Pepsin digestion, protein resistance

Asero, R., Mistrello, G., Roncarolo, D. et al. 2000. Lipid transfer protein A pan-allergen in plant-derived foods that is highly resistant to pepsin digestion. Int Arch Allergy Immunol 122 20-32. 

All types of hotulinum toxin are sensitive to low pH such as that existing in the stomach as well as to pepsin, the protease produced in the stomach. However, the toxins produced by Clostridia are complexed with other proteins and the complexes are relatively resistant to digestion in the stomach. They dissociate at the more alkaline pH of the small intestine, where the dissociated toxin can be... 

The sulfur atom binds readily to heavy meted ions. Under oxidizing conditions, two cysteines can join together in a disulfide bond to form the amino acid cystine. When cystines are part of a protein, insulin for example, this stabiUzes tertiary structure and makes the protein more resistant to denaturation disulfide bridges are therefore common in proteins that have to function in harsh environments including digestive enzymes (e.g., pepsin and chymotrypsin) and structural proteins (e.g., keratin). Disulfides are also found in peptides too small to hold a stable shape on their own (e.g., insulin). 

Fibrous proteins that have supporting or protective function in the animal, of which there are three types (1) collagens in the skin, tendons and bones, which are resistant to pepsin and trypsin, and are converted to water-soluble gelatin by boiling with water (2) elastins in the tendons and arteries, which are not converted to gelatin and (3) ceratins, which comprise the horns, hoofs, feathers, scales, and nails, and which are insoluble in dilute acids and alkalis and are not attacked by any animal digestive enzymes. 

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