C H A P T E R 3 5 , F IG U R E 7
Schematic diagram of an immunoglobulin. (A) The simplest immunoglobulin (Ig) molecule, IgG consists of four polypeptide chains: two
heavy chains of -50,000 Da and two light chains of -25,000 Da. More complex Ig molecules are dimers or pentamers that consist of similar basic structures but may
have slightly different heavy and/or light chains. Each of the two heavy chains (shown in blue) contains four immunoglobulin domains, a variable VH donain at the N ter-
minus, and three “constant” domains, CHI, CH2, and CH3. Each of the light chains contains two domains, VL and CL. An immunoglobulin domain is 110-120 amino
acids in length. The V domains of both chains contain three regions described as hypervariable regions (orange bands) within the VH and VL domains. Each domain con-
tains one intrachain disulfide bridge. The CHI and the CL1 domains are linked by a single, interchain disulfide bridge. Two disulfide bridges join the two heavy chains
near the N-terminal residues of the CH2 domain. A single, Asn-linked, oligosaccharide chain is present within the CH2 domains of each of the heavy chains. A stretch of
amino acids between the CHI and CH2 domains forms a “hinge” region where the two heavy chains are connected by two disulfide bridges. The hinge region acts as a
flexible “tether” between the CHI and CH2 domains. NMR studies show this region to be very flexible, which permits the Ig molecule to assume various shapes, from
the “Y” shown in the figure to a “T” with the “arms” perpendicular to the Fc region (see below). The antigen-binding residues are located at the upper “tips” of the Y-
shaped Ig molecule. In the folded Ig molecule, the residues of the hypervariable regions are actually located at the tips, as can be seen in Figure 35-8. The portion of the
IgG molecule above the dashed lines is the variable region; that portion below the dashed lines is the constant region. (B) Immunoglobulin secondary structure. The sec-
ondary structures of all immunoglobulin domains are essentially the same. Each domain contains seven antiparallel |3 sheets. The cystine residues (disulfide bonds) are
space-filled in the figure. The open, accessible residues within the hinge are also evident in the structure. The figure is derived from the coordinates published by E.A.
M ol. Im m u n o l.
31:169, 1994. (C) Space-filling model for IgG. Chains are color coded the same throughout. The polypeptide regions comprising the hinge are
readily accessible to proteolytic enzymes. Papain cleaves at the N-terminal side of the uppermost disulfide bridge in the hinge to create three fragments; two Fab frag-
ments containing the VH and CHI domains linked to the VL and CL domains by a single disulfide bond. The Fc fragment contains two CH2 and two CH3 domains of the
heavy chains. 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 regions that binds
to the cellular Fc receptor. This receptor binding sire is sometimes referred to as the “biological activity” of the Ig molecule. Cleavage by pepsin produces a fragment
(Fab')2 and several fragments from the Fc region of the Ig chains. The figure is derived from the coordinates published by E. A. Padlan,
M ol. Im m u n o l.