What Is Noncompetitive Inhibition?

In enzyme kinetics, noncompetitive inhibition is a type of reversible enzyme inhibition where an inhibitor binds to an enzyme at a site other than the active site, reducing the enzyme’s overall activity.

Unlike competitive inhibition — where inhibitors fight for the active site — noncompetitive inhibitors change the enzyme’s shape or conformation, making it less effective even when substrate is present.

To understand this quantitatively, we use two important constants: α (alpha) and α′ (alpha prime).

Understanding Alpha (α) and Alpha Prime (α′)

These constants describe how the inhibitor affects binding to the enzyme and the enzyme-substrate complex.

Where:

• [I] = inhibitor concentration
• Kᵢ = inhibition constant for inhibitor binding to the enzyme
• Kᵢ′ = inhibition constant for inhibitor binding to the enzyme–substrate complex

How to Calculate Alpha and Alpha Prime — Step by Step

Let’s walk through an example.

Example:

Suppose you are studying an enzyme with the following data:

• Inhibitor concentration, [I] = 0.5 mM
• Kᵢ = 0.25 mM
• Kᵢ′ = 1.0 mM

Step 1: Calculate α

α=1+[I]Kiα=1+Ki​[I]​α=1+0.50.25=1+2=3α=1+0.250.5​=1+2=3

Step 2: Calculate α′

α′=1+[I]Ki′α′=1+Ki​′[I]​α′=1+0.51.0=1+0.5=1.5α′=1+1.00.5​=1+0.5=1.5

Result:

• α = 3
• α′ = 1.5

What Does This Mean?

In noncompetitive inhibition:

• α and α′ are not equal, indicating that the inhibitor affects both the free enzyme and the enzyme–substrate complex differently.
• Because both are greater than 1, the inhibitor reduces enzyme activity.

When α = α′ → pure noncompetitive inhibition
When α ≠ α′ → mixed inhibition

How This Appears in the Lineweaver–Burk Plot

In a Lineweaver–Burk (double-reciprocal) plot:

• The Vmax decreases (since the enzyme’s catalytic ability is reduced).
• The Km may stay the same or change slightly, depending on whether α equals α′.
• Lines intersect left of the y-axis, a hallmark of mixed or noncompetitive inhibition.

Pro Tip: How to Recognize Noncompetitive Inhibition Experimentally

1. Plot 1/V vs 1/[S] at different [I] values.
2. If all lines intersect left of the y-axis but above the x-axis, it’s likely noncompetitive inhibition.
3. Use nonlinear regression or software like GraphPad Prism to estimate α and α′ directly.

Quick Summary Table

FAQ

Q1. Why are α and α′ important?
They allow biochemists to quantify how inhibitors affect enzyme kinetics and distinguish between different inhibition types.

Q2. What if α′ < α?
Then the inhibitor binds more tightly to the enzyme–substrate complex than to the free enzyme, suggesting mixed inhibition.

Q3. Do α and α′ have units?
No — both are dimensionless constants.

Final Thoughts

Learning how to calculate alpha (α) and alpha prime (α′) is crucial for mastering enzyme kinetics. Whether you’re working in a lab or studying for an exam, understanding these constants helps you interpret how inhibitors modulate enzyme function — a concept that’s essential in drug design, metabolic engineering, and biochemistry.