Antibodies are glycoprotein molecules also called immunoglobulins. As mentioned earlier, the basic structure is a Y-shaped molecule that has two functional parts—the arms and the stem (figure 16.4a). The two identical arms, called the Fab regions, bind antigen. The stem is the Fc region, functioning as a "red flag" that enlists other components of the immune system. These names were assigned following early studies that showed that enzymatic digestion of antibodies yielded two types of fragments—fragments that were antigen-binding (Fab) and fragments that could be crystallized (Fc). ■ glycoprotein, p. 29
There are five major classes of human immunoglobulins (Ig): IgM, IgG, IgA, IgE, and IgD (table 16.1). All have the same basic Y-shaped structure, called an antibody monomer, but some form multimers of that basic subunit. For example, IgM is a pentamer made up of five connected monomers; IgA characteristically forms a dimer of two monomers. The various classes of antibody molecules differ in both their structural characteristics and corresponding biological properties. The specialized attributes of each class will be described shortly.
All antibodies have a similar molecular structure, consisting of two high molecular weight polypeptide chains, called the heavy chains, and two lower molecular weight polypeptide chains, called the light chains (see figure 16.4b) Disulfide bonds (S—S) join the two chains, creating a Y-shaped molecule with identical halves. Disulfide bonds also join the two halves of the molecule. At the fork of the Y is a flexible or "hinge" region. In some antibody classes, the arms of the Y move when antigen is attached, sometimes far enough out to form a T-shape. Recent evidence indicates that other parts of some antibody classes may be pliable as well. For example, some antibodies have been shown to bind to proteins inside crevices on the surface of rhi-novirus particles, the cause of the common cold. ■ virion, p. 323
The heavy and light chains of the antibody molecule each form several tightly folded globular regions termed domains; all immunoglobulin domains consist of approximately 110 amino acids. The heavy chain of IgG is composed of four immunoglobulin domains, and the light chain is composed of two immunoglobulin domains (figure 16.5). Several other structures of the immune system, including T-cell receptors, have similar globular configurations.
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