chemistry

Half sphere exposure

Half sphere exposure is a chemistry 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 Half sphere exposure rather than just read about it. In short: Half Sphere exposure (HSE) is a protein solvent exposure measure that was first introduced by Hamelryck (2005). Like all solvent exposure measures it measures how buried amino acid residues are in a protein.

Key takeaways

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

Reference excerpt

Half Sphere exposure (HSE) is a protein solvent exposure measure that was first introduced by Hamelryck (2005). Like all solvent exposure measures it measures how buried amino acid residues are in a protein. It is found by counting the number of amino acid neighbors within two half spheres of chosen radius around the amino acid. The calculation of HSE is found by dividing a contact number (CN) sphere in two halves by the plane perpendicular to the Cβ-Cα vector. This simple division of the CN sphere results in two strikingly different measures, HSE-up and HSE-down. HSE-up is defined as the number of Cα atoms in the upper half (containing the pseudo-Cβ atom) and analogously HSE-down is defined as the number of Cα atoms in the opposite sphere. If only Cα atoms are available (as is the case for many simplified representations of protein structure), a related measure, called HSEα, can be used. HSEα uses a pseudo-Cβ instead of the real Cβ atom for its calculation. The position of this pseudo-Cβ atom (pCβ) is derived from the positions of preceding Cα−1 and the following Cα+1. The Cα-pCβ vector is calculated by adding the Cα−1-Cα0 and Cα+1-Cα0 vectors. HSE is used in predicting discontinuous B-cell epitopes. Song et al. have developed an online webserver termed HSEpred to predict half-sphere exposure from protein primary sequences. HSEpred server can achieve the correlation coefficients of 0.72 and 0.68 between the predicted and observed HSE-up and HSE-down measures, respectively, when evaluated on a well-prepared non-homologous protein structure dataset. Moreover, residue contact number (CN) can also be accurately predicted by HSEpred webserver using the summation of the predicted HSE-up and HSE-down values, which has further enlarged the application of this new solvent exposure measure. Recently, Heffernan et al. has developed the most accurate predictor for both HSEα and HSEβ based on a big dataset by using multiple-step iterative deep neural-network learning. The predicted HSEa shows a higher correlation coefficient to the stability change by residue mutants than predicted HSEβ and ASA. The results, together with its easy Ca-atom-based calculation, highlight the potential usefulness of predicted HSEa for protein structure prediction and refinement as well as function prediction.

References

Illustrations

Half sphere exposure: Half Sphere Exposure (HSE) construction. This simple, two-dimensional measure of solvent exposure counts the number of neighbors in two domes (with radius R typically equal to 10 or 12 Å) around the Cα atom. It is simple and extremely fast to compute, and superior to the widely used Contact Number measure. The HSE value pair (up and down) of the example above is (3,5). — Half Sphere Exposure (HSE) construction. This simple, two-dimensional measure of solvent exposure counts the number of neighbors in two domes (with radius R typically equal to 10 or 12 Å) around the Cα atom. It is simple and extremely fast to compute, and superior to the widely used Contact Number measure. The HSE value pair (up and down) of the example above is (3,5).

Worked examples

Example 1 — a first encounter with Half sphere exposure

Start with the simplest possible case. Write down what Half sphere exposure claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, 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 Half sphere exposure 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 Half sphere exposure 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 Half sphere exposure

In research: Half sphere exposure appears in chemistry 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 Half sphere exposure 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: Half sphere exposure is common in secondary-school and first-year university syllabi. It links to neighbouring topics Amino acids, Nitrogen cycle, Protein structure, so understanding it makes those chapters shorter.
In everyday life: Look for Half sphere exposure 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.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Half sphere exposure” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Half sphere exposure in 20 minutes

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

Frequently asked questions

What is Half sphere exposure in simple terms?

Why does Half sphere exposure matter?

Because it connects several chemistry 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 Half sphere exposure?

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 Half sphere exposure.