Step by Step
1/v
What the Lineweaver-Burk plot is
The Lineweaver-Burk plot (double reciprocal plot) is a graphical representation of enzyme kinetics. It plots 1/v (reciprocal of reaction velocity) on the y-axis against 1/[S] (reciprocal of substrate concentration) on the x-axis. This transforms the hyperbolic Michaelis-Menten curve into a straight line — making Km and Vmax easy to read off directly.
Memory trick: "Double reciprocal" = flip both axes. Hyperbola → straight line.
Y
y-intercept = 1/Vmax
Where the line crosses the y-axis (1/v axis), the value is 1/Vmax. Therefore Vmax = 1 ÷ (y-intercept). A lower y-intercept means a higher Vmax.
Memory trick: Y = 1/Vmax. "Y is for Vmax" — just take the reciprocal of the y-intercept.
X
x-intercept = −1/Km
Where the line crosses the x-axis (1/[S] axis), the value is −1/Km. Therefore Km = −1 ÷ (x-intercept). Note the negative sign — the x-intercept is always on the negative side of the x-axis for real enzymes.
Memory trick: X = −1/Km. Negative x-intercept gives you Km when you flip and negate.
Inh
Identifying inhibition type on the plot
Competitive inhibition: lines cross the y-axis at the same point (same Vmax) but have different x-intercepts (different apparent Km — shifted left). Non-competitive inhibition: lines cross the x-axis at the same point (same Km) but have different y-intercepts (different Vmax — higher y-intercept = lower Vmax).
Memory trick: Competitive = same y, different x. Non-competitive = same x, different y.
Applied Walkthrough
1
An enzyme's Lineweaver-Burk plot shows y-intercept = 0.5 and x-intercept = −2.
2
Vmax = 1/0.5 = 2 (units/min). Km = 1/2 = 0.5 mM.
3
A competitive inhibitor is added. The new line has the same y-intercept (0.5) but a different, less negative x-intercept — confirming Vmax unchanged, Km increased.
4
A non-competitive inhibitor is added instead. The new line has the same x-intercept (−2) but a higher y-intercept — confirming Km unchanged, Vmax decreased.
Exam Application
Exams show Lineweaver-Burk plots and ask you to identify the type of inhibition, read off Km and Vmax, or explain what changes when an inhibitor is added. Master the two rules: competitive = same y-intercept (same Vmax); non-competitive = same x-intercept (same Km). The slope = Km/Vmax increases with either type of inhibition.
⚠ Common Trap
Students confuse which intercept gives Km vs Vmax. Remember: y-intercept = 1/Vmax (the easy one — y is for Vmax). x-intercept = −1/Km (negative, so negate when calculating Km). Also: a higher y-intercept means a LOWER Vmax (reciprocal relationship — don't forget to flip).
✓ Quick Self-Check
1. What does the y-intercept of a Lineweaver-Burk plot equal?
1/Vmax — so Vmax = 1 ÷ (y-intercept).
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2. What does the x-intercept equal?
−1/Km — so Km = −1 ÷ (x-intercept).
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3. On a Lineweaver-Burk plot, how does competitive inhibition appear?
The lines intersect on the y-axis (same Vmax) but have different x-intercepts (apparent Km increased).
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4. How does non-competitive inhibition appear on the Lineweaver-Burk plot?
The lines intersect on the x-axis (same Km) but have different y-intercepts (Vmax decreased — higher y-intercept).
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5. If the y-intercept = 0.25, what is the Vmax?
Vmax = 1/0.25 = 4 (in whatever units velocity is measured).
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