What are the units of enzyme activity?

Question

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?

Answer 1

This refers to the turnover number (a.k.a k_cat 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].

V_o = [ES]k₂

• V_o - 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:

V_max = [E]_Tk₂

• V_max - V_max 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 k_cat 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.

Summary

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 V_max.