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My textbook says that catalase is the fasting acting known mammalian enzyme and it can act on 40 million molecules of hydrogen peroxide per second.

Does that mean that is acting on that number of molecules during normal conditions or does it mean that one “act” takes one 40 millionth of a second?

This fact is truly incredible. The speed at which these molecules are clicking in and out of the active sites must be near light speed.

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Yes, it means that each individual enzyme molecule, under ideal conditions, is able to perform its reaction in one 40 millionth of a second.

But, the reactants aren't moving at even close to the speed of light. 40,000,000 reactions per second works out to 25 nanoseconds per complete reaction ($2H_2O_2$ entry into active site, catalysis reaction, and $2H_2O+O_2$ leaving, clearing the way for the next set of reactants to bind). The speed of light is 299792458 m/s, which means it travels approximately 7.5 m in 25 nanoseconds. Catalase is about 6x10 nm in size. Even if the molecular reactants had to each travel 10 nm, that's still about 1 billionth the speed to light.

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  • $\begingroup$ Wow , numbers both big and small are truly mind boggling in science. $\endgroup$
    – Kantura
    Jun 9, 2021 at 20:05
  • $\begingroup$ That's one of the reasons we use scientific notation. 2.5e-8 seconds is a little easier to wrap your head around than 25 billionths of a second. $\endgroup$
    – MattDMo supports the strike
    Jun 10, 2021 at 14:24
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    $\begingroup$ l upvoted your answer, but now think you have fallen into a trap over the speed of light. See my comment to the poster. Think about it. Even if you say that catalase is affecting collisions, the colliding molecules are not moving any faster, they are just colliding more frequently. Speed and catalysis are both rates but speed is distance per time, whereas catalysis is events per time. $\endgroup$
    – David
    Jun 10, 2021 at 15:37
  • $\begingroup$ @David I think you might have misunderstood the last part of my answer. I wasn't trying to conflate the act of catalysis itself with the speed of light. Based on the last sentence of the OP's question, I was just trying to show that even if the reactants had to travel the entire length of the enzyme to get to the active site in 25 nanoseconds (a much longer distance than they likely would have to travel in reality), they still would only be moving at 1 billionth the speed of light, which travels on the order of ~10m in the same length of time. Does that make sense? $\endgroup$
    – MattDMo supports the strike
    Jun 10, 2021 at 19:31
  • $\begingroup$ Sure. But I'd like to make the OP think more carefully about his use of words in science. $\endgroup$
    – David
    Jun 10, 2021 at 20:26

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