Immunoglobulin Structure And Function

An Ab or Ig is composed of peptide chains with carbohydrate pendant groups. A schematic of the Ab IgG is shown in Figure 5.5. The peptide chains form the quaternary structure of the Ig, while the carbohydrate moieties serve

Immunoglobulin Structure And Function

Variable Regions

Figure 5.5 • Structure of immunoglobulin G (IgG), showing antigen-binding regions and key elements of the molecule.

Variable Regions

Figure 5.5 • Structure of immunoglobulin G (IgG), showing antigen-binding regions and key elements of the molecule.

as antigen-recognition groups and probably as conformation-stabilizing units. The general structure of the Ig looks something like a Y, with the antigen-binding regions at the bifurcated end. In this area are peptide sequences that are "programmable" by the immune system to allow the Ig to recognize a large number of antigens.

Treatment with either of two enzymes, papain or pepsin, digests an Ab into fragments that are useful in understanding its molecular structure. Papain clips the Ab into two fragments that contain the antigen-binding regions. These fragments have been termed the Fab, or antigen-binding, fragment. The remaining part of the Ab after papain digestion contains two peptide chains linked by a disulfide bond. Treatment of the same Ab with pepsin yields the two Fab units joined by the disulfide bond, plus two of the distal peptide chains. These distal units have been crystallized and, hence, are termed the Fc fragment (for "crystalliz-able"). The disulfide bond, therefore, provides a demarcation between the two molecular regions. The nomenclature of an Ab includes a high-molecular weight, or heavy, chain on the inside and a low-molecular weight, or light, chain on the outside.

IMPORTANT FEATURES OF ANTIBODY MOLECULAR STRUCTURE4'6

As stated previously, the tip end of the Fab region binds antigen. There are two of these regions, so we say that the Ab is bivalent and can bind two antigen molecules. The overall amino acid sequence of the Ab dictates its conformation. The peptide sequence for most antibodies is similar, except for the hypervariable regions. The amino acid sequence at the end of the heavy chain (Fc) determines the class of the Ig (i.e., IgG, IgM, etc.). All antibodies resemble each other in basic shape, but each has a unique amino acid sequence that is complementary to the antigen in a "lock-and-key" interaction (antigen-Ab specificity). Some, such as IgM, are pentamers of IgG (Fig. 5.6). In reality, the lock-and-key model is too simplistic, and an induced fit model is preferred.

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