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1.3.3 Reverse turns and loops

In compact globular proteins, a polypeptide often makes a sharp turn called a reverse turn. For instance, these turns often link adjacent strands in antiparallel β pleated sheet (as represented in Figure 12a). Also known as β bends, reverse turns involve four amino acid residues with a hydrogen bond between the C=O group of the first residue and the N–H group of the fourth (Figure 14a and b).

Figure 14 Reverse turns (β bends). (a) Type I β bend. (b) Type II β bend.

Type I and type II β bends differ in respect of the torsion angles for the residues. Looking at the Cα3–N bond in Figure 14a and b, you should be able to confirm that there is a difference of 180º in the torsion angle about this bond (ɸ3) in the type I and type II bends. Notice also that the β bends are tighter than the turn in the α helix structure. The second residue in the turn is frequently proline, which readily adopts the required conformation for such a turn. Another type of turn is the omega loop. This loop contains between six and 16 residues in a compact structure with a pinched-in shape resembling the Greek upper case character Ω (omega) (Figure 15). The side-chains of omega loops fill the inside of the loop.

Figure 15 Space-filling model of a polypeptide backbone (side-chains and H atoms not shown) in an omega (Ω) loop structure (residues 40–54 of cytochrome c from tuna (pdb file 3cyt)).

Reverse turns and omega loops tend to be located at the surface of proteins where they play an important part in the recognition role of proteins, such as the recognition of specific antigens by antibodies.


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