The Asx turn
The name "Asx" is used here to represent either of the amino acids aspartate (Asp) or asparagine (Asn).
Types
Four types of Asx turn can be distinguished:
types I, I’, II and II’. These categories correspond to those of the better-known hydrogen-bonded
, which have four residues and a hydrogen bond between the CO of residue i and the NH of residue i+3. Asx turns and
have structurally similar hydrogen-bonded loops and exhibit sidechain-mainchain mimicry in the sense that the sidechain of residue i of the Asx turn mimics the mainchain of residue i of the
beta turn. Regarding their occurrence in proteins, they differ in that type I is the commonest of the four
while type II’ is the commonest of the Asx turns.
Occurrence
Asx and
both occur frequently at the
N-termini of α-helices.
as part of
or
such that the Asx, serine or threonine is the
N cap residue. They are thus often regarded as helix capping features.
Related motifs
Similar motifs occur with
serine or
threonine as residue i, which are called
.
In spite of serine and threonine having one less sidechain atom, such that the sidechain-mainchain mimicry of β turns is imperfect, these features occur in proteins as the four types in numbers approaching those of Asx turns. They also exhibit a tendency to substitute each other over evolutionary time.
A proportion of Asx turns are accompanied by a mainchain–mainchain hydrogen bond that qualifies them as .
External links
