Reverse turns are specific sites in proteins at which the polypeptide chain changes its overall direction1-6. This category of secondary structure enables the chain to turn a corner, and its frequent occurrence 7,8 is the geometric basis for the ultimate globular shape of the protein. β-Turns in particular are comprised of four consecutive residues with a stereochemistry9 that constrains the turn to be polar. In consequence, turns are almost always situated at the surface of the protein, in contact with solvent water. We have searched proteins of known structure and find that, on occasion, a turn may be buried within the hydrophobic interior of the molecule. In every instance of a buried turn, one or more solvent molecules were also found in a hydrogen-bonded complex with main-chain atoms of the turn residues. These bound water molecules appear to function as an integral part of the protein structure.
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