Beta-Turns (Molecular Biology)

A b-turn is a type of nonregular secondary structure in proteins that causes a change in direction of the polypeptide chain. In the b-turn, a hydrogen bond is formed between the backbone carbonyl oxygen of one residue (i) and the backbone amide NH of the residue three positions further along the chain (i + 3) (Fig. 1) (1). This (i) to (i + 3) interaction distinguishes b-turns from g-turns, which have an (i) to (i + 2) hydrogen bond. There are several different types of b-turns, classified according to the backbone dihedral angles of the two intervening residues, (i + 1) and (i + 2). The type I and II turns are the most common, together accounting for about two-thirds of all b-turns, but they have very different amino acid preferences (2). Type I turns prefer Asn, Asp, or Ser in the (i) position; Asp, Ser, Thr or Pro in (i + 1); Asp, Ser, Asn, or Arg in (i + 2); and Gly, Trp, or Met in (i + 3). In contrast, type II turns have a preference for Pro at position (i + 1), Asn or Gly at position (i + 2) and Gln or Arg at (i + 3) (2). Other b-turn types include type I ‘ (related to type I by inversion of the sign of the f and y angles of the Ramachandran Plot), II’ (related to type II in the same way), IV, VIa, VIb, and VIII. Types I ‘ and II ‘ are almost exclusively found in hairpins; that is, they usually connect two adjacent antiparallel b-strands. The type IV b-turn is a miscellaneous type that includes any (i) to (i + 3) hydrogen-bonded turn in a protein structure where the f and y angles are different (by >40°) from the values that define the other b-turn types (Table 1). Types VIa and VIb have a proline cis-peptide bond at the (i + 2) position; in type VIa the proline adopts a backbone conformation close to that of a-helical residues, whereas in type VIb the proline has a conformation similar to that of

Figure 1. Schematic representations of a b-turn in a protein structure. (Left) The b-turn is shown connecting two b-strands in a hairpin motif. (Right) The detailed atomic structure of the b-turn is shown. Residues are numbered 1 to 4 for (i) to (i + 3). The hydrogen bond between the backbone carbonyl oxygen of residue 1 and the backbone amide nitrogen of residue 4 is shown as a dotted line. This example is a type I b-turn, because the backbone angles for the (i + 1) residue are f = -63° and y = -33°; and for the (i + 2) residue, f is -86° and y = -3°. Oxygen atoms and nitrogen atoms are shown as dark spheres. This figure was generated by Molscript (3) and Raster3D (4, 5).

Table 1. Classification of Beta-Turn Types