Re: How come pepsin is not broken down in pH of 2

Hi Cheryl,

Not only does pepsin remain active at pH 2, but pH 2 is optimum for pepsin 
activity.  Pepsinogen, the precursor of pepsin, cleaves below pH 5 to form 
pepsin.  Why pepsin is stable at low pH is not completely clear, though.  
Pepsin denatures at approximately pH 6.  What this suggests is that pepsin 
retains its tertiary molecular structure only when there is a high 
concentration of hydrogen ions, and at pH 6 there are not enough hydrogen 
ions to keep the structure stable.  At pH 2, there are apparently the 
perfect amount of hydrogen ions to keep the structure folded properly. (The 
enzyme's isoelectric point approximately 1.5, which means that it is 
electrically neutral around that pH.)   
  
You should check out the website at this address: http://bssv01.lancs.ac.uk/StuWork/BIOS316/Bios31698/Pepsin/Pepsin.htm
It has 3D structures of both pepsin and pepsinogen, along with a nice 
summary of pepsin.  This website also mentions some specific biochemical 
aspects of the enzyme, such as the amino acids that are important in the 
active site of the enzyme.  I hope this helps!

Sarah Earley
CU Boulder