Enzyme Purification Calculator

Calculate specific activity, fold purification, and percent yield for protein purification tables.

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The Purification Table

Enzyme purification is tracked with two numbers that move in opposite directions. Specific activity rises as contaminating protein is removed; yield falls as some target enzyme is inevitably lost at each step.

Specific activity = total units / mg protein     Fold purification = SAstep / SAcrude
StepTypical foldTypical yieldBasis of separation
Crude extract1×100%—
Ammonium sulfate2–5×70–90%Solubility
Ion exchange10–50×50–80%Net charge
Gel filtration5–20×70–90%Size
Affinity100–1,000×50–90%Specific binding

Affinity chromatography dominates because it separates on a property unique to the target rather than a general physical characteristic. A single affinity step often achieves what several conventional steps cannot.

Reading the Trade-Off

A step giving high fold purification but poor yield may still be worth using if the protein is abundant; a low-fold step with excellent recovery may be better for a scarce one. The judgement is about what remains at the end, not about maximising either number alone.

Specific activity plateaus when the preparation reaches homogeneity — further steps lose material without raising it. That plateau is the practical signal to stop, and it should be confirmed by a single band on SDS-PAGE rather than by the activity figure alone.

Occasionally specific activity rises unexpectedly across a step that should not purify much. This often means an inhibitor was removed rather than that protein was separated, and it is worth investigating rather than accepting.

Worked Examples

Example 1: Affinity column: 9000 units, 0.9mg protein
SA=10,000 units/mg vs crude 0.1
Result: Fold=100,000x — excellent affinity result
90% yield at 100,000-fold purification
Example 2: Ion exchange: 7000 units, 140mg
SA=50 units/mg, fold=500x
Result: 70% yield at 500-fold
Typical ion exchange step result
Example 3: Yields compound
Four steps at 70% recovery each
Result: 24% overall yield
0.704 = 0.24. Adding a marginal step for a small purity gain can cost more material than it is worth.
Example 4: Reaching the plateau
Specific activity unchanged across an additional column
Result: Preparation is already homogeneous
Further steps lose protein without improving purity. Confirm with SDS-PAGE and stop.

Common Mistakes

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Chasing fold purification at any yield cost

A 1,000-fold step recovering 5% may leave less usable protein than a 50-fold step recovering 80%. Judge by what remains at the end.

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Assuming high specific activity means pure

It plateaus at homogeneity, but a co-purifying inactive contaminant can also inflate the apparent value. Confirm purity by SDS-PAGE.

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Forgetting that yields multiply

Four steps at 70% each leave 24% of the original activity. Step count matters as much as individual performance.

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Ignoring an unexpected activity increase

Specific activity rising more than a step should allow often indicates inhibitor removal rather than protein separation.

Frequently Asked Questions

Specific activity definition?
Units enzyme activity per mg total protein. Activity unit: 1 μmol substrate converted per minute. Specific activity increases as purification proceeds. Homogeneous enzyme: maximum specific activity.
Purification table?
Step | Total protein | Total activity | Specific activity | Fold | Yield%. Yield must decrease each step. Fold purification increases. Final product: high fold + acceptable yield.
What is specific activity?
Enzyme units per milligram of total protein. It rises as contaminating protein is removed and plateaus when the preparation is homogeneous.
Why is affinity chromatography so effective?
It separates on a binding property unique to the target rather than a general physical characteristic, often achieving 100 to 1,000-fold purification in one step.
How do yields combine across steps?
They multiply. Four steps at 70% each leave 24% of the original activity, so minimising step count matters.
What does it mean if specific activity rises unexpectedly?
Often that an inhibitor has been removed rather than protein separated. It is worth investigating rather than assuming exceptional purification.