A protein is a polypeptide with more than a hundred amino acid residues, having a molecular mass higher than 10,000u. However, the distinction is not sharp, and polypeptides with fewer amino acids might be called proteins if they exhibit a well-defined conformation, such as insulin with 51 amino acids.
What distinguishes a protein from a polypeptide, and under what conditions might a polypeptide be referred to as a protein?
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A protein differs from a polypeptide in its complexity and functionality. A polypeptide is a chain of amino acids linked by peptide bonds. When a polypeptide assumes a specific three-dimensional structure, often involving multiple polypeptide chains, and exhibits biological activity, it is termed a protein. Proteins are larger and more intricate, with defined functions, while polypeptides may lack tertiary or quaternary structures. The transition from polypeptide to protein is context-dependent; a polypeptide may be considered a protein if it displays functional complexity, interaction with cofactors, and involvement in cellular processes, emphasizing its role beyond a mere linear chain of amino acids.