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Proteins Blueprint to Machine

Protein structure is organized hierarchically into four distinct levels—primary, secondary, tertiary, and quaternary—that determine the molecule's shape and function. The primary structure consists of a specific linear sequence of amino acids linked by rigid, planar peptide bonds; this sequence ultimately dictates how the protein folds into its unique native conformation. Local spatial arrangements of the polypeptide backbone, stabilized by hydrogen bonds, form secondary structures such as alpha helices and beta conformations, which are defined by specific dihedral angles around the single bonds of the backbone. The overall three-dimensional folding of a single polypeptide chain constitutes its tertiary structure, while the association of multiple polypeptide subunits forms the quaternary structure. Although covalent disulfide bonds can provide stability, protein conformation is primarily stabilized by numerous weak noncovalent interactions, with the hydrophobic effect playing a predominant role by driving nonpolar amino acid side chains to cluster in the protein's interior away from water.