Constant Region Domains

The immunoglobulin constant-region domains take part in various biological functions that are determined by the amino acid sequence of each domain.

CH1 AND CL DOMAINS

The CH1 and CL domains serve to extend the Fab arms of the antibody molecule, thereby facilitating interaction with antigen and increasing the maximum rotation of the Fab arms. In addition, these constant-region domains help to hold the VH and VL domains together by virtue of the interchain disulfide bond between them (see Figure 4-6). Also, the CH1 and CL domains may contribute to antibody diversity by allowing more random associations between VH and VL domains than would occur if this association were driven by the

VH/VL interaction alone. These considerations have important implications for building a diverse repertoire of antibodies. As Chapter 5 will show, random rearrangements of the immunoglobulin genes generate unique VH and VL sequences for the heavy and light chains expressed by each B lymphocyte; association of the VH and VL sequences then generates a unique antigen-binding site. The presence of CH1 and CL domains appears to increase the number of stable VH and VL interactions that are possible, thus contributing to the overall diversity of antibody molecules that can be expressed by an animal.

HINGE REGION

The 7,8, and a heavy chains contain an extended peptide sequence between the CH1 and CH2 domains that has no homology with the other domains (see Figure 4-8). This region, called the hinge region, is rich in proline residues and is flexible, giving IgG, IgD, and IgA segmental flexibility. As a result, the two Fab arms can assume various angles to each other when antigen is bound. This flexibility of the hinge region can be visualized in electron micrographs of antigen-antibody complexes. For example, when a molecule containing two dinitrophenol (DNP) groups reacts with anti-DNP antibody and the complex is captured on a grid, negatively stained, and observed by electron microscopy, large complexes (e.g., dimers, trimers, tetramers) are seen. The angle between the arms of the Y-shaped antibody molecules differs in the different complexes, reflecting the flexibility of the hinge region (Figure 4-12).

Antibody Hinge

FIGURE 4-12

Experimental demonstration of the flexibility of the hinge region in antibody molecules. (a) A hapten in which two dini-trophenyl (DNP) groups are tethered by a short connecting spacer group reacts with anti-DNP antibodies to form trimers, tetramers, and other larger antigen-antibody complexes. A trimer is shown schematically. (b) In an electron micrograph of a negatively stained preparation of these complexes, two triangular trimeric structures

Dnp Immunoglobulin Structure

are clearly visible. The antibody protein stands out as a light structure against the electron-dense background. Because of the flexibility of the hinge region, the angle between the arms of the antibody molecules varies. [Photograph from R. C. Valentine and N. M. Green, 1967, J. Mol. Biol. 27:615; reprinted by permission of Academic Press Inc. (London) Ltd.]

FIGURE 4-12

Experimental demonstration of the flexibility of the hinge region in antibody molecules. (a) A hapten in which two dini-trophenyl (DNP) groups are tethered by a short connecting spacer group reacts with anti-DNP antibodies to form trimers, tetramers, and other larger antigen-antibody complexes. A trimer is shown schematically. (b) In an electron micrograph of a negatively stained preparation of these complexes, two triangular trimeric structures are clearly visible. The antibody protein stands out as a light structure against the electron-dense background. Because of the flexibility of the hinge region, the angle between the arms of the antibody molecules varies. [Photograph from R. C. Valentine and N. M. Green, 1967, J. Mol. Biol. 27:615; reprinted by permission of Academic Press Inc. (London) Ltd.]

Two prominent amino acids in the hinge region are proline and cysteine. The large number of proline residues in the hinge region gives it an extended polypeptide conformation, making it particularly vulnerable to cleavage by proteolytic enzymes; it is this region that is cleaved with papain or pepsin (see Figure 4-3). The cysteine residues form interchain disul-fide bonds that hold the two heavy chains together. The number of interchain disulfide bonds in the hinge region varies considerably among different classes of antibodies and between species. Although ^ and € chains lack a hinge region, they have an additional domain of 110 amino acids (Ch2/Ch2) that has hingelike features.

OTHER CONSTANT-REGION DOMAINS

As noted already, the heavy chains in IgA, IgD, and IgG contain three constant-region domains and a hinge region, whereas the heavy chains in IgE and IgM contain four constant-region domains and no hinge region. The corresponding domains of the two groups are as follows:

IgA, IgD, IgG

IgE, IgM

CH1/CH1

CH1/CH1

Hinge region

CH2/CH2

CH2/CH2

CH3/CH3

CH3/CH3

Ch4/CH4

Although the CH2/CH2 domains in IgE and IgM occupy the same position in the polypeptide chains as the hinge region in the other classes of immunoglobulin, a function for this extra domain has not yet been determined.

X-ray crystallographic analyses have revealed that the two Ch2 domains of IgA, IgD, and IgG (and the CH3 domains of IgE and IgM) are separated by oligosaccharide side chains; as a result, these two globular domains are much more accessible than the others to the aqueous environment (see Figure 4-8b). This accessibility is one of the elements that contributes to the biological activity of these domains in the activation of complement components by IgG and IgM.

The carboxyl-terminal domain is designated CH3/ CH3 in IgA, IgD, and IgG and CH4/CH4 in IgE and IgM. The five classes of antibody and their subclasses can be expressed either as secreted immunoglobulin (sIg) or as membrane-bound immunoglobulin (mIg). The carboxyl-terminal domain in secreted immunoglobulin differs in both structure and function from the corresponding domain in membrane-bound immunoglobulin. Secreted immunoglobulin has a hydrophilic amino acid sequence of various lengths at the carboxyl-terminal end. The functions of this domain in the various classes of secreted antibody will be described later. In membrane-bound immunoglobulin, the carboxyl-terminal domain contains three regions:

■ An extracellular hydrophilic "spacer" sequence composed of 26 amino acid residues

■ A hydrophobic transmembrane sequence

■ A short cytoplasmic tail

The length of the transmembrane sequence is constant among all immunoglobulin isotypes, whereas the lengths of the extracellular spacer sequence and the cytoplasmic tail vary.

B cells express different classes of mIg at different developmental stages. The immature B cell, called a pre-B cell, expresses only mIgM; later in maturation, mIgD appears and is coexpressed with IgM on the surface of mature B cells before they have been activated by antigen. A memory B cell can express mIgM, mIgG, mIgA, or mIgE. Even when different classes are expressed sequentially on a single cell, the antigenic specificity of all the membrane antibody molecules expressed by a single cell is identical, so that each antibody molecule binds to the same epitope. The genetic mechanism that allows a single B cell to express multiple immunoglobu-lin isotypes all with the same antigenic specificity is described in Chapter 5.

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Responses

  • Meghan
    Why is it that the hinge of igg heavy chain is between ch1 and ch2?
    7 years ago
  • Kaija
    How proline residue show flexibility in hinge region of antibody structure?
    5 years ago
  • blanco
    Does IgD have four constant domains?
    1 year ago

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