Which of the following amino acids contains carboxylic acid in their side chain?

Aspartate is the amino acid that contains a carboxylic acid in its side chain. In biochemistry, amino acids are categorized based on the properties of their side chains, also known as R groups. Aspartate has a side chain that includes an additional carboxylic acid group (-COOH), which gives it acidic properties and allows it to participate in various biochemical reactions, especially in metabolic pathways and enzyme functions.

In contrast, serine contains a hydroxyl group (-OH) in its side chain, which makes it polar but not acidic. Lysine has an amino group (-NH2) in its side chain, characterized by its basic properties, and glycine has a simple hydrogen atom as its side chain, making it the simplest amino acid without any functional group that contributes acidity like a carboxylic acid. Thus, among the options given, aspartate is uniquely notable for having a carboxylic acid functional group in its structure.