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A cystine knot is a protein structural motif containing three disulfide bridges (formed from pairs of cysteine residues). The sections of polypeptide that occur between two of them form a loop through which a third disulfide bond passes, forming a rotaxane-like substructure. The cystine knot motif stabilizes protein structure and is conserved in proteins across various species. There are three types of cystine knot, which differ in the topology of the disulfide bonds:
The growth factor cystine knot was first observed in the structure of nerve growth factor (NGF), solved by X-ray crystallography and published in 1991.; The GFCK is present in four superfamilies. These include nerve growth factor, transforming growth factor beta (TGF-β), platelet-derived growth factor, and glycoprotein hormones including human chorionic gonadotropin. These are structurally related due to the presence of the cystine knot motif but differ in sequence. All GFCK structures that have been determined are dimeric, but their dimerization modes in different classes are different. The vascular endothelial growth factor subfamily, categorized as part of the platelet-derived growth factor superfamily, includes proteins that are angiogenic factors.
The presence of the cyclic cystine knot (CCK) motif was discovered when Cyclotide were isolated from various plant families. The CCK motif has a cyclic backbone, triple stranded beta sheet, and cystine knot conformation.
Inhibitor cystine knot (ICK) is a structural motif with a triple stranded antiparallel beta sheet linked by three disulfide bonds, forming a knotted core. It is often found in many venom Peptide such as those of snails, spiders, and scorpions. Peptide K-PVIIA, which contains an ICK, can undergo a successful enzymatic backbone cyclization. The disulfide connectivity and the common sequence pattern of the ICK motif provides the stability of the peptides that support cyclization.
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