Mucins are large glycoproteins with an average molecular mass of 2 106 Daltons [27] that
assemble into a protective gel-like layer that extends upwards to 150 μm of the epithelial surface [28].
Mucin glycoproteins consist of linear or branched oligosaccharide chains attached to a protein core
containing a very distinct composition of amino acids. Serine (Ser) and threonine (Thr) are even more
prevalent in core proteins, making up 41% of the molecule in humans [29] and 57% in rats [30]. Both
Ser and Thr have side chains that contain hydroxyl groups that are involved in linking oligosaccharide
chains to the protein via O-glycosidic bonds [31]. Core proteins also contain a high percentage of
proline (Pro), aspartic acid (Asp) and glutamic acid (Glu). The oligosaccharide side chains are formed
by five sugars: fucose (Fu), galactose (Gal), N-acetylgalactosamine (GalNAc), N-acetylglucosamine
(GlcNAc), and sialic acid (SA). These sugars form linear or branched arrays of 2–12 sugar residues
that are attached via an O-glycosidic bond between Ser or Thr in the protein and GalNAc.