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The mucin glycoproteins contribute to the mucus barrier throughout the body. Close integration with innate and immune defensive systems demands controlled turnover while retaining continuous defensive functions.

Members of the mucin family show tissue specificity. They comprise secreted gel-forming and membrane-associated forms, each with a characteristic peptide domain based structure. This molecular organisation enables formation of secreted, gel networks and the cell surface glycocalyx. Mucins carry a large range of tissue specific O-glycan chains linked through N-acetyl-D-Galactosamine, to Serine and threonine residues in Variable number tandem repeat domains. A smaller number of N-glycans are responsible for processing of the mucin peptide during biosynthesis.

Mucin turnover is a balance between biosynthesis, oligomerisation, network formation, physical disruption and enzymes which degrade glycan chains, the peptide backbone and generate and cleave disulphide bridges in mucin monomers, dimer and trimers. Mucin network formation is also dependent on proteins and peptides which establish a stable adherent mucus gel, such as trefoil peptides, gastrokines, transferrin, secretory IgA. Interaction of the resident gastrointestinal microflora with the host mucus layer is especially important and forms an integral part of the dynamic defensive barrier. In the gut there are continuous dietary based interactions, including peptidases and proteases, bile salts and bacterial enzymes. Evidence suggests that the formation of mucin fragments are a fundamental part of the turnover process and that these are generated on a tissue and mucin specific basis.

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