section 35.6

Immunoglobulin Structure

and

Function

815

FIGURE 35-8

(Also see color figure.) Complementarity-determining regions. The actual locations of the complementarity-determining

regions (CDRs) of the VH and VL chains are shown in this fragment. The orientation is approximately 90° clockwise

relative to the orientations in Figure 35-7. The CDR residues are primarily found in the hairpin turns that connect the /3

sheets of the VH and VL domains. The cystallographic structure used for showing the CDR residues is derived from the

structure of a Fab fragment interacting with the capsular polysaccharide from

H. influenzae B

published in the Protein

Data Bank file 1HOU. The three orientations of the structure show (1) the backbone /3 sheets of the immunoglobulin

domain; (2) a side view that shows how the CDR regions overlap; and (3) a view that shows the extensive area that the

CDR regions provide for making contact with an epitope of the antigen.

CH3 domains of the two heavy chains (designated Fc, for

fragment, crystallizable) and two products designated Fab.

The Fab comprises the VL and CL domains of one light

chain linked through a disulfide bridge to the VFI and CFI1

domains of a heavy chain. Two Fab fragments are formed

from each immunoglobulin molecule. Because the Fab

fragment contains the antigen binding sites, it has been a

very useful tool for understanding antigen-antibody struc-

tural relationships. Proteolytic cleavage by the enzyme

pepsin produces a fragment that contains the Fab regions

of both heavy and light polypeptide chains. This fragment

is designated F(ab/2. These fragments were particularly

useful in the elucidation of the amino acid sequences and

the crystal structures of immunoglobulin molecules. Be-

cause of the great flexibility of the hinge region, crystals of

the complete IgG molecule are not suitable for x-ray crys-

tallography. The models shown in Figure 35-7 are thus a

composite of F(ab

) 2

and Fc structures.

The initial discovery and description of the constant,

variable, and hypervariable domains and elucidation

of the primary structures of the immunoglobulins was

made possible because of the disease multiple myeloma.

In multiple myeloma, a single B-cell clone (mono-

clone) synthesizes large quantities of structurally iden-

tical or monoclonal antibody molecules. Isolation of

immunoglobulin from multiple myeloma patients who

were producing structurally different immunoglobulins *

1

6

9

provided sufficient quantities of unique immunoglob-

ulins for protein sequence analysis. These sequence

comparisons led to identification of the hypervariable

regions and their relationship to antibody specificity.

This seminal accomplishment provided the basis for the

now established relationships between the primary struc-

tures of the hypervariable regions and antibody epitope

specificity.

Immunoglobulin molecules possess the ability to recog-

nize structurally diverse epitopes because of the enormous

variety of complementary tertiary structures that differ-

ent amino acid sequences provide. Although the hyper-

variable regions are separated by intervening sections of

several amino acids in the linear protein sequence, the

folded polypeptide presents the hypervariable regions on

one “face” of the immunoglobulin “arm” (Figure 35-8).

Complementarity between antibody and antigen is respon-

sible for the specificity and affinity of the antibody the epi-

tope that is being recognised. Complementarity is deter-

mined by the amino acid sequences of the hypervariable

regions; thus leading to these regions being now desig-

nated the complementarity-determining regions (CDRs).

Binding of small antigens, e.g., haptens, to antibodies

occurs when hypervariable region amino acid residues

form a pocket into which the hapten fits. Such binding

is similar to that between enzymes and small-molecule

substrates, i.e., a lock-and-key fit between antigen and

The antigen-binding regions of each Fab retain their ability to bind to the epitope for which they are specific. The Fc

fragment contains a region that binds to the cellular Fc receptor. This receptor binding site is sometimes referred to as

the “biological activity” of the Ig molecule. Cleavage by pepsin produces a fragment (Fab'h and several fragments from

the Fc region of the Ig chains. The figure is derived from the coordinates published by E.A. Padlan,

Mol. Immunol.

31,

169, 1994.