Antibody structures

Each IgG antibody molecule consists of four polypeptide chains (two identical light chains and two identical heavy chains joined by disulfide bonds) and has two antigen-binding sites (i.e. is bivalent). 

Each light chain folds into two domains, one for the variable region and one for the constant region. Each IgG heavy chain folds into four domains, one for the variable region and three in the constant region. 

Light and Each molecule of immunoglobulin G (IgG) is Y-shaped and consists of four 

Variable and Comparison of the amino acid sequences of many immunoglobulin polypep- 

Instant Notes in Biochemistry 2nd Edition, B.D. Hames N.M. Hooper, (c) 2000 BIOS Scientific Publishers Ltd, Oxford. 

Class Subunit structure Molecular weight [kDa] Sugar content p (w/w)] Serum concentration [mg mr ] Halflives in blood m 

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Stanfield, R.L., et al. Major antigen-induced domain rearrangements in an antibody. Structure 1 83-93,... 

Figure 17.2 An example of prediction of the conformations of three CDR regions of a monoclonal antibody (top row) compared with the unrefined x-ray structure (bottom row). LI and L2 are CDR regions of the light chain, and HI is from the heavy chain. The amino acid sequences of the loop regions were modeled by comparison with the sequences of loop regions selected from a database of known antibody structures. The three-dimensional structure of two of the loop regions, LI and L2, were in good agreement with the preliminary x-ray structure, whereas HI was not. However, during later refinement of the x-ray structure errors were found in the conformations of HI, and in the refined x-ray structure this loop was found to agree with the predicted conformations. In fact, all six loop conformations were correctly predicted in this case. (From C. Chothia et al.. Science 233 755-758, 1986.)...

Although amine-reactive protocols, such as SATA thiolation, result in nearly random attachment over the surface of the antibody structure, it has been shown that modification with up to 6 SATAs per antibody molecule typically results in no decrease in antigen binding activity (Duncan et al., 1983). Even higher ratios of SATA to antibody are possible with excellent retention of activity. 

Enbrel is a product now approved for medical use that is based upon this strategy. The product is an engineered hybrid protein consisting of the extracellular domain of the TNF p75 receptor fused directly to the Fc (constant) region of human IgG (see Box 13.2 for a discussion of antibody structure) The product is expressed in a CHO cell line from which it is excreted as a dimeric soluble protein of approximately 150 kDa. After purification and excipient addition (mannitol, sucrose and trometamol), the product is freeze-dried. It is indicated for the treatment of rheumatoid arthritis and is usually administered as a twice-weekly s.c. injection of 25 mg product reconstituted in WFI. Enbrel functions as a competitive inhibitor of TNF, a major pro-inflammatory cytokine. Binding of TNF to Enbrel prevents it from binding to its true cell surface receptors. The antibody Fc component of the hybrid protein confers an extended serum half-life on the product, increasing it by fivefold relative to the soluble TNF receptor portion alone. 

Recombinant DNA technology has provided an alternative (and successful) route of reducing the innate immunity of murine monoclonals. The genes for all human immunoglobulin sub-types have been cloned, and this has allowed generation of various hybrid antibody structures of reduced immunogenicity. 

Whilst all immunoglobulins conform to the generalised pattern described above (i.e. each molecule comprises two L chains and two H chains), there are five different classes of antibodies that represent variations on this common theme. The L chains of these molecules may be cor A (classified according to their structure) the H chains are classified as a, 8, e,y or /x in IgA, IgD, IgG, IgE and IgM, respectively. Furthermore, some of these antibodies possess ancillary protein chains that form polymeric antibody structures. 

From the results reported to date, it seems that the manner in which haptens are attached to carrier proteins leads to significant differences in certain cases. Clearly, haptens designed with aromatic moieties between the linkage to the immunogenic carrier protein and the TSA motif often have better antibody recognition. Recently, Hilvert pointed out that on both micro and macro levels, mechanistic improvements arise as a function of time. The differences in time scales for the evolution of natural enzymes and antibodies — millions of years versus weeks or months — also appear to be an explanation of the low efficiency of antibody catalysts. He also highlighted that the unique immunoglobulin fold has not been adopted by nature as one of the common scaffolds on which to build enzyme catalytic machinery. Therefore, antibody structure itself places limitations on the kind of reactions amenable to catalysis. 

First, based on the concept of catalytic antibodies as first described by Jencks, the use of a TSA of the target reaction to elicit antibodies during an immune response has been developed and identified as a classic strategy in the search for catalytic antibodies. Structural studies have in particular shown that this strategy has led to... 

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