| Themes > Science > Life Sciences > General Biology > Immunology > Recognition Systems in Immunity > The Major Histocompatibility Complex > Structure of MHC proteins | |||||||||||||||||||||||||||||||||||||||||||||
The MHC genes and their products are grouped into 2 classes on the basis of their chemical structure and biological properties. The two MHC proteins have a similar secondary and tertiary structure with subtle functional differences. Class I molecules are made up of one heavy chain (45 kD) and a light chain called ß2-microglobulin (12 kD) that contributes to the overall structure of the protein. Class II molecules do not contain ß2-microglobulin and consist of two (alpha and ß) chains of similar size (34 and 30 kD). Click here to see a cartoon of the primary structures of MHC class I and class II Both classes of MHC molecule fold up to produce very similar 3-D structures. Each has 2 MHC-unique domains which fold together to form a peptide binding platform whose base is formed by a ß-pleated sheet structure with the sides consisting of two alpha-helices. This structure forms a cleft or groove which accommodates a peptide. In both classes the peptide binding "MHC superdomain" is supported by a pair of immunoglobulin-like (IgSF) domains. The differences between the 2 classes are the linear connectivity of the polypeptide chains and the dimensions of the peptide-binding groove which accommodates 8-9 amino acids in class I but is open-ended for class II. Click here to see a
picture of the 3D structure of an MHC molecule
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