The term random coil denotes a protein or fragment that has no recognizable secondary structure. It comes from English and is not translated in the German-language scientific literature, but used in the English original: random = 'random', 'random' and coil = 'ball', 'spiral', 'twist'.
In contrast to the secondary structures α-helices or β-pleated sheets , random coil structures may look 'randomly' arranged to the observer . They are just as important for the protein structure and thus the protein function as the regularly arranged forms of the primary structure . Among other things, they enable the primary structure to bend backwards and thus allow the formation of very compact protein structures. Random coil structures play an important role on the outside of membrane proteins . Furthermore, they act as 'hinges' that enable protein subunits, for example in transport proteins or enzymes, to move against each other.
The balls are often not spherical, but rather irregularly shaped and can contain areas that are not balled. Stable bonds within random coil structures are the peptide bonds of the primary structure , i.e. the amino acid chain (primary structure). Ionic bonds, hydrogen bonds, dipole-dipole and van der Waals forces are in principle also possible between amino acid residues of a protein to stabilize random coil structures. In fact, they are jointly responsible for the arrangement within this structure.
The amino acid proline often contributes to the formation of random coil structures because its ring-shaped structure does not fit into a normally shaped α-helix or a β-sheet ('structure breaker', 'helix breaker'). But proline does not always have to be a helix breaker automatically. There are z. B. in collagen or plant structural proteins are known secondary structures that consist almost exclusively of proline and are referred to as polyproline-II helix. The denaturation of proteins, for example by heat, leads to the "unfolding" of the existing secondary structures and converts them completely into random coils. The primary structure, i.e. the amino acid sequence, is retained. A common example of this is cooking an egg.