200
min^-1
3.333333
s^-1
0.3
s
1.666667
µmol/s
200
min^-1
3.333333
s^-1
0.3
s
1.666667
µmol/s
The Kcat Calculator determines the turnover number of an enzyme, which represents the maximum number of substrate molecules converted to product per enzyme molecule per unit time. Also known as the catalytic constant, kcat is a fundamental measure of enzyme catalytic power that allows comparison between different enzymes regardless of their concentration.
By dividing the maximum reaction velocity (Vmax) by the total enzyme concentration, you obtain kcat, which describes how efficiently a single enzyme active site processes substrate when fully saturated. This parameter is critical in enzymology, drug design, and metabolic engineering.
The turnover number is calculated from Vmax and total enzyme concentration:
kcat = Vmax / [Et]
Where Vmax is the maximum velocity and [Et] is the total enzyme concentration. The result is converted from per minute to per second for convenience. Typical kcat values range from less than 1 s⁻¹ for slow enzymes to over 10⁶ s⁻¹ for the fastest enzymes like carbonic anhydrase.
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With Vmax of 100 µmol/min and 0.5 µmol enzyme, kcat is 200 per minute or about 3.3 per second. This is a moderately active enzyme.
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A high Vmax relative to a low enzyme concentration gives kcat of 500,000 per minute or about 8,333 per second, characteristic of a very efficient catalytic enzyme.
Most enzymes have kcat values between 1 and 10,000 s⁻¹. Some enzymes like carbonic anhydrase have remarkably high kcat values near 10⁶ s⁻¹, while others involved in regulatory processes may have kcat values below 1 s⁻¹. The value depends on the reaction mechanism and the specific enzyme.
Vmax depends on enzyme concentration; doubling the enzyme doubles Vmax. Kcat is an intrinsic property of the enzyme, independent of how much enzyme is present. It tells you how fast a single enzyme molecule can work, making it a better metric for comparing catalytic abilities of different enzymes.
Total enzyme concentration [Et] is needed because kcat describes catalysis per enzyme molecule. You must know how many enzyme molecules produced the observed Vmax to calculate the per-molecule rate. This requires careful determination of active enzyme concentration, often using active-site titration.
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