biology

Trefoil knot fold

Trefoil knot fold is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Trefoil knot fold rather than just read about it. In short: The trefoil knot fold is a protein fold 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. including methyltransferase proteins involve…

Key takeaways

Trefoil knot fold belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Trefoil knot fold to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Trefoil knot fold from memory before moving on to harder problems.

Reference excerpt

The trefoil knot fold is a protein fold 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. including methyltransferase proteins involved in posttranscriptional RNA modification in all three domains of life, including bacterium Thermus thermophilus and proteins, in archaea[1] and in eukaryota. 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 in which it appears. Before the discovery of the first knotted protein, it was believed that the process of protein folding could not efficiently produce deep knots in protein backbones. Studies of the folding kinetics of a dimeric protein from Haemophilus influenzae have revealed that the folding of trefoil knot proteins may depend on proline isomerization. Computational algorithms have been developed to identify knotted protein structures, both to canvas the Protein Data Bank for previously undetected natural knots and to identify knots in protein structure predictions, where they are unlikely to accurately reproduce the native-state structure due to the rarity of knots in known proteins. 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 cysteines. 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.

Trefoil domain

Trefoil (P-type) domain is a cysteine-rich domain 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; spasmolytic polypeptide (SP) (TFF2), a protein of about 115 residues that inhibits gastrointestinal motility and gastric acid secretion; intestinal trefoil factor (ITF) (TFF3); Xenopus laevis stomach proteins xP1 and xP4; xenopus integumentary mucins 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 sperm-binding protein B (ZP-B); intestinal sucrase-isomaltase (EC 3.2.1.48 / EC 3.2.1.10), a vertebrate membrane bound, multifunctional enzyme complex which hydrolyzes sucrose, maltose and isomaltose; and lysosomal alpha-glucosidase (EC 3.2.1.20).

Examples Human gene encoding proteins containing the trefoil domain include:

acid alpha-glucosidase, MGAM, TFF1, TFF2, TFF3, and ZP4.

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.

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

Illustrations

Worked examples

Example 1 — a first encounter with Trefoil knot fold

Start with the simplest possible case. Write down what Trefoil knot fold claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Trefoil knot fold before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Trefoil knot fold ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Trefoil knot fold

In research: Trefoil knot fold appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Trefoil knot fold in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Trefoil knot fold is common in secondary-school and first-year university syllabi. It links to neighbouring topics Protein domains, Protein folds, Protein tandem repeats, so understanding it makes those chapters shorter.
In everyday life: Look for Trefoil knot fold outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Trefoil knot fold in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Trefoil knot fold means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Trefoil knot fold out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Trefoil knot fold in simple terms?

Why does Trefoil knot fold matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Trefoil knot fold?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Trefoil knot fold.