Proteins play the central role in most biological processes both because of their ability to fold into well-defined three-dimensional structures and because of their nanometer-scale size, which allows them to engulf small molecules and to grip other biomacromolecules. Proteins achieve their structures and size by virtue of the dozens or hundreds of alpha-amino acids that they comprise. Each a-amino acid is only 0.24 nanometers in length. Dozens or more are required to create functional domains that fold into well-defined three-dimensional structures. Still more are required to create the functional multidomain structures that serve as receptors, catalysts, transcription factors, and other species that make up so many of the working parts of living cells.
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