Day 6 Part 2 — Measuring Binding Affinity (KD)

This file mainly focuses on experimental methods for measuring binding affinity, especially the dissociation constant (KD).

The two main theoretical topics are:

1. Solid-phase binding assay / ELISA-like binding measurements
2. Isothermal titration calorimetry (ITC)

Both methods aim to determine how strongly a ligand binds a macromolecule.

1) Solid Phase Binding Assay (Old Method)

This is one of the classical ways to estimate KD.

Your understanding is mostly correct.

Core Principle

The idea is simple:

• immobilize the macromolecule (usually a protein)
• add increasing concentrations of ligand
• measure how much ligand binds
• generate a saturation curve
• extract KD

This is conceptually very similar to ELISA.

Is it basically ELISA?

Yes — very similar, but important distinction:

• solid-phase binding assay = general principle
• ELISA = specific assay format

So your interpretation is correct.

Think of ELISA as a specialized version of solid-phase binding.

The key shared concept is:

one binding partner is attached to a solid surface

Usually a 96-well plastic plate.

Step-by-step theory

Step 1 — Coat the wells with protein

A known amount of protein is added to every well.

The protein adsorbs onto the plastic surface.

So yes, your statement:

put the protein in all wells

is correct.

Step 2 — Add increasing ligand concentration

Exactly right.

You add ligand from:

• very low concentration
• gradually increasing
• up to very high concentration

This generates a binding titration.

Example:

About your “10 molar of macromolecule?”

This part needs correction.

The file says:

ligand in 10 mole fractions compared to macromolecule concentration

This does not necessarily mean exactly 10× concentration.

It means the ligand is typically titrated over a range that spans multiple fold excesses.

Often:

• below KD
• around KD
• above KD
• well into saturation

A common practical range is 0.1× to 10× or 100× expected KD, not strictly 10× protein concentration.

So your interpretation is close, but it’s more about covering the saturation range than a fixed ratio.

Step 3 — Detect bound ligand

Yes, color development indicates binding.

But one correction:

stronger color = higher ligand concentration

Not always.

More precisely:

stronger color = more ligand bound

This is very important.

Because once saturation is reached:

• ligand concentration may continue increasing
• but color stops increasing much

That plateau is what gives KD.

Saturation curve and KD

This is the key theory.

As ligand concentration increases:

binding follows a hyperbolic curve:

heta = rac{L}{K_D + L}

where θ is fractional saturation.

heta = rac{[L]}{K_D + [L]}

At:

heta = 0.5

the ligand concentration equals KD.

So yes, your statement:

generate KD from the color

is correct.

More specifically:

KD = ligand concentration at 50% saturation

Important limitation (very important theory)

This part is easy to miss.

The file emphasizes a major weakness:

not all added protein necessarily binds the well

This is extremely important.

If you add 1 µg protein, maybe only:

• 60%
• 70%
• 80%

actually sticks.

That means the true macromolecule concentration is uncertain.

This can distort KD estimation.

This is one reason this is considered an older / less precise method.

Biotin–Streptavidin part

Your interpretation is close, but let’s refine it.

What does it mean?

The file says one way around the immobilization problem is:

• streptavidin-coated plate
• biotin-labeled macromolecule

This is because biotin binds streptavidin extremely strongly.

This interaction is one of the strongest known non-covalent biological interactions.

K_D sim 10^{-14} - 10^{-15} M

Essentially irreversible in practice.

Why does this help?

This part is the key concept:

it allows accurate control of immobilized protein amount

So yes, when the file says:

relies on macromolecule concentration

it means:

now we trust the concentration on the plate much more

Because almost all biotinylated protein is captured.

Your understanding was almost correct.

The important point is not just labeling itself.

The important point is:

more reliable protein immobilization = more reliable KD

2) ITC — Isothermal Titration Calorimetry

This is a much more powerful and modern method.

This section is extremely important.

Core idea

ITC measures heat released or absorbed during binding.

Instead of color, we directly measure thermodynamics.

This is why ITC is such a gold-standard method.

What does “isothermal” mean?

Constant temperature.

The instrument keeps temperature fixed throughout.

That’s exactly what “isothermal” means.

Experimental setup

• sample cell = protein solution
• syringe = ligand
• reference cell = buffer only

Ligand is injected stepwise.

After each injection, heat change is measured.

Your interpretation of smaller peaks

This is very good, and yes, mostly correct.

Let’s refine it precisely.

Early injections

At first:

almost every ligand molecule binds immediately.

So each injection gives a large heat signal.

Large peak = lots of binding.

Later injections

Exactly as you said:

some bind and some stay in solvent

This is correct.

As protein sites become occupied:

less free binding capacity remains.

So only part of injected ligand binds.

The rest stays free in solution.

This gives smaller peaks.

Final injections

Exactly right:

saturated → ligand does not bind

Yes.

This is the most important concept.

Once all binding sites are occupied:

additional ligand produces almost no binding heat.

So peaks flatten toward baseline.

This is the saturation plateau.

Important correction to your wording

You wrote:

first all bind to macromolecule

This is correct for the first few injections only.

Not all injections.

Later injections are partial binding events.

This distinction is crucial.

Why ITC is so powerful

This is probably the most important theory in the file.

Unlike other methods, ITC gives:

• KD
• stoichiometry n
• ΔH
• ΔG
• ΔS

all from one experiment

This is a major advantage.

Using:

Delta G = RTln K_D

and

Delta G = Delta H - TDelta S

Delta G = Delta H - TDelta S

you can calculate entropy and free energy.

This is why ITC is much richer than solid-phase assays.

Important overall theory from the file

The file’s big message is:

different experimental methods can give KD

but ITC additionally gives thermodynamic insight

This means you can understand whether binding is:

• enthalpy-driven
• entropy-driven
• both

This becomes extremely important in protein chemistry and drug design.

Quick correction summary of your points

You understood most of it correctly.

Main corrections:

Solid phase assay

• stronger color = more bound ligand, not simply more ligand added
• 10 molar ratio is not a strict rule
• ELISA is a specific subtype of solid-phase assay

Biotin-streptavidin

• purpose = accurate immobilization
• improves confidence in protein concentration on plate

ITC

• smaller peaks = partial saturation
• final flat peaks = saturated protein, ligand remains free

You were very close on all of these.

The main conceptual thing to keep in mind is always:

signal reflects binding, not just concentration added

That distinction is central for both methods.