Characteristically, the mucus is composed of a number of components: glycoproteins, proteins, lipids, electrolytes, inorganic salts, water, enzymes, mucopolysaccharides, among others. The mucin molecule of a polypeptide backbone, which is attached to the pendant sugar groups at periodic intervals on the peptide chain. The molecular weights of these glycoproteins vary from 2 x 106 to 14 x 106 daltons (3). In general, a major portion of the peptide backbone is covered with carbohydrates grouped in various combinations. Galactose, fucose, N-acetylglucosamine, N-acetylgalactosamine, and N-acetylneuraminic acid (sialic acid) are typically found in the mucin molecules. These carbohydrates may constitute as much as 70-90% of the total mucin weight (7). The sugar molecules can carry sulfate residues via ester linkages. Each carbohydrate chain terminates in either a sialic acid
(pKa = 2.6) or with an L-fucose group. Hence, the mucin molecules behave as anionic polyelectrolytes as neutral pH (8). Because of the rather large number of sugar groups, the mucin molecule is capable of picking up almost 50-80 times its own weight in water. The oligosaccharides form a protective coat over the glycoprotein backbone by preventing the enzymatic action of proteases (9). Numerous sugar hydroxyl groups of mucin molecules have the potential to interact with other polymers by hydrogen bonds. The positions and relative amounts of amino acids in the glycoprotein backbone are important to the matrix structure of the mucus, since they confer to the overall tertiary structure and folding of the glycoprotein.
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