and is called – Fab (for antigen binding) fragment. Each molecule of immunoglobulin is split by papain into three parts, one Fc and two Fab pieces. When treated with pepsin, a 5S fragment is obtained, which is composed essentially of two Fab fragments held together in position. This fragment is called F(ab)2. The Fc portion is digested by pepsin into smaller fragments.
Immunoglobulins are glycoproteins, each molecule consisting of two pairs of polypeptide chains of different sizes. The smaller chains are called ‘light’ (L) chains and larger ones ‘heavy’ (H) chains. The L chain has a molecular weight of approximately 25000 and the H chain of 50,000. The L chain is attached to the H chain by a disulfide bond. The two-H chains are joined together by 1-5 S-S bonds, depending on the class of immunoglobulins.
The H chains are structurally and antigenically distinct for each class and are designated by the Greek letter corresponding to the immunoglobulin class, like –
The L chains are similar in all classes of immunoglobulins. They occur in two varieties kappa ( k ) and lambda ( l ). A molecule of immunoglobulin may have either kappa or lambda chains, but never both together.
The antigen-combining site of the molecule is at its aminoterminus. It is composed of both L and H chains. Of the 214-aminoacid residues that make up the L chain, about 107 that constitute the carboxyterminal half occur only in a constant sequence. This part of the chain is therefore called the ‘constant region’. Only two sequence patterns are seen in the constant region- those determining the kappa and lambda specificities. On the other hand the aminoacid sequence in the aminoterminal half of the chain is highly variable, the variability determining the immunological specificity of the antibody molecule. It is therefore called the ‘variable region’. The H chain also has ‘constant’ and ‘variable’ regions. While in the L chain the two regions are of equal length, in the H chains the variable region constitutes approximately only a fifth of the chain and is located at its aminoterminus. The infinite range of the antibody specificity of immunoglobulins depends on the variability of the aminoacid sequences at the ‘variable regions’ of the H and L chains, which form the antigen combining sites.
The aminoacid sequences of the variable regions of the L and H chains are not uniformly variable along their length but consist of relatively invariable and some highly variable zones. The highly variable zones numbering three in the L and four in the H chains are called hypervariable regions or hot spots and are involved with the formation of the antigen binding sites.
The Fc fragment is composed of the carboxyterminal portion of the H chains. It does not possess antigen-combing activity but determines the biological properties of the immunoglobulin molecule. The portion of the H chain present in the Fab fragment is called the Fd piece.
Each immunoglobulin peptide chain has internal disulfide links in addition to interchain disulfide bonds, which bridge the H and L chains. These intrachain disulfide bonds form loops and each of the loop is compactly folded to form a globular domain, each domain having a separate function. The variable region domains, VL and VH are responsible for the formation of a specific antigen-binding site. The area of the H chain in the C region between the first and second C region domains (CH1 and CH2) is the hinge region. It is more flexible and is more exposed to enzymes and chemicals.