Pepsin is an Enzyme

• Scientists and researchers John N. Mills and Jordan Tang describe pepsin as "an enzyme containing 327 amino-acid residues in a single polypeptide chain" in their scientific analysis of pepsin and its molecular weight. Pepsin contains 21 different amino acids, totaling 327 amino-acid molecules, which string together to form proteins.
  
  

Types of Amino Acids

• Mills and Tang list the following as the 21 amino acids which make up pepsin: lysine, histidine, arginine, aspartic acid, asparagine, threonine, serine, phosphoserine, glutamic acid, glutamine, proline, glycine, alanine, half-cystine, valine, methionine, isoleucine, leucine, tyrosine, phenylalanine and tryptophan.
  
  

Gastric Juice

• Pepsin is part of the stomach's "gastric juice." Hydrochloric acid converts pepsinogen to pepsin, which, in turn, breaks down proteins to peptides. Pepsinogen production is stimulated by the presence of gastrin in the blood. Mucous protects the stomach from hydrochloric acid and pepsin. Biology professor Michael Gregory of SUNY explains that the digestive action of pepsin can be responsible for ulcers on the stomach wall when bacteria on portions of the stomach lining prevents it from secreting mucous.
  
  .