{{Short description|Feature in proteins and polypeptides}} The '''Asx turn'''<ref>{{cite book|last=Richardson|first=JS|chapter=The anatomy and taxonomy of protein structure|year=1981|volume=34|pages=167–339 |pmid=7020376|doi=10.1016/S0065-3233(08)60520-3|isbn=9780120342341|title=Advances in Protein Chemistry Volume 34}}</ref><ref>{{cite journal|last=Tainer|first=JA|author2=Getzoff ED|authorlink2=Elizabeth D. Getzoff|title=Determination and analysis of the 2 A-structure of copper, zinc superoxide dismutase|journal=Journal of Molecular Biology|year=1982|volume=160|pages=181–217 |pmid=7175933 |doi=10.1016/0022-2836(82)90174-7|issue=2}}</ref><ref>{{cite journal|last=Rees|first=DC|author2=Lewis M|title=Refined crystal structure of carboxypeptidase a at 1.54 Å resolution|journal=Journal of Molecular Biology|year=1983|volume=168|pages=367–387 |pmid=6887246|doi=10.1016/S0022-2836(83)80024-2|issue=2}}</ref><ref>{{cite journal|last=Eswar|first=N|author2=Ramachandran C|title=Secondary structures without backbone: An analysis of backbone mimicry by polar side chains in proteins|journal=Protein Engineering|year=1999|volume=12|issue=6|pages=447–455|doi=10.1093/protein/12.6.447|pmid=10388841|doi-access=free}}</ref><ref>{{cite journal|last=Chakrabarti|first=P|author2=Pal D|title=Interrelationships of side-chain and main-chain conformations in proteins|journal=Progress in Biophysics and Molecular Biology|year=2001|volume=76|issue=1–2|pages=1–102|doi=10.1016/s0079-6107(01)00005-0|pmid=11389934|doi-access=free}}</ref><ref>{{cite journal|last=Duddy|first=WJ|author2=Nissink WMJ|title=Mimicry by asx- and ST-turns of the four main types of β-turn in proteins|journal=Protein Science|year=2004|volume=13|issue=11|pages=3051–3055|doi=10.1110/ps.04920904|pmid=15459339|last3=Allen|first3=Frank H.|last4=Milner-White|first4=E. James|pmc=2286581}}</ref><ref>{{cite journal|last=Thakur|first=AK|author2=Kishore R|title=Characterization of β-turn and asx-turns mimicry in a model peptide : Stabilization via C-H•••O interaction|journal=Biopolymers|year=2006|volume=81|issue=6|pages=440–449|doi=10.1002/bip.20441|pmid=16411188|s2cid=27091571 }}</ref> is a structural feature in proteins and polypeptides. It consists of three amino acid residues (labeled i, i+1 and i+2) in which residue i is an aspartate (Asp) or asparagine (Asn) that forms a hydrogen bond from its sidechain CO group to the mainchain NH group of residue i+2. About 14% of Asx residues present in proteins belong to Asx turns.

The name "Asx" is used here to represent either of the amino acids aspartate (Asp) or asparagine (Asn).

== Types ==

thumb|right|An Asx turn with an aspartate at residue i. One of the sidechain oxygens of the aspartate forms a hydrogen bond (dotted line) with the mainchain NH group of residue i+2. Colors: red, oxygen; grey, carbon; blue, nitrogen. Hydrogen atoms are omitted. Four types of Asx turn can be distinguished:<ref>{{cite journal|last=Duddy|first=WJ|author2=Nissink WMJ|title=Mimicry by asx- and ST-turns of the four main types of beta turn in proteins|journal=Protein Science|year=2004|volume=13|issue=11|pages=3051–3055|doi=10.1110/ps.04920904|pmid=15459339|last3=Allen|first3=Frank H.|last4=Milner-White|first4=E. James|pmc=2286581}}</ref> types I, I’, II and II’. These categories correspond to those of the better-known hydrogen-bonded beta turns, which have four residues and a hydrogen bond between the CO of residue i and the NH of residue i+3. Asx turns and beta turns 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 beta turns while type II’ is the commonest of the Asx turns.

== Occurrence == Asx and ST turns both occur frequently at the N-termini of α-helices.<ref>{{cite journal|last=Doig|first=AJ|author2=Macarthur MW|title=Structures of N-termini of helices in proteins|journal=Protein Science|year=1997|volume=6|issue=1|pages=147–155|doi=10.1002/pro.5560060117|pmid=9007987|pmc=2143508|last3=MacArthur|first3=Malcolm W.|last4=Thornton|first4=Janet M.}}</ref><ref>{{cite journal|last=Presta|first=LG|author2=Rose GD |title=Helix Caps|journal=Science|year=1988|volume=240|issue=4859|pages=1632–1641|doi= 10.1126/science.2837824|pmid=2837824|bibcode=1988Sci...240.1632P}}</ref><ref>{{cite journal|last=Aurora|first=R|author2=Rose GD |title=Helix Capping|journal=Protein Science|year=1998|volume=7|issue=1|pages=21–38|doi= 10.1002/pro.5560070103|pmid=9514257|pmc=2143812}}</ref><ref>{{cite journal|last=Gunasekaran|first=K|author2=Nagarajam HA |title=Stereochemical punctuation marks in protein structure|journal=Journal of Molecular Biology|year=1998|volume=275|issue=5|pages=917–932|doi= 10.1006/jmbi.1997.1505|pmid=9480777|last3=Ramakrishnan|first3=C|last4=Balaram|first4=P|s2cid=35919397}}</ref> as part of Asx motifs or ST motifs 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 ST turns.<ref>{{cite journal|last=Duddy|first=WJ|author2=Nissink WMJ|title=Mimicry by asx- and ST-turns of the four main types of β-turn in proteins|journal=Protein Science|year=2004|volume=13|issue=11|pages=3051–3055|doi=10.1110/ps.04920904|pmid=15459339|last3=Allen|first3=Frank H.|last4=Milner-White|first4=E. James|pmc=2286581}}</ref> 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.<ref>{{cite journal|last=Wan|first=W-Y|author2=Milner-White EJ|title=A Recurring Two-Hydrogen-bond Motif Incorporating a Serine or Threonine Residue is found both at α-Helical N Termini and in Other Situations|journal=Journal of Molecular Biology|year=2009|volume=286|issue=5|pages=1651–1662|doi=10.1006/jmbi.1999.2551|pmid=10064721}}</ref>

A proportion of Asx turns are accompanied by a mainchain&ndash;mainchain hydrogen bond that qualifies them as Asx motifs.

==References== <references />

==External links== *[http://motif.gla.ac.uk/motif/index.html Motivated Proteins] *[http://www.ebi.ac.uk/pdbe-site/pdbemotif/ PDBeMotif] * [http://motif.gla.ac.uk/motif/index.html]<ref>{{cite journal|last=Leader|first=DP|author2=Milner-White EJ|title=Motivated Proteins: A web application for studying small three-dimensional protein motifs|journal=BMC Bioinformatics|year=2009|volume=10|pages=60|doi=10.1186/1471-2105-10-60|pmid=19210785|pmc=2651126 |doi-access=free }}</ref> * [http://www.ebi.ac.uk/pdbe-site/pdbemotif/].<ref>{{cite journal|last=Golovin|first=A|author2=Henrick K|title=MSDmotif: exploring protein sites and motifs|journal=BMC Bioinformatics|year=2008|volume=9|pages=312|doi=10.1186/1471-2105-9-312|pmid=18637174|pmc=2491636|doi-access=free}}</ref>

Category:Protein structural motifs