Class antigen-binding site

Figure 15.19 Schematic representation of the peptide-binding domain of a class I MHC protein. The al and a2 domains are viewed from the top of the molecule, showing the empty antigen-binding site as well as the surface that is contacted by a T-cell receptor. (Adapted from P.J. Bjdrkman et al.. Nature 329 506-512, 1987.)...

The fundamental structure of immunoglobulins was first established by Gerald Edelman and Rodney Porter. Each chain is made up of identifiable domains some are constant in sequence and structure from one IgG to the next, others are variable. The constant domains have a characteristic structure known as the immunoglobulin fold, a well-conserved structural motif in the all /3 class of proteins (Chapter 4). There are three of these constant domains in each heavy chain and one in each light chain. The heavy and light chains also have one variable domain each, in which most of the variability in amino acid residue sequence is found. The variable domains associate to create the antigen-binding site (Fig. 5-24). 

Humans have five classes of immunoglobulins, each with different biological functions. The most abundant class is IgG, a Y-shaped protein with two heavy and two light chains. The domains near the upper ends of the Y are hypervariable within the broad population of IgGs and form two antigen-binding sites. 

Buckle et al., 1997), and the strongly binding peptide (Chen and Sigler, 1999). They all bind as an extended chain or /l-hairpin to a groove between two helices, reminiscent of the MHC class I antigen-binding site. All these peptides are unstructured in solution, but adopt the extended conformation when bound to the apical domain. 

B14. Bjorkman, P. J., Saper, M. A., Samraoui, B., Bennett, W. S., Strominger, J. L., and Wiley, D. C. The foreign antigen binding site and T cell recognition regions of class I histocompatibility antigens. Nature (London) 329, 512-518 (1987). 

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