Functional Definition:

Ionizable groups are functional groups that can lose or gain a proton but are typically uncharged.

While the concept may seem basic now it is extremely important as we begin to talk about protein structure and peptides. Peptides are long chains of amino acids that help form the primary protein structure. Each amino acid has terminals or functional groups at its ends; an amine (N-terminal) and a carboxylic acid (c-terminal). The functional groups that hang off of the amino acid chain as well as its terminals all have pKa values that determine protonation and deprotonation. As the pH in an organism changes the ionizable groups loose or gain protons determining things like folding and denaturation.