Beta turn

A β turn is a type of non-regular secondary structure in proteins that causes a change in direction of the polypeptide chain.Beta turns (β turns, β-turns, β-bends, tight turns, reverse turns), also called Venkatachalam-turns, are very common motifs in proteins and polypeptides. Each consists of four amino acid residues (labelled i, i+1, i+2 and i+3). They can be defined in two ways: 1. By the possession of an intra-main-chain hydrogen bond between the CO of residue i and the NH of residue i+3. Alternatively, 2. By having a distance of less than 7Å between the Cα atoms of residues i and i+3. The hydrogen bond criterion is the one most appropriate for everyday use, partly because it gives rise to four distinct categories; the distance criterion gives rise to the same four categories but yields additional turn types. Two websites are available for finding and examining hydrogen-bonded beta turns in proteins, Motivated Proteins: [1] and PDBeMotif [2].

Beta turns defined by hydrogen bond criterion:

The hydrogen bond criterion for beta turns, applied to polypeptides whose amino acids are linked by trans peptide bonds, gives rise to just four categories, as shown by Venkatachalam in 1968. They are called types I, II, I’ and II’. All occur regularly in proteins and polypeptides but type I is most common, because it most resembles an alpha helix, occurring within 3/10 helices and at the ends of some classic alpha helices.

The four types of beta turn are distinguished by the Φ,ψ angles of residues i+1 and i+2 as shown in the Table below giving the typical average values. Glycines are especially common at amino acids with positive Φ angles; for prolines such a conformation is sterically impossible but they occur frequently at amino acid positions where Φ is negative.

The main chain atoms of type I and I’ β turns are enantiomers (mirror images) of one another. So are the main chain atoms of type II and II’ β turns.

Type I and II β turns exhibit a relationship to one another because they potentially interconvert by the process of peptide plane flipping (180° rotation of the CONH peptide plane with little positional alteration to side chains and surrounding peptides). The same relationship exists between type I’ and II’ β turns. Some evidence has indicated that these interconversions occur in beta turns in proteins such that crystal or NMR structures merely provide a snapshot of β turns that are, in reality, interchanging. In proteins in general all four beta turn types occur frequently but I is commonest, followed by II, I' and II' in that order. Beta turns are especially common at the loop ends of beta hairpins; they have a different distribution of types from the others; type I' is commonest, followed by types II', I and II.

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