carboxyl
Summary
A carboxyl group is –COOH: a carbon double-bonded to one O and single-bonded to an OH. In amino acids it is the acidic group at the C-terminus and in Asp and Glu side chains. Its pKa is well below 7.4, so at physiological pH it is deprotonated to carboxylate (–COO⁻).
Detail
Meaning: A carboxyl group is –COOH: a carbon double-bonded to one oxygen and single-bonded to an OH group. It appears at the C-terminus of a polypeptide, on the α-carbon of a free amino acid, and in the side chains of aspartate and glutamate. It is acidic because its conjugate base, carboxylate (–COO⁻), is stabilized by resonance. The α-carboxyl pKa is near 2 and acidic side chains are near 4, so at pH 7.4 these groups are deprotonated and negative.
Equation: pH = pKa + log([A⁻]/[HA]), where A⁻ is carboxylate and HA is –COOH (concentrations in mol/L; pH and pKa have no units). When pH exceeds pKa, carboxylate predominates.
On the MCAT: Questions ask for the net charge of an amino acid or peptide at a given pH, the reading of a titration curve, or peptide bond formation, where the carboxyl of one residue joins the amino group of the next and water is lost. Salt bridges between carboxylate and protonated lysine or arginine also appear in protein folding.
Common confusion: Carboxyl is not the same as carbonyl. A carbonyl is just C=O; a carboxyl has a carbonyl and a hydroxyl on the same carbon. Also, at physiological pH the group is –COO⁻, not neutral –COOH.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.