While proteins are made up of amino acids, which contain carbon, hydrogen, oxygen, and nitrogen, nitrogen is the key element that distinguishes proteins from other biomolecules.

Here's why:

* Amino acids: The building blocks of proteins are amino acids. Each amino acid contains an amino group (-NH2), which is the source of nitrogen.

* Peptide bonds: Proteins are formed when amino acids link together through peptide bonds. These bonds are formed by the removal of a water molecule, and the nitrogen from one amino acid forms a bond with the carbon of the next amino acid.

Therefore, without nitrogen, it is impossible to build the amino acids that make up proteins.

While other minerals like iron, zinc, and magnesium are essential for various metabolic processes involved in protein synthesis, nitrogen is the only one directly incorporated into the protein molecule itself.