Brief Introduction to Antibody Structure

The central dogma of molecular biology is that DNA codes for the amino acid sequence of the protein chain. This precise amino acid sequence contains the information needed for the protein to fold into its complicated three-dimensional structure which in turn determines the function of the protein. The purpose of antibodies is to rid the organism of non-self molecules or cells (antigens) and they do this by recognising and binding to markers and carrying out effector functions which activate the immune system of the organism in various ways. These functions of antibodies are reflected in their structure.

The basic structure of all antibody or immunoglobulin (Ig) molecules consists of 4 protein chains shaped like a capital letter "Y" and linked by disulphide bonds. There are two pairs of chains in the molecule: heavy and light. There are two classes (isotypes) of the light chain called kappa and lambda. Heavy chains have five different isotypes which divide the Igs into five different classes, each with different effector functions (in humans IgG1-4, IgA1-2, IgD, IgM, IgE). Each class of heavy chain can combine with either of the light chains, except in camelids (camels and llamas) whose antibodies sometimes lack light chains.

The most common immunoglobulin class is IgG. Its basic monomer structure is shown in Figure 1. Each chain is divided into regions or domains consisting of around 110 amino acid residues. The light chain has two domains and the heavy has four. The N-terminal domain at the tip of the arms of the "Y" on both the heavy and light chain are known to be variable in amino acid sequence composition and are thus called variable domains (VL and VH). The other domains are called constant for a similar reason (CL, CH1, CH2, CH3).

The variable domains show three regions of hypervariability in sequence called the complementarity determining regions (CDRs). They differ in length and sequence between different antibodies and are mainly responsible for the specificity (recognition) and affinity (binding) of the antibodies to their target markers. Proteolytic digestion of antibodies releases different fragments termed Fv (Fragment variable), Fab (Fragment antigen binding) and Fc (Fragment crystallisation) (illustrated in Figure 1). Note that antibody engineering can join the separate segments of the heavy and light chains in the Fv with a flexible peptide linker to form a single-chain Fv (scFv).

The linear protein chain in each of the domains folds into a characteristic 3D structure called the immunoglobulin fold. It consists of two sheets, formed by the protein chain, packed against each other. In the constant domains, each sheet consists of three and four strands respectively. In Figure 2, these two sheets are coloured red and green while the rest of the protein chain is coloured blue. The two sheets are pinned to one another by a disulphide bond shown in yellow. The constant domains of the Fc fragment are responsible for mediating the effector functions of the antibody.

The variable domain has a similar structure, except that there are nine strands; four and five in each sheet respectively. These provide a framework which supports the three CDR loops. Figure 3 shows a variable domain. One sheet is coloured blue, the other green. Two of the CDRs are show in red. The third CDR, which also contains the fourth strand from the green sheet and the fifth strand from the blue sheet is shown in yellow. The variable domains, one from the heavy chain and one from the light chain, form the two antigen binding sites, one at the tip of each arm of the "Y" (Figure 4). In this case, the picture shows a Fab fragment, with the light chain (blue) consisting of two domains (VL and CL) and the heavy chain (green) also consisting of two domains (VH and CH1). The CDRs on the tips of the variable domains are shown in red and the antigen (in this case lysozyme) shown in yellow. Antibody Humanisation consists of grafting these red CDRs from xenogeneic antibodies into human antibodies using genetic engineering techniques (see The Design Cycle).

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