Reverse Turns Turns

An energetically economical and space-saving way of changing direction in a polypeptide chain involves four amino acid residues, which are often joined to chains in the /-sheet conformation with a 180° chain reversal. Reverse turns (or loops) consist mostly of hydrophilic residues. Reverse turns are common structural elements; approximately one-third of the amino acid residues of proteins is found in this conformation. The best-known examples are /?-hairpins that link two adjacent strands of an antiparallel /¡-pleated sheet. They are stabilized by hydrogen bonds connecting the C = O on residue i to the NH on residue i + 2 or i + 3. At least, 15 different variants of reverse turns have been described and categorized (23). Because of the geometric constraints required for the turn, certain classes of turns have preferences for specific amino acids (glycine or proline) at specific positions. /-Turns linking contiguous /-strands are of considerable interest to biochemists because they are usually at the protein surface and often are involved in molecular recognition and antigenicity.

Examples of secondary structures are shown in Figure 2; the ribbon-like coils at both right and left sides are a-helices; parallel ^-sheets are at the center; a /-turn links the antiparallel /f-sheet pair at the upper-right-hand corner (this is the reason for naming of /-turn).

Sleeping Sanctuary

Sleeping Sanctuary

Salvation For The Sleep Deprived The Ultimate Guide To Sleeping, Napping, Resting And  Restoring Your Energy. Of the many things that we do just instinctively and do not give much  of a thought to, sleep is probably the most prominent one. Most of us sleep only because we have to. We sleep because we cannot stay awake all 24 hours in the day.

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