Protein

A protein is a biological macromolecule built from one or more chains of amino acids joined together by peptide bonds. These amino acid chains are called polypeptides. Proteins are among the major functional molecules of cells and can serve as enzymes, structural components, receptors, transporters, molecular motors, and signaling molecules. Their wide range of functions reflects differences in their amino acid sequences, structures, and molecular interactions .¹

The order of amino acids in a polypeptide chain is its primary structure. Interactions within the chain and with its surroundings often cause the polypeptide to adopt a particular three-dimensional structure, or conformation. This structure influences how the protein interacts with other molecules and therefore contributes to its function. Some proteins consist of a single polypeptide chain, while others function as complexes containing multiple chains .¹

Protein is related to, but not identical with, the terms peptide and polypeptide. All involve amino acids linked by peptide bonds, but peptides generally refer to shorter chains, while polypeptide refers to an amino acid chain without necessarily implying that it functions as a complete protein. There is no single universally accepted size boundary separating a peptide from a protein .^1,2