Re: How do digestive enzymes work?

Briony,
   Most digestive enzymes work by breaking up proteins into smaller 
chemical units that your body can then use to synthesize other compounds.  
Proteins are chemical compounds that are made up of very long chains of 
amino acids.  Each amino acid is linked to the next one in the chain by a 
peptide bond.  You can think of a peptide bond as being like a link in a 
chain.  If you cut apart the links, you will one-by-one break the long 
chain into smaller and smaller pieces.  This is what digestive enzymes do 
to proteins.  In digestion the protein is broken up into the individual 
amino acids that make it up.  There are 20 amino acids that make up all 
proteins.  Our bodies use these amino acids to synthesize our own proteins, 
as well as to make many other important molecules.

The lock and key description refers to the fact that enzymes (which are 
themselves proteins) perform chemistry in a small cavity called the active 
site, or catalytic site.  You may think of this as the lock.  The key is 
the chemical compound that the enzyme performs chemistry upon – in this 
case, the protein that the enzyme is going to digest.  The protein must be 
able to fit into the active site of the enzyme in a proper way in order for 
the enzyme to work; sort of like how only the key with the right shape fits 
into a lock.  Often this is determined by the protein's shape; the shape of 
the protein must properly fit the shape of the enzyme’s active site.  This 
is one reason we have a number of digestive enzymes.  They all perform the 
same chemical reaction of breaking the peptide bonds in proteins, but 
different digestive enzymes recognize different shapes of target proteins.