Re: How do proteins or amino acids act as buffers in the human body?

Chai,
  To first understand how amino acids and proteins function as buffers in 
the body, you need to understand a little about the structure of an amino 
acid. If we look at the general structure of an amino acid, you can see 
that it has an amine (NH3) group on one end and a carboxyl (COOH) group on 
the other end.
                 Example: glycine     3HN--CH2--COOH

  If the amino acid is in solution, as the pH changes the amino and carboxy 
hyrogens (protons) will be affected.  

using glycine as an example again:
				 LOW pH				NEUTRAL pH			  HIGH pH
             +3HN--CH2--COOH  ->  +3HN--CH2--COO-  ->  2HN--CH2--COO  
 		       +1 net charge       neutral charge      -1 net charge
					        pH 2.3				 pH 9.7
  As you can see, as the pH changes a hydrogen can be lost or gained 
affecting the charge. Glycine is a simple example since it doesn't have any 
R groups attached to the carbon.  Many R groups can also gain or lose 
hydrogens, such as arginine, glutamic acid, ect.  This means that at any 
particular pH an amino acid may have a net + or - charge.  To complicate 
this a little bit more, proteins are made up of many amino acids each of 
which contributes to the proteins overall charge.  Remember that there will 
be only 1 terminal amine and carboxy group, BUT there will be many R groups 
affecting the charge. 

  We now consider a basic definition of a buffer as a solution that 
consists of a mixture of a weak acid and it conjugate base.  A good buffer 
will tend to resist changes in pH upon addition of moderate amounts of 
strong acid or base.  Amino acids that have this capability are glycine and 
histidine.  For proteins to act as buffers, you must take into account all 
of the R groups and the pH that they will gain or lose protons.  

  To investigate this further, I suggest that you obtain a general 
chemistry and biochemistry textbook.  I would especially recommend readings 
on hemoglobin and bisphosphoglycerate.

                           Jeff Stiefel