A carboxyl group is a molecule that consists of 1 carbon atom, 2 oxygen atoms and 1 hydrogen atom in its unionised form. The functional group can be viewed as -COOH. When ionised it loses the hydrogen atom therefore becoming negative in charge. More commonly carboxyl groups are seen as part of a larger molecule such as an amino acids. In proteins (due to the negative charge) the carboxyl group can form peptide bonds or in enzymes the negative charge of the carboxyl group can be used to disrupt the bonds in a substrate and drive a reaction.