20,000
M^-1 s^-1
0.02
10^6 M^-1 s^-1
0.0002
fraction
0.02
%
20,000
M^-1 s^-1
0.02
10^6 M^-1 s^-1
0.0002
fraction
0.02
%
The Catalytic Efficiency calculator computes the ratio of kcat to Km, which is the best measure of how efficiently an enzyme converts substrate to product under physiological conditions where substrate concentration is typically below Km. This ratio, known as the specificity constant, captures both the catalytic power and substrate binding ability of an enzyme in a single value.
Enzymes with kcat/Km approaching 10⁸ to 10⁹ M⁻¹s⁻¹ are considered catalytically perfect, operating at or near the diffusion-controlled limit where every encounter between enzyme and substrate leads to product. This metric is essential for comparing enzyme performance and for enzyme engineering efforts.
Catalytic efficiency is the ratio of the turnover number to the Michaelis constant:
Catalytic Efficiency = kcat / Km
The units are M⁻¹s⁻¹ (per molar per second). This ratio reflects the rate of catalysis at low substrate concentrations where [S] is much less than Km. The calculator also flags whether the enzyme approaches the diffusion-controlled limit of approximately 10⁸ to 10⁹ M⁻¹s⁻¹.
Inputs
Results
A kcat of 100 s⁻¹ and Km of 5 mM gives an efficiency of 2 × 10⁴ M⁻¹s⁻¹. This is a moderately efficient enzyme, well below the diffusion limit.
Inputs
Results
Carbonic anhydrase has a kcat/Km of about 2.3 × 10¹⁰ M⁻¹s⁻¹, exceeding the diffusion limit. It is one of the fastest known enzymes.
The diffusion-controlled limit is the theoretical maximum rate at which enzyme and substrate can encounter each other in solution, typically 10⁸ to 10⁹ M⁻¹s⁻¹. Enzymes with kcat/Km near this limit convert virtually every substrate molecule they encounter, and their rate is limited only by how fast substrate diffuses to the active site.
kcat alone only measures how fast a saturated enzyme works. In cells, enzymes are rarely saturated. kcat/Km captures performance at physiological substrate concentrations, combining binding affinity and catalytic speed. It also determines substrate specificity when an enzyme can act on multiple substrates.
Enter Km in molar (M) for this calculator. If your Km is in millimolar (mM), divide by 1000 first. For example, Km = 5 mM should be entered as 0.005 M. Correct units are essential because the efficiency value spans many orders of magnitude.
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