chemistry

Ligand efficiency

Ligand efficiency 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 Ligand efficiency rather than just read about it. In short: Ligand efficiency is a measurement of the binding energy per atom of a ligand to its binding partner, such as a receptor or enzyme. Ligand efficiency is used in drug discovery research programs to assist in narrowing focus to lead compounds with optimal combinations of physicochemical properties and pharmacological properties.

Key takeaways

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

Reference excerpt

Ligand efficiency is a measurement of the binding energy per atom of a ligand to its binding partner, such as a receptor or enzyme. Ligand efficiency is used in drug discovery research programs to assist in narrowing focus to lead compounds with optimal combinations of physicochemical properties and pharmacological properties. Mathematically, ligand efficiency (LE) can be defined as the ratio of Gibbs free energy (ΔG) to the number of non-hydrogen atoms of the compound:

LE = -(ΔG)/N where ΔG = −RTlnKi and N is the number of non-hydrogen atoms. It can be transformed to the equation:

LE = 1.4(−log IC50)/N

Other metrics Some suggest that better metrics for ligand efficiency are percentage/potency efficiency index (PEI), binding efficiency index (BEI) and surface-binding efficiency index (SEI) because they are easier to calculate and take into account the differences between elements in different rows of the periodic table. PEI is a relative measure for comparing compounds tested in the same conditions (e.g. a single-point assay) and are not comparable at different inhibitor concentrations. Also for BEI and SEI, similar measurements must be used (e.g. always using pKi).

PEI = (% inhibition at a given compound concentration as fraction: 0 – 1.0) / (molecular weight, kDa) BEI = (pKi, pKd, or pIC50) / (molecular weight, kDa) SEI = (pKi, pKd, or pIC50) / (PSA/100 Å) where pKi, pKd and pIC50 is defined as −log(Ki), −log(Kd), or −log(IC 50), respectively. Ki and IC50 in mol/L. The authors suggest plotting compounds SEI and BEI on a plane and optimizing compounds towards the diagonal and so optimizing both SEI and BEI which incorporate potency, molecular weight and PSA. There are other metrics which can be useful during hit to lead optimization: group efficiency (GE), lipophilic efficiency/lipophilic ligand efficiency (LipE/LLE), ligand lipophilicity index (LLEAT) ligand efficiency dependent lipophilicity (LELP), fit quality scaled ligand efficiency (LEscale), size independent ligand efficiency (SILE). Group efficiency (GE) is a metric used to estimate the binding efficiency of groups added to a ligand. Unlike ligand efficiency which evaluates the efficiency of the entire molecule, group efficiency measures the relative change of the Gibbs free energy (ΔΔG), caused by addition or modification of groups, normalized by the change in the number heavy atoms in those groups (ΔN), using the equation:

GE = -(ΔΔG)/ΔN

See also Drug design Drug discovery hit to lead

References

Worked examples

Example 1 — a first encounter with Ligand efficiency

Start with the simplest possible case. Write down what Ligand efficiency 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 Ligand efficiency 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 Ligand efficiency 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 Ligand efficiency

In research: Ligand efficiency 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 Ligand efficiency 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: Ligand efficiency is common in secondary-school and first-year university syllabi. It links to neighbouring topics Drug discovery, Medicinal chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Ligand efficiency 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 Ligand efficiency in 20 minutes

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

Frequently asked questions

What is Ligand efficiency in simple terms?

Why does Ligand efficiency 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 Ligand efficiency?

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 Ligand efficiency.