find the missing link: carboxyl group + amino group + side group =? glucose fatty acid amino acid nucleotide

Brief Explanations:

An amino - acid is composed of a carboxyl group, an amino group, and a side - chain (side group). Glucose is a monosaccharide, fatty acid has a carboxyl group and a long hydrocarbon chain, and a nucleotide has a sugar, a phosphate group, and a nitrogenous base.

Answer:

Amino acid