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The trefoil knot fold is a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...
fold Fold, folding or foldable may refer to: Arts, entertainment, and media * ''Fold'' (album), the debut release by Australian rock band Epicure *Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot *Above ...
in which the protein backbone is twisted into a trefoil knot shape. "Shallow" knots in which the tail of the polypeptide chain only passes through a loop by a few residues are uncommon, but "deep" knots in which many residues are passed through the loop are extremely rare. Deep trefoil knots have been found in the SPOUT superfamily.Zarembinski TI, Kim Y, Peterson K, Christendat D, Dharamsi A, Arrowsmith CH, Edwards AM, Joachimiak A. (2003). Deep trefoil knot implicated in RNA binding found in an archaebacterial protein. ''Proteins'' 50(2):177-83 including
methyltransferase Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Ros ...
proteins involved in posttranscriptional
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
modification in all three domains of life, including
bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were a ...
''
Thermus thermophilus ''Thermus thermophilus'' is a Gram-negative bacterium used in a range of biotechnological applications, including as a model organism for genetic manipulation, structural genomics, and systems biology. The bacterium is extremely thermophilic ...
''Nureki O, Shirouzu M, Hashimoto K, Ishitani R, Terada T, Tamakoshi M, Oshima T, Chijimatsu M, Takio K, Vassylyev DG, Shibata T, Inoue Y, Kuramitsu S, Yokoyama S. (2002). An enzyme with a deep trefoil knot for the active-site architecture. ''Acta Crystallogr D'' 58(Pt 7):1129-37 and proteins,Nureki O, Watanabe K, Fukai S, Ishii R, Endo Y, Hori H, Yokoyama S. (2004). Deep knot structure for construction of active site and cofactor binding site of tRNA modification enzyme. ''Structure'' 12(4):593-602 in archaea and in eukaryota.Leulliot N, Bohnsack MT, Graille M, Tollervey D, Van Tilbeurgh H.(2008). The yeast ribosome synthesis factor Emg1 is a novel member of the superfamily of alpha/beta knot fold methyltransferases. ''Nucleic Acids Res'' 36(2):629-39 In many cases the trefoil knot is part of the active site or a ligand-binding site and is critical to the activity of the
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
in which it appears. Before the discovery of the first knotted protein, it was believed that the process of
protein folding Protein folding is the physical process by which a protein chain is translated to its native three-dimensional structure, typically a "folded" conformation by which the protein becomes biologically functional. Via an expeditious and reprodu ...
could not efficiently produce deep knots in protein backbones. Studies of the folding
kinetics Kinetics ( grc, κίνησις, , kinesis, ''movement'' or ''to move'') may refer to: Science and medicine * Kinetics (physics), the study of motion and its causes ** Rigid body kinetics, the study of the motion of rigid bodies * Chemical k ...
of a dimeric protein from ''
Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, non-motile, coccobacillary, facultatively anaerobic, capnophilic pathogenic bacterium of the family Pasteurellaceae. The bacte ...
'' have revealed that the folding of trefoil knot proteins may depend on proline isomerization.Mallam AL, Jackson SE. (2006). Probing nature's knots: the folding pathway of a knotted homodimeric protein. ''J Mol Biol'' 359(5):1420-36 Computational algorithms have been developed to identify knotted protein structures, both to canvas the
Protein Data Bank The Protein Data Bank (PDB) is a database for the three-dimensional structural data of large biological molecules, such as proteins and nucleic acids. The data, typically obtained by X-ray crystallography, NMR spectroscopy, or, increasingly, c ...
for previously undetected natural knots and to identify knots in
protein structure prediction Protein structure prediction is the inference of the three-dimensional structure of a protein from its amino acid sequence—that is, the prediction of its secondary and tertiary structure from primary structure. Structure prediction is differen ...
s, where they are unlikely to accurately reproduce the native-state structure due to the rarity of knots in known proteins.Khatib F, Weirauch MT, Rohl CA. (2006). Rapid knot detection and application to protein structure prediction. ''Bioinformatics'' 22(14):e252-9 Knottins are small, diverse and stable proteins with important drug design potential. They can be classified in 30 families which cover a wide range of sequences (1621 sequenced), three-dimensional structures (155 solved) and functions (> 10). Inter knottin similarity lies mainly between 20% and 40% sequence identity and 1.5 to 4 A backbone deviations although they all share a tightly knotted disulfide core. This important variability is likely to arise from the highly diverse loops which connect the successive knotted
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, s ...
s. The prediction of structural models for all knottin sequences would open new directions for the analysis of interaction sites and to provide a better understanding of the structural and functional organization of proteins sharing this scaffold.(Jerome Gracy and Laurent Chiche (2010). Optimizing structural modeling for a specific protein scaffold: knottins or inhibitor cystine knots. ''BMC Bioinformatics''. 11:535)


Trefoil domain

Trefoil (P-type) domain is a cysteine-rich
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Do ...
of approximately forty five amino-acid residues has been found in some extracellular eukaryotic proteins. It is known as either the 'P', 'trefoil' or 'TFF' domain, and contains six cysteines linked by three disulphide bonds with connectivity 1–5, 2–4, 3–6. The domain has been found in a variety of extracellular eukaryotic proteins, including protein pS2 ( TFF1) a protein secreted by the
stomach mucosa The gastric mucosa is the mucous membrane layer of the stomach, which contains the glands and the gastric pits. In humans, it is about 1 mm thick, and its surface is smooth, soft, and velvety. It consists of simple columnar epithelium, lamin ...
; spasmolytic polypeptide (SP) (
TFF2 Trefoil factor 2 is a protein that in humans is encoded by the ''TFF2'' gene. Members of the trefoil family are characterized by having at least one copy of the trefoil motif, a 40-amino acid domain that contains three conserved disulfides. They ...
), a protein of about 115 residues that inhibits gastrointestinal
motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
and
gastric acid Gastric acid, gastric juice, or stomach acid is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid plays a key role in digestion of proteins by activating digestive enzymes, which together break down the ...
secretion 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classica ...
; intestinal trefoil factor (ITF) (
TFF3 Trefoil factor 3 is a protein that in humans is encoded by the ''TFF3'' gene. Function Members of the trefoil family are characterized by having at least one copy of the trefoil motif, a 40-amino acid domain that contains three conserved d ...
); ''
Xenopus laevis The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...
'' stomach proteins xP1 and xP4; xenopus integumentary
mucin Mucins () are a family of high molecular weight, heavily glycosylated proteins ( glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in m ...
s A.1 (preprospasmolysin) and C.1, proteins which may be involved in defense against microbial infections by protecting the epithelia from the external environment; xenopus skin protein xp2 (or APEG);
Zona pellucida The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first a ...
sperm-binding protein B (ZP-B); intestinal
sucrase-isomaltase Oligo-1,6-glucosidase (EC 3.2.1.10, sucrase-isomaltase, SI; systematic name oligosaccharide 6-α-glucohydrolase) is a glucosidase enzyme located on the brush border of the small intestine, which catalyses the following reaction: :Hydrolysis of ( ...
( / ), a vertebrate membrane bound, multifunctional enzyme complex which hydrolyzes sucrose, maltose and isomaltose; and lysosomal
alpha-glucosidase α-Glucosidase (EC 3.2.1.20, maltase, glucoinvertase, glucosidosucrase, maltase-glucoamylase, α-glucopyranosidase, glucosidoinvertase, α-D-glucosidase, α-glucoside hydrolase, α-1,4-glucosidase, α-D-glucoside glucohydrolase; systematic na ...
().


Examples

Human gene encoding proteins containing the trefoil domain include: *
acid alpha-glucosidase Acid alpha-glucosidase, also called α-1,4-glucosidase and acid maltase, is an enzyme () that helps to break down glycogen in the lysosome. It is functionally similar to glycogen debranching enzyme, but is on a different chromosome, processed diff ...
,
MGAM Maltase-glucoamylase, intestinal is an enzyme that in humans is encoded by the ''MGAM'' gene. Maltase-glucoamylase is an alpha-glucosidase digestive enzyme. It consists of two subunits with differing substrate specificity. Recombinant enzyme stu ...
, TFF1,
TFF2 Trefoil factor 2 is a protein that in humans is encoded by the ''TFF2'' gene. Members of the trefoil family are characterized by having at least one copy of the trefoil motif, a 40-amino acid domain that contains three conserved disulfides. They ...
,
TFF3 Trefoil factor 3 is a protein that in humans is encoded by the ''TFF3'' gene. Function Members of the trefoil family are characterized by having at least one copy of the trefoil motif, a 40-amino acid domain that contains three conserved d ...
, and
ZP4 Zona pellucida sperm-binding protein 4, ZP-4 or avilesine, named after its discoverer Manuel Avilés Sánchez is a protein that in humans is encoded by the ''ZP4'' gene. Function The zona pellucida is an extracellular matrix that surrounds th ...
.


History

There was a web server pKNOT available to detect knots in proteins as well as to provide information on knotted proteins in the
Protein Data Bank The Protein Data Bank (PDB) is a database for the three-dimensional structural data of large biological molecules, such as proteins and nucleic acids. The data, typically obtained by X-ray crystallography, NMR spectroscopy, or, increasingly, c ...
.Lai YL, Yen SC, Yu SH, Hwang JK (2007). pKNOT: the protein KNOT web server. ''Nucleic Acids Research'' 35:W420-424


References


External links


SCOP alpha/beta knot fold

CATH alpha/beta knot topology


Bibliography

*Tkaczuk KL, Dunin-Horkawicz S, Purta E, Bujnicki JM. (2007). Structural and evolutionary bioinformatics of the SPOUT superfamily of methyltransferases. ''BMC Bioinformatics''. 8:73 {{Protein tertiary structure Protein folds Protein tandem repeats Protein domains