A cystine knot is a protein

structural motif In a chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a common three-dimensional structure that appears in a variety of different, evolutionarily unrelated molecules. A structural motif does not have t ...

containing three disulfide bridges (formed from pairs of

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, some ...

residues). The sections of

polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

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) * inhibitor cystine knot (ICK) common in spider and snail toxins * Cyclic Cystine Knot, or cyclotide The growth factor cystine knot was first observed in the structure of

nerve growth factor Nerve growth factor (NGF) is a neurotrophic factor and neuropeptide primarily involved in the regulation of growth, maintenance, proliferation, and survival of certain target neurons. It is perhaps the prototypical growth factor, in that it was ...

(NGF), solved by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

and published in 1991 by Tom Blundell in

Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...

.; The GFCK is present in four superfamilies. These include nerve growth factor,

transforming growth factor beta Transforming growth factor beta (TGF-β) is a multifunctional cytokine belonging to the transforming growth factor superfamily that includes three different mammalian isoforms (TGF-β 1 to 3, HGNC symbols TGFB1, TGFB2, TGFB3) and many other ...

(TGF-β),

platelet-derived growth factor Platelet-derived growth factor (PDGF) is one among numerous growth factors that regulate cell growth and division. In particular, PDGF plays a significant role in blood vessel formation, the growth of blood vessels from already-existing blood v ...

, and glycoprotein hormones including

human chorionic gonadotropin Human chorionic gonadotropin (hCG) is a hormone for the maternal recognition of pregnancy produced by trophoblast cells that are surrounding a growing embryo (syncytiotrophoblast initially), which eventually forms the placenta after implantatio ...

. 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 cyclotides were isolated from various plant families. The CCK motif has a cyclic backbone, triple stranded

beta sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a ge ...

, and cystine knot conformation. Novel proteins are being added to the cystine knot motif family, also known as 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

with a triple stranded antiparallel beta sheet linked by three disulfide bonds, forming a knotted core. The ICK motif can be found under the category of

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature ...

, such as animals and plants. It is often found in many venom

peptides Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

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.

Drug implications

The stability and structure of the cystine knot motif implicates possible applications in drug design. The hydrogen bonding between the disulfide bonds of the motif and beta-sheet structures gives rise to highly efficient structure stabilization. In addition, the size of the motif is approximately 30 amino acid residues.Kolmar, Harald. “Biological Diversity and Therapeutic Potential of Natural and Engineered Cystine Knot Miniproteins.” Current Opinion in Pharmacology, vol. 9, no. 5, 2009, pp. 608–614., doi:10.1016/j.coph.2009.05.004. These two characteristics make it an attractive biomolecule to be used for drug delivery as it exhibits thermal stability, chemical stability, and proteolytic resistance. The biological activities of these molecules are partially due to the unique interlocking arrangement and cyclized peptide backbone which contains a conserved sequence shared among circulins. Circulins have previously been identified in a screen for anti-HIV activity.K.R. Gustafson, R.C. Sowder II, L.E. Henderson, I.C. Parsons, Y. Kashman, J.H. Cardellina II, J.B. McMahon, R.W. Buckheit Jr., L.K. Pannell, M.R. Boyd Circulins A and B: novel HIV-inhibitory macrocyclic peptides from the tropical tree Chassalia parvifolia J. Am. Chem. Soc., 116 (1994), pp. 9337-9338 Studies have shown that cystine knot proteins can be incubated at temperatures of 65 °C or placed in 1 N HCl/1N NaOH without loss of structural and functional integrity.Craik, David J., et al. “The Cystine Knot Motif in Toxins and Implications for Drug Design.” Toxicon, vol. 39, no. 1, 2001, pp. 43–60., doi:10.1016/s0041-0101(00)00160-4. Its resistance from oral and some intestinal proteases suggest possible use for oral delivery. Possible future applications include pain relief as well as antiviral and antibacterial functions.

References

{{reflist Protein structure