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I was looking at this graph of turnip peroxidase activity and I saw that they use units of 1/sec for enzyme activity. What does this unit intuitively represent and how is it calculated?

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The Wikipedia "Enzyme Assay" page looks pretty good. –  Darlingtonia Aug 20 '12 at 21:35
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This refers to the turnover number (a.k.a kcat or k2) of an enzyme and is usually calculated using Michaelis-Menten kinetics. Jump to the summary at the end if you want a simple answer. If you want a more thorough answer, consider the following chemical equation:

[E] + [S] ⇌ [ES] → [E] + [P]

This says that a certain concentration of enzyme mixed with a certain concentration of substrate will first combine and form a complex depending on the enzyme's affinity for the substrate. Then the enzyme complex will generate a product depending on the enzyme's ability to convert the transition state into product. The k1 constant is the arrow moving from [E] + [S] → [ES] and the k-1 is the arrow moving from [ES] → [E] + [S] (they oppose each other). The k2 constant is the arrow moving from [ES] → [E] + [P].

Vo = [ES]k2

  • Vo - this is the rate that product is being formed, which can be measured.
  • [ES] - this is the concentration of enzyme-substrate complexes.
  • k2 - this is a constant value comparing the two.

This can also be expressed in terms of maximum rate:

Vmax = [E]Tk2

  • Vmax - Vmax is catalytic rate when [E]T = [ES] (when all of the enzyme present is bound to substrate/saturated).
  • [E]T - [E]T is the total enzyme concentration. [E]T = [E] + [ES].
  • k2 - the kcat is the catalytic constant. It is a constant indicating how quickly an enzyme can convert substrates into products. It is easily observed by this equation.


So k2 is basically an indicator of how efficiently or quickly an enzyme operates. Not all enzymes follow standard Michaelis-Menten kinetics. For example, the allosteric properties of some enzymes cause a non-linear saturation curve. Because of this, the turnover number is commonly referred to. The turnover number's units of s-1 indicate one product molecule per second, so a turnover rate of 3000 means you can create 3000 products in 1 second at Vmax.

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