ST turn explained

The ST turn is a structural feature in proteins and polypeptides.^[1] 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 bond from its sidechain oxygen group to the mainchain NH group of residue i + 2.^{[2] [3]}

Similar motifs occur with aspartate or asparagine as residue i, called asx turn. Four types of asx turn and ST turn can be distinguished: types I, I’, II and II’. These categories correspond (via sidechain-mainchain mimicry of residue i) to those of the more abundant 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. 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 ST and asx turns.

Asx and ST turns both occur frequently at the N-termini of α-helices,^{[4] [5] [6] [7]} as part of asx motifs or ST motifs, with the asx, serine or threonine as the N cap residue. They are thus often regarded as helix capping features.

Evidence for a functionally relevant ST turn is provided in the CDR3 region of the T-cell receptor (B chain, V domain) ^[8]

A proportion of ST turns are accompanied by a mainchain-mainchain hydrogen bond that qualifies them as ST motifs.

External links

• Motivated Proteins
• PDBeMotif

Notes and References

1. Duddy. WJ. Nissink WMJ . Mimicry by asx- and ST-turns of the four main types of β-turn in proteins. Protein Science. 2004. 13. 11. 3051–3055. 10.1110/ps.04920904. 15459339. Allen. Frank H.. Milner-White. E. James. 2286581.
2. Leader. DP. Milner-White EJ . Motivated Proteins: A web application for studying small three-dimensional protein motifs. BMC Bioinformatics. 2009. 10. 60. 10.1186/1471-2105-10-60. 19210785. 2651126 . free .
3. Golovin. A. Henrick K . MSDmotif: exploring protein sites and motifs. BMC Bioinformatics. 2008. 9. 312. 10.1186/1471-2105-9-312. 18637174. 2491636. free.
4. Doig. AJ. Macarthur MW . Structures of N-termini of helices in proteins. Protein Science. 6. 1. 147–155. 10.1002/pro.5560060117. 9007987. 2143508. 2008. MacArthur. Malcolm W.. Thornton. Janet M..
5. Presta. LG. Rose GD . Helix Caps. Science. 1988. 240. 4859. 1632–1641. 10.1126/science.2837824. 2837824. 1988Sci...240.1632P.
6. Aurora. R. Rose GD . Helix Capping. Protein Science. 1998. 7. 1. 21–38. 10.1002/pro.5560070103. 9514257. 2143812.
7. Gunasekaran. K. Nagarajam HA . Stereochemical punctuation marks in protein structure. Journal of Molecular Biology. 1998. 275. 5. 917–932. 10.1006/jmbi.1997.1505. 9480777. Ramakrishnan. C. Balaram. P. 35919397.
8. Yassai. MB. Demus W. Gorski J. Structural and Mechanistic Implications of Rearrangement Frequencies within Human TCRBV Genes. J Immunol. 199. 3. 1142–1152. 2017. 10.4049/jimmunol.1601450. 28659354. 5659713.