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Enzyme Inhibition
Four Mechanisms

How molecules slow enzymes by changing \( K_m \) and \( V_{max} \) — competitive, noncompetitive, uncompetitive, and mixed.

Biochemistry

What Is an Inhibitor?

An enzyme inhibitor is a molecule that binds to an enzyme and reduces its catalytic activity. Inhibitors are central to drug design, metabolic regulation, and toxin biology.

The Michaelis–Menten Equation

\( V_{max} \)
Maximum reaction rate when all enzyme is saturated with substrate.
\( K_m \)
Substrate concentration at half of \( V_{max} \). A lower \( K_m \) means tighter binding.

Reading the Saturation Curve

\([S]\) substrate conc. \(v\) (rate) \(V_{max}\) \(\frac{V_{max}}{2}\) \(K_m\)

Competitive Inhibition

Active site Inhibitor Substrate (blocked out)

Noncompetitive Inhibition

Substrate still binds Inhibitor at separate site

Uncompetitive Inhibition

Substrate bound first Inhibitor binds ONLY the ES complex

Mixed Inhibition

A mixed inhibitor binds an allosteric site on both the free enzyme and the ES complex, but with different affinities. It is the general case that contains the others as special limits.

The Lineweaver–Burk Plot

Taking the reciprocal of Michaelis–Menten gives a straight line. The y-intercept is \( \frac{1}{V_{max}} \), the x-intercept is \( -\frac{1}{K_m} \), and the slope is \( \frac{K_m}{V_{max}} \). Each inhibitor type shifts these intercepts in a characteristic way.

Lineweaver–Burk Signatures

\(\frac{1}{[S]}\) \(\frac{1}{v}\) No inhibitor Competitive Noncompet.

Competitive vs. Uncompetitive

Competitive
Binds free enzyme at the active site. \( K_m \uparrow \), \( V_{max} \) same. Overcome by more substrate.
Uncompetitive
Binds ES complex only. \( K_m \downarrow \), \( V_{max} \downarrow \). More substrate makes it worse.

How to Identify the Type

1
Check \( V_{max} \). Unchanged → competitive. Decreased → noncompetitive, uncompetitive, or mixed.
2
Check \( K_m \). Increased → competitive or mixed. Decreased → uncompetitive. Unchanged → noncompetitive.
3
Read the Lineweaver–Burk lines. Where do they intersect — y-axis, x-axis, or off-axis?
4
Confirm with mechanism. Which form of the enzyme does the inhibitor bind?

Worked Example 1

An inhibitor raises the measured \( K_m \) from \( 2\,\text{mM} \) to \( 6\,\text{mM} \), but \( V_{max} \) stays at \( 100\,\mu\text{M/s} \). What type is it?

Worked Example 2

A drug lowers \( V_{max} \) from \( 80 \) to \( 40\,\mu\text{M/s} \) and also lowers \( K_m \) from \( 5 \) to \( 2.5\,\text{mM} \). Both parameters drop by the same factor. Which inhibition is this?

Worked Example 3

On a Lineweaver–Burk plot, the inhibited and uninhibited lines intersect exactly on the x-axis. \( V_{max} \) is reduced. What type?

Common Mistakes

✕ Reversing Km direction
Competitive raises \( K_m \); uncompetitive lowers it. Don't swap these.
✕ "More substrate always helps"
True only for competitive. For uncompetitive, more substrate worsens inhibition.
✕ Confusing the binding site
Only competitive binds the active site; the rest use allosteric sites.

Summary Table

Type \( V_{max} \) \( K_m \) Binds
Competitivesame↑free E
Noncompetitive↓sameE and ES
Uncompetitive↓↓ES only
Mixed↓↑ or ↓E and ES

Question 1 of 5

A competitive inhibitor affects which kinetic parameters?

Question 2 of 5

An inhibitor that binds only the enzyme–substrate complex is called:

Question 3 of 5

On a Lineweaver–Burk plot, lines intersecting on the y-axis indicate:

Question 4 of 5

For which inhibitor type does adding more substrate make inhibition worse?

Question 5 of 5

A noncompetitive inhibitor produces which combination?