Cystine knot

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 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: 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 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 (GFCK) was first observed in the structure of Nerve Growth Factor, solved by X-ray crystallography and published in 1991 by Tom Blundell in Nature. The GFCK comprises four superfamilies. These include nerve growth factor, transforming growth factor beta, 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 presence of the cyclic cystine knot (CCK) motif was discovered when cyclotides were isolated from various plant families. The CCK motif has a cyclic backbone, triple stranded beta sheet, and cystine knot conformation. There are currently novel proteins being added to the cystine knot motif family, which are called the C-terminal cystine knot (CTCK) proteins. They share approximately 90 amino acid residues in their cysteine-rich C terminal regions. Inhibitor cystine knot (ICK) is a structural motif with a triple stranded antiparallel beta sheet linked by three disulfide bonds, forming a knotted core. ICK motif can be found under the category of phylum, such as animals and plants. It is usually found in many venom peptides, which is in the venoms of snails, spiders, and scorpions. Peptide K-PVIIA that contains an ICK can undergo a successful enzymatic backbone cyclization. The disulfide connectivity and the common sequence pattern of ICK motif provide the stability of the peptides that supports cyclization.