Step by Step
1
The definition of Km
Km is the substrate concentration at which an enzyme reaches half of its maximum reaction velocity (½ Vmax).
2
Low Km means high affinity
If an enzyme reaches half-Vmax at a low substrate concentration, this means it binds its substrate tightly — achieving significant activity even when substrate is scarce.
3
High Km means low affinity
Conversely, if an enzyme needs a high substrate concentration just to reach half-Vmax, this indicates weaker binding — the enzyme requires substantial substrate before it can work efficiently.
4
An important property of Km
Km is an intrinsic property of the enzyme-substrate pair and does NOT change with enzyme concentration — it reflects binding affinity, not how much enzyme is present.
Applied Walkthrough
1
An enzyme with a low Km reaches half its maximum velocity even at a low substrate concentration — indicating tight binding and high affinity for its substrate.
2
By contrast, an enzyme with a high Km requires a much higher substrate concentration before reaching that same half-maximal velocity — reflecting weaker binding and lower affinity.
3
Crucially, Km remains constant regardless of how much enzyme is present in a reaction — doubling the enzyme concentration would double the reaction's Vmax, but would not change its Km at all.
4
This distinction between Km (a measure of binding affinity) and Vmax (a measure of maximum reaction rate) becomes especially important when analyzing how different types of inhibitors affect enzyme kinetics, covered in the following two lessons.
Exam Application
Exams test whether you understand Km as substrate concentration at half-maximal velocity, whether you can correctly interpret low Km (high affinity) versus high Km (low affinity), and whether you understand that Km doesn't change with enzyme concentration.
⚠ Common Trap
The most common trap is confusing Km with Vmax — Km specifically reflects binding affinity (how much substrate is needed to reach half-maximal velocity), while Vmax reflects the maximum reaction rate itself; these are two distinct kinetic parameters, not interchangeable measures.
✓ Quick Self-Check
1. What is Km, precisely?
The substrate concentration at which an enzyme reaches half its maximum velocity (½ Vmax).
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2. What does a low Km indicate about an enzyme's substrate binding?
High affinity — tight binding, reaching half-Vmax even at low substrate concentration.
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3. What does a high Km indicate?
Low affinity — weaker binding, requiring high substrate concentration to reach half-Vmax.
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4. Does Km change with enzyme concentration?
No — Km is an intrinsic property of the enzyme-substrate pair, independent of enzyme concentration.
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5. What does change with enzyme concentration, if not Km?
Vmax.
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