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The ST turn is a structural feature in proteins and polypeptides. Each consists of three amino acid residues in which residue i is a serine or threonine that forms a hydrogen bond from its sidechain oxygen group to the mainchain NH group of residue i + 2.
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 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, 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