This is a chain-like molecule that has a cluster of atoms, called a carboxyl group, at one end of the chain.
From the economist.com
The researchers found that they could attach erythropoietin to the carboxyl group, and thus to the antibody.
From the economist.com
In the chemical synthesis, the two carboxyl groups of aspartic acid are joined into an anhydride, and the amino group is protected by a compound that will prevent further reactions of that group.