β – Turns (secondary structure of proteins)
Definition: Turns and bends refer to short segments of amino acids that join two units of secondary structure such as two adjacent strands of an antiparallel β sheet.
Other names: β bend, reverse turns or hairpin turns
Location: Beta-turn has several unsaturated backbone hydrogen bond donors and acceptors. The peptide groups of the central two amino acid residues in the turn can hydrogen-bond with water. So beta turns are often found near the surface of a protein (it is polar).
Structure: A β turn involves four aminoacid residues, in which the carbonyl oxygen of first residue is hydrogen-bonded to the hydrogen of N-H group of fourth residue, resulting in a tight 180-degree turn.
The peptides groups of the central two residues do not participate in any inter residue hydrogen bonding.
Aminoacids found: Gly (small and flexible) and Pro (Peptide bonds involving the imino nitrogen of proline readily assume the cis configuration) residues often occur in turns.
Function: Permits the change of direction of the peptide chain to get a folded structure. It connects the ends of two adjacent segments of an antiparallel sheet.
Turns are classified into Types I and Type II according to the (phi, psi) angles of the two central residues (residue 2 and 3).
Why they are called β turns: Usually they connect adjacent beta strands in a beta sheet.
Why they called reverse turns: Because the polypeptide chain makes a 180 0 change in direction when they are connected by turns.
Why they are included under secondary structure: Because they are highly ordered structure stabilized by internal hydrogen bonds.