The Asx turn is a structural feature in

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respondi ...

and

polypeptides Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

. It consists of three

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

residues (labeled i, i+1 and i+2) in which residue i is an

aspartate Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...

(Asp) or

asparagine Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...

(Asn) that forms a

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...

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

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 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 turn The ST turn is a structural feature in proteins and polypeptides. Each consists of three amino acid residues (labeled ''i'', ''i'' + 1 and ''i'' + 2) in which residue ''i'' is a serine (S) or threonine (T) that forms a hydrogen b ...

s both occur frequently at the N-termini of

α-helices The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues earli ...

. as part of Asx motifs or

ST motif The ST motif is a commonly occurring feature in proteins and polypeptides. It consists of four or five amino acid residues with either serine or threonine as the first residue (residue ''i''). It is defined by two internal hydrogen bonds. One is bet ...

s such that the Asx, serine or threonine is the

N cap The term N cap (N-cap, Ncap) describes an amino acid in a particular position within a protein or polypeptide.{{cite journal, last=Leader, first=DP, author2=Milner-White EJ , title=The structure of the ends of helices in globular proteins, journal= ...

residue. They are thus often regarded as helix capping features.

Related motifs

Similar motifs occur with

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − for ...

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO ...

as residue i, which are called

s. 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 Asx motifs.

References

External links

Motivated ProteinsPDBeMotif

{{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 Protein structural motifs