Karnatak PGET 2007 Microbiology MCQ Answer 7

The correct answer is ? a/c

Apart from the large number of allelic forms of MHC molecules there are two main structural types; these are known as MHC class I molecules and MHC class II molecules (fig 1.) .

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• Class I molecules have a three domain transmembrane structure; the peptide-binding site is shared by the two domains furthest from the cell membrane – a fourth domain is supplied by a separate molecule β2-microglobulin.
• Class II molecules have 2 transmembrane polypeptide chains; the peptide-binding site is shared by the two domains furthest from the cell membrane.

The overall structure of the peptide-binding site is very similar for both class I and class II MHC molecules; the base is made of β-pleated sheet, as is found in an immunoglobulin domain – the sides of the groove that holds the peptide are α-helices. The peptides are held very securely in place by hydrogen bonding to the backbone of the peptide (which allows a very wide range of peptide sequences to bind), while some of the sidechains of the peptide fit into pockets in the MHC protein (which provides the allelic specificity of peptide binding).

In class I MHC both ends of the peptide are anchored into the protein and so only short peptides (8 to 10 residues) can be accommodated, while in class II MHC the ends protrude from the binding groove, allowing longer peptides (15 to 25 residues) to bind.

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