Can anybody tell me the molecular basis of why protective function of mucin is due to their high viscosity?
Mucins are heavily glycosylated proteins and one of the major constituents of mucus in various cavities like mouth, alimentary canal, vagina etc. They protect the epithelial cells in these sites by forming a a gel like barrier between the environment and the cells. They protect cells from noxious substances or mechanical damage that may happen due to friction. They may also interact with pathogens and may have a protective role there.
I found this old review which is nice.
Mucins contain terminal sialic acids on their O-glycans that directly interact with bacterial receptors. which essentially trap the bacteria on the gel created by the secreted mucins. Alternatively, the sialic acids can act as decoys binding to bacterial receptors providing a type of molecular protection to underlying receptors on the cell surface.
Following on from Rusty's answer, the same is also true for viruses such as influenza i.e. binding to mucins can reduce infection of respiratory epithelial cells. Mucins also have an antioxidant capacity and can bind hydroxyl radicals to help reduce oxidative stress towards cells.
I never heard of that antioxidant binding function for mucins. Do you have a reference on that? Id like to see it.
Here is a reference that describes it. I used to do some work with antioxidants and sialic acids are pretty important with my research now.http://www.ncbi.nlm.nih.gov/pubmed/17485090
Thanks. Interesting article. I had never seen that DNA nick assay for determining radical damage in vitro. Back when I studied iron-generated free radical damage, we always used the thiobarbituric acid reactivity assay. This would have made my life much easier!
Im also curious as the to the chemistry behind this reaction with mucins. Terminal sialic acids are negatively charged and oxygen radicals also carry a charge, so how are they interacting? At first I thought it must be some attack on the oxygen that connects that sialic to the sugar backbone (glycosidic bond) similar to a Beta-elimination reaction. However the authors couldn't detect any free sialic acid. They conclude..
"Thus, it is unlikely the mechanism of the OH scavenging ability of mucin is based on the hydrolysis of glycosyl bond of sialic acid by OH. "
That is a surprising result. What is the protective effect? My guess is its not in the sialic acid, but in buried parts of the chain that disappear after you remove the sialic acids.