What is the building block of a protein molecule?

The building block of a protein molecule is an amino acid. Amino acids are organic compounds that combine to form proteins, which are essential macromolecules in living organisms. Each amino acid consists of a central carbon atom, an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group) that determines the unique properties of each amino acid.

In the context of proteins, amino acids link together through peptide bonds to create long chains. These chains then fold into specific three-dimensional shapes to perform various functions in the body, such as acting as enzymes, structural components, or signaling molecules.

The other options represent different types of biomolecules. Nucleotides are the building blocks of nucleic acids such as DNA and RNA, fatty acids are components of lipids, and monosaccharides are the simplest carbohydrates, which serve as energy sources and structural units in polysaccharides. Each of these plays an important role in biological systems but does not contribute directly to the structure of proteins.