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Protein Purification
Chromatography Overview

Learn how scientists isolate and purify proteins from complex mixtures using different chromatography techniques based on size, charge, and binding affinity.

Biochemistry

Why Purify Proteins?

Cells contain thousands of different proteins mixed together. To study any single protein: its structure, function, or use it as a drug: we must first separate it from everything else. Purification lets us work with a single, defined molecule.

Research
Determine 3D structure and enzyme kinetics
Medicine
Produce therapeutic proteins like insulin
Industry
Use enzymes in food processing and diagnostics

What Is Chromatography?

Chromatography is a separation technique where a mixture is passed through a stationary phase (packed in a column) while carried by a mobile phase (buffer solution). Different proteins interact with the stationary phase to different degrees, causing them to elute at different times.

Sample mixture Eluate (collected fractions) Stationary phase (beads) Mobile phase (buffer)

Key Terminology

1
Stationary phase: The material packed inside the column (resin/beads) that proteins interact with.
2
Mobile phase: The buffer solution that carries proteins through the column.
3
Elution: Washing proteins off the column, often with a gradient of salt or pH.
4
Fraction: A small collected volume of eluate. Each fraction may contain different proteins.
5
Resolution: The ability to distinguish (separate) two protein peaks from each other.

Three Major Types

Each chromatography method separates proteins based on a different physical property. Choosing the right method depends on what makes your target protein unique.

Size Exclusion
Separates by molecular size: large proteins elute first, small ones enter pores and elute later
Ion Exchange
Separates by net charge: proteins bind to oppositely charged beads and are eluted by a salt gradient
Affinity
Separates by specific binding: the column has a ligand that binds only the target protein

Size-Exclusion Chromatography

L Elution volume Absorbance Large Small

Ion-Exchange Chromatography

− Bead − − − − + − Cation Exchange (negative beads) + Bead + + + + − + Anion Exchange (positive beads) - - - = electrostatic binding

Affinity Chromatography

Bead Ligand Target Washed away 1. Load & Bind Target binds ligand 2. Wash Remove contaminants Bead Target 3. Elute Compete off target

Comparing the Three Types

Property Size Exclusion Ion Exchange Affinity
Basis of separation Molecular size Net charge Specific binding
Binding to column? No Yes Yes
Elution method Continuous flow Salt / pH gradient Competing ligand
Selectivity Low Medium Very high
Typical use Final polish / buffer exchange Early purification step Capture step

Measuring Purity: The Purification Factor

After each chromatography step, we calculate specific activity to track how pure our protein has become. Specific activity equals enzyme activity divided by total protein:

A rising specific activity with each step means contaminating proteins are being removed, increasing the proportion of our target protein.

Worked Example: Purification Table

You are purifying enzyme X from a bacterial lysate. After each step, you measure total activity and total protein:

Step Total Protein (mg) Total Activity (U) Specific Activity Purification Factor
Crude extract 1000 10,000 10 U/mg 1.0×
Ion exchange 200 8,000 40 U/mg 4.0×
Affinity 10 7,000 700 U/mg 70×

Tracking Yield

While we want high purity, we also want to recover as much protein as possible. The yield tells us what fraction of the original target enzyme we still have:

Using our previous example after the affinity step:

Step-by-Step Purification Strategy

1
Lyse cells: Break open cells to release all proteins into solution (crude extract).
2
Centrifuge: Remove cell debris and insoluble material by spinning at high speed.
3
Ion-exchange chromatography: Bind target by charge, wash away uncharged contaminants, elute with salt gradient.
4
Affinity chromatography: Capture target with a specific ligand for dramatic purity increase.
5
Size-exclusion chromatography: Polish the sample, remove aggregates, and exchange into final buffer.
6
Verify purity: Run SDS-PAGE gel to confirm a single band at the expected molecular weight.

Verifying Purity with SDS-PAGE

Crude IEX Affinity 250 kDa Target 10 kDa + − Migration

Reading an Elution Profile

Elution Volume (mL) Absorbance (280 nm) 0 50 100 150 200 [NaCl] Contam. 1 Target Contam. 2 Collect these fractions

Common Mistakes

✕ Confusing Elution Order in SEC
In size-exclusion chromatography, large proteins elute first. Students often assume small molecules come out first because they are "faster."
✕ Mixing Up Ion-Exchange Types
A cation exchanger has negative beads (binds cations). An anion exchanger has positive beads (binds anions). The name refers to what it catches, not what it is.
✕ Ignoring Yield
High purification factor is meaningless if yield drops to 1%. Always track both specific activity and % yield at each step.

Question 1 of 5

In size-exclusion chromatography, which protein elutes first from the column?

Question 2 of 5

A protein with a net negative charge at pH 7 would bind to which type of ion-exchange column?

Question 3 of 5

A crude extract has a total activity of 5,000 U and total protein of 500 mg. After one purification step, total activity is 4,000 U and total protein is 50 mg. What is the purification factor?

Question 4 of 5

Which chromatography technique provides the highest selectivity in a single step?

Question 5 of 5

In an ion-exchange purification, bound proteins are typically eluted by: