Re: Why does pepsin in the digestive system, not break down itself.

Dear CP:

Thanks for your interesting question.

Pepsin, as you know, is an enzyme secreted in the stomach which digests 
proteins by cleaving peptide bonds, which link the amino acids that 
comprise a protein.  Most enzymes, including pepsin, are proteins.

The answer to you question is that pepsin does indeed digest other pepsin 
molecules.  However, during a meal, pepsin molecules will be far outnumbered 
by other proteins, thus a pepsin molecule will run into many other peptide 
bonds to cleave before it runs into another pepsin molecule.  Also, pepsin 
will be continually secreted by the stomach during a meal.

Side point:  it is possible that pepsin digests itself less readily than 
other proteins.  This is because pepsin is most active at cleaving the 
peptide bond between two hydrophobic amino acids.  The amino acids in 
pepsin are arranged such most of the amino acids on the surface are 
hydrophilic.
                                     
Ray Hsu
hsurm@medicine.wustl.edu