catsittingstill

But my recent encounters with the flu and such got me thinking about

Mucus

Mucus is composed mainly of glycoproteins called mucins, along with water, secreted immunoglobulins, general anti-microbial enzymes such as lysozyme, and salts.

Mucins start out as apomucins, rod-shaped proteins that have three sections—a "head" rich in the amino acid cysteine, a central section rich in serine, threonine, and proline, and a cysteine rich "tail." These sections are important because cysteines can link to each other, either within a protein or across proteins, so the cysteine rich head and tail mean that mucins link to each other easily. The central section, with its serines, threonines and prolines, is glycosylated—that is sugars are added to the serines and threonines, growing into long branching chains (polysaccharides). The central section has so many glycosylated sites that it becomes positively "furry" with polysaccharides. Once the polysaccharides are added, apomucin becomes mucin.

Polysaccharides bind water molecules via weak bonds called H-bonds; so mucins have a lot of water bound to them. This is part of what gives mucus its slippery, slimy texture. Furthermore, when water molecules are bound to polysaccharides it's much harder for the first few water molecules to cling together in tiny ice crystals (nucleate) as a prelude to freezing solid. It turns out that mucin-like proteins, also rich in polysaccharides, form the "antifreeze" proteins in some arctic fish.

Some polysaccharides also function on the surface of cells, as molecular "nametags" for other cells. However some microbes have evolved to take advantage of that—sporting proteins that bind the "nametags" of the cells they need to attack. Some secreted mucins have many copies of these polysaccharides, "distracting" the pathogen and persuading it to bind an unimportant protein, rather than its usual prey.

And mucus is more than just the stuff that makes your nose run or your voice hoarse. Saliva contains quite a bit of mucus, which gives it the excellent lubricating qualities that make it possible to swallow crackers and peanut butter (or drive a popcorn hull between your teeth).

References:
Essentials of Glycobiology (edited by Ajit Varki, Richard Cummings, Jeffrey Esko, Hudson Freeze, Gerald Hart, Jamey Marth) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York 1999.

http://www.vivo.colostate.edu/hbooks/molecules/mucins.html accessed 2/29/08.