biology

OB-fold

OB-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 OB-fold rather than just read about it. In short: In molecular biology, the OB-fold (oligonucleotide/oligosaccharide-binding fold) is a small protein structural motif observed in different proteins that bind oligonucleotides or oligosaccharides. It was originally identified in 1993 in four unrelated proteins: staphylococcal nuclease, anticodon binding domain of aspartyl-tRNA synthetase, and the B-subunits of heat-labile enterotoxin and verotoxin-1.

Key takeaways

OB-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 OB-fold to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of OB-fold from memory before moving on to harder problems.

Reference excerpt

In molecular biology, the OB-fold (oligonucleotide/oligosaccharide-binding fold) is a small protein structural motif observed in different proteins that bind oligonucleotides or oligosaccharides. It was originally identified in 1993 in four unrelated proteins: staphylococcal nuclease, anticodon binding domain of aspartyl-tRNA synthetase, and the B-subunits of heat-labile enterotoxin and verotoxin-1. Since then it has been found in multiple proteins many of which are involved in genome stability. This fold is often described as a Greek key motif.

Structure The OB-fold consists of a five-stranded β-sheet coiled to form a closed β-barrel, capped by an α-helix located at one end and a binding cleft at the other. The α-helix packs against the bottom layer of residues, roughly perpendicular to the barrel axis. The β-sheet structure protrudes beyond this layer and packs around the sides of the helix. The binding specificities of each OB-fold depend on the different length, sequence, and conformation of the loops connecting the β-strands.

Structural determinants OB-fold domains have several key structural determinants. These common features arise from physical principles governing protein structure rather than from sequence homology.

β-sheet structure: The closed β-sheet has specific parameters that determine geometrical features like mean radius and average angle between strand directions and barrel axis.

β-bulges: Most structures have a common β-bulge in the first strand. β-bulges provide small increases in barrel radius and required coiling of β-strands.

Interior residue packing: The interior of the closed β-sheet has a regular three-layer structure of residues, with each β-strand contributing one residue to each layer.

β-barrel deformation: Many β-barrels are similarly flattened, with an elliptical cross-section.

Barrel-helix interface: A cavity on the barrel axis is filled by a large hydrophobic residue from the helix.

Binding site location: In some proteins, the binding sites are located on the side surface of the β-barrel where three loops come together, in such a way they are partially wrapped by the binding partner. In others, the binding cleft at the side of the barrel opposite to the helix functions as binding site.

Function OB-folds are versatile binding domains that can interact with single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), RNA, proteins, phospholipids and oligosaccharides. In genome guardian proteins, OB-folds play crucial roles in DNA binding and recognition, protein-protein interactions and catalytic functions in multi-subunit complexes.

Examples of proteins containing this domain Single-stranded DNA binding protein (SSB) Replication Protein A (RPA) RecG helicase RuvA (part of the Holliday junction branch migration complex) Minichromosome maintenance (MCM) proteins DNA ligase III RecO (recombination mediator)

Relationship to SH3 domains OB-folds are structurally similar to Src homology 3 (SH3) domains, with their β-strands superimposing with less than 2 Å difference. This structural similarity is important for understanding OB-fold function and regulation, as SH3 domains bind to PXXP-containing ligands in a pocket similar to the ssDNA binding pocket of many OB-folds.

Evolution and distribution The OB-fold may represent a stable folding motif that appeared early in protein evolution, with its wide occurrence due to its adaptability to different functions and sequences. OB-fold proteins present great versatility, which likely contributed to the development and widespread adoption of the fold in genome guardian proteins. They can adopt various oligomerisation states and quaternary structures, allowing for complex and dynamic interactions. The OB-fold has flexibility in binding to a variety of substrates through variations in loop sizes, compositions, and insertions, showing a modular nature. In some cases, it can provide catalytic functions to multi-subunit complexes, expanding its utility beyond just binding. Its structural similarity to SH3 domains allows OB-folds to participate in protein-protein interactions, enabling regulation and complex formation.

References

External links InterPro: Nucleic acid-binding, OB-fold (IPR012340)

Illustrations

Worked examples

Example 1 — a first encounter with OB-fold

Start with the simplest possible case. Write down what OB-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 OB-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 OB-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 OB-fold

In research: OB-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 OB-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: OB-fold is common in secondary-school and first-year university syllabi. It links to neighbouring topics Protein domains, Protein folds, Protein superfamilies, so understanding it makes those chapters shorter.
In everyday life: Look for OB-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 OB-fold in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what OB-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 OB-fold out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is OB-fold in simple terms?

Why does OB-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 OB-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 OB-fold.