Immunoglobulin’s: Structure, Antigen and Complement Biding Sites

One of the body’s most important defense mecha­nisms against infection is the production by the reticu­loendothelial tissues of a class of proteins called antibodies or immunoglobulin’s.

These proteins circulate in the bloodstream, where they make up part of the “gamma globulin” fraction of blood plasma.

The production of the immunoglobulin’s is stimulated by chemical substances, called antigens, present in or released from the infecting agent (e.g., constituents of bacterial membranes or the coats of vi­ruses) and recognized as being foreign or alien to the body.

This response is called the “immune response.” Although antigens can belong to virtually any chemi­cal category, they usually are proteins, polysac­charides, or nucleic acids.

The immunoglobulin molecules that are produced during an immune re­sponse have the capacity to bind to the antigens, the reaction between the two being highly specific; that is, each type of immunoglobulin that is produced in the body reacts with a particular antigen and no other. A major source of the antibodies that circulate in the blood are the “plasma cells” derived by the differenti­ation of a class of white blood cells known as B lym­phocytes.

The binding of antibodies to antigens that are in the surface of invading microorganisms (such as bacteria) triggers a series of reactions that leads to the destruc­tion of the foreign cells. These reactions involve a host of other blood plasma proteins collectively referred to as complement and that are normally present in the plasma (i.e., unlike the immunoglobulin’s, they are not synthesized in response to the appearance of an anti­gen).

Attachment of the immunoglobulin molecules to the surface antigens of the infecting cells is followed by the sequential binding and activation of the comple­ment proteins. Acting in a manner similar to digestive enzymes, the complement proteins create holes in the bacterial surface. This produces a “Donnan effect” in which ions and water enter the for­eign cell, causing it to swell and eventually rupture (i.e., lyse).

In effect, the immunoglobulin’s by binding to the surface antigens have served to identify the cells that are to be attacked and destroyed by comple­ment. Each antibody has more than one antigen- binding site. Therefore, when free antigen molecules are encountered in the body, they can be cross-linked to form a large precipitable complex.

When antibodies react simultaneously with antigens in the surfaces of separate cells, the cells are agglutinated, that is, they are clumped together into small masses. Precipitated antigens and agglutinated cells are ultimately en­gulfed and disposed of by phagocytic white blood cells. The immune response appears to be of rather recent evolutionary origin, because immunoglobulin produc­tion is characteristic only of vertebrates.

Immunoglobulin Structure:

The human body is capable of synthesizing more than a million different kinds of immunoglobulin molecules, each capable of reacting with a different antigen, but all of them appear to share the same fundamental quaternary structure. During the early stages of an infection, the response to the antigen involves the pro­duction of a specific class of immunoglobulin’s known as immunoglobulin M or IgM, having a molecular weight of about 1,000,000. Later, the amount of IgM gradually declines as another class or “isotype” of immunoglobulin called IgG appears.

Although other immunoglobulin’s are also present in blood (see Table 4-11), IgG represents the most abundant form (as much as 80%). IgG has been more extensively studied than the other immunoglobulin’s and much of the de­scription that follows is based on results obtained from studies of IgG.

IgG molecules with a molecular weight of about 150,000 are composed of four polypeptide chains of two different kinds—a pair of identical high- molecular-weight chains called “heavy” or H chains and a pair of identical lower-molecular-weight chains called “light” or L chains. The L chains have a molecular weight of 20,000 to 25,000 and consist of about 214 amino acids. The H chains have a molecular weight of 50,000 to 55,000 and contain about 450 amino acids. In associating with each other, the four chains form a molecule whose quaternary structure resembles that of the letter “Y” (Fig. 4-35).

Each arm of the Y contains a complete L chain and part of an H chain and the leg of the Y contains the remaining parts of the H chains. Near its C-terminus, each L chain is linked to an H chain by a disulfide bridge, and two additional disulfide bridges link the H chains to­gether.Each of the four polypeptide chains that form an immunoglobulin is divided into separate regions called “domains.” There are two domains in the L chains and four in the H chains.

Within each of the domains, folding of the polypeptide chain produces two parallel planes each containing several segments with folded beta structure. In each folded beta structure, neigh­boring stretches of the polypeptide have opposite polarity (i.e., they are antiparallel). The two parallel planes in each domain are held together by disulfide bridges and also by van der Waals interactions be­tween hydrophobic side chains of .amino acids in each sheet.

In each of the arms of the immunoglobulin, the H and L chains associate to establish a globular quater­nary structure maintained principally by van der Waals interactions between hydrophobic side chains of amino acids in each chain; some hydrogen bonding be­tween chains also occurs.

Antigen- and Complement-Binding Sites:

Studies employing digestive enzymes such as trypsin and papain have revealed which portions of an im­munoglobulin molecule combine with the antigens and which part binds complement. These digestive en­zymes characteristically cleave immunoglobulin’s in a region (called the “hinge” region) located on the C- terminal side of the disulfide bridge that links each L chain to an H chain (see Fig. 4-35).

Two types of frag­ments are produced by such enzymatic cleavage, one of which retains the capacity to bind the antigen (called the F_ab fragment). Each of the two F_ab frag­ments that is produced consists of a complete L chain and the N-terminal half of an H chain. The other type of fragment consists of the C-terminal halves of both H chains and is called the F_c fragment because it can be crystallized. The antigen appears to be bound near the ends of the two F_ab units (the shaded area in Fig. 4-35).

The angle between the F_ab and F_c units is varia­ble because of the flexibility of the hinge regions of the molecule. As a result, the overall shape of an im­munoglobulin can vary between that of the letter “Y” and the letter “T.”

This flexibility is unquestionably related to the immunoglobulin’s capacity to bind to the antigens. The complement proteins associate with the F_c fragment. Immunoglobulin’s contain a small amount of carbohydrate (i.e., the immunoglobulin’s are glycoproteins). The carbohydrate portion consists of a short chain of sugars covalently bonded to an asparagine residue and sandwiched along the interface be­tween the F_ab and F_c units.