amphoteric
Summary
Amphoteric describes a species that can act as either an acid or a base. In biochemistry the standard example is an amino acid: its carboxyl group can donate a proton and its amino group can accept one. Its net charge therefore depends on pH, and at the pI it is a zwitterion.
Detail
Meaning: An amphoteric species can act as an acid or as a base. An amino acid is the standard example: its carboxyl group (pKa near 2) donates a proton and its amino group (pKa near 9 to 10) accepts one. At low pH both groups are protonated (net +1). At high pH both are deprotonated (net −1). In between it is a zwitterion with +1 and −1 charges. Water and HCO₃⁻ are amphoteric too.
Equation: pI = (pKa1 + pKa2)/2, where pI is the pH of zero net charge and pKa1 and pKa2 (unitless) are the values on either side of the neutral form. With an acidic side chain use the two lowest pKa values. With a basic side chain use the two highest.
On the MCAT: Amino acid titration curves show flat buffering regions at each pKa, with the pI midway between the two values around the neutral form. Questions ask for the net charge at a given pH, or which way a protein moves in electrophoresis: toward the electrode of opposite charge, stopping at its pI.
Common confusion: A zwitterion is not uncharged. It carries a positive and a negative charge that cancel. The same amino acid is a cation, a zwitterion or an anion depending on pH.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Campbell Biology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.