Amino Acids Are Polyprotic

Amino acids from which proteins are built have an acidic carboxylic acid group, a basic amino group, and a variable substituent designated R  

A zwitterion is a molecule with both positive and negative charges. 

At low pH, both the ammonium group and the carboxyl group are protonated. At high pH, neither is protonated. The substituent may also have acidic or basic properties. Acid dissociation constants of the 20 common amino acids are given in 

In general, a diprotic acid has two acid dissociation constants, designated and K 2 (where K K,2)-. 

For a triprotic system, with three acidic protons, the corresponding relations are 

---