Gamma-aminobutyric acid (GABA), a four-carbon nonprotein
amino acid, is a significant component of the free
amino acid pool. GABA has an amino group on the g-carbon
rather than on the a-carbon, and exists in an unbound form. It is
highly soluble in water: structurally it is a flexible molecule that
can assume several conformations in solution, including a cyclic
structure that is similar to proline1. GABA is zwitterionic (carries
both a positive and negative charge) at physiological pH values
(pK values of 4.03 and 10.56)