Cystine knot

A cystine knot is a protein structural motif where two disulfide bridges (cystines - formed from pairs of cysteine molecules) are formed. The sections of polypeptide that occur between them then form a loop through which a third disulfide bond passes, forming a rotaxane substructure. It occurs in many proteins across many species and provides considerable structural stability.

This motif was first observed in the structure of Nerve Growth Factor, solved by X-ray crystallography and published in 1991 by Tom Blundell in Nature.