|MadSci Network: Biochemistry|
Rick, Sorry for the delay in response as I was on vacation. The short answer to your question is that mucin is secreted throughout the gastrointestinal tract from mucous neck cells and in an alkaline medium in the stomach mucosa. Further the viscosity of the medium that contains mucin is rather insoluble and forces the pepsin and hydrochloric acid into an aqueous phase further protecting the stomach lining. mucin: polypeptide glycoprotein with 4 subunits connected by disulfide bonds that is resistant to degradation by pepsin and other proteolytic enzymes due to the saccharide moieties attached to OH amino acid side chains (ie. serine, threonine, & tyrosine). Note that tyrosine is one of the side chain aromatics required by pepsin (see below). FYI regarding pepsin: Cleaves peptide bonds adjacent to aromatic side chains (phenylalanine, tyrosine & tryptophan). Pepsinogen I (acid secreting region) and Pepsinogen II (pyloric region) are precursors. Pepsinogen interestingly can also be released by the mucous neck cells in small quantites. Pepsinogen, however, is normally released in larger quantities during digestion of proteins by the peptic cells. Sources: Ganong Review of Physiology Guyton Medical Physiology Stryer Biochemistry
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