# When an enzyme is diluted with water, what is slowing down the rate of reaction?

I'm specifically talking about the decomposition of hydrogen peroxide by bovine catalase. The number of catalase molecule obviously doesn't change, so there's the same number of catalase molecule within an undiluted and a diluted solution. Also, seeing as water is not an enzyme inhibitor, and doesn't change the pH or temperature, then shouldn't the rate of reaction remain the same regardless of concentration, since every environmental conditions & the number of catalase molecule in the solution before dilution is the same after it is diluted?

[…] there's the same number of catalase molecule within an undiluted and a diluted solution. Also, seeing as water is not an enzyme inhibitor, […]

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You are right in that the absolute amount of catalase molecules does not change upon dilution, but the concentration does!

Reactions are about collisions of the reactants and in the case of dilutions, there are more collisions with innocent bystanders than with an other reactant.

• I've only got rudimentary of chemistry knowledge, so I'm paraphrasing your idea into terms I can understand. What's happening is the water molecule is interfering with the active site of the enzyme, preventing it from working at full capacity, thus lowering the rate of reaction. Therefore, more diluted solution means more water molecules means more interference, and an overall lower rate of reaction. Is this a correct understanding? – L to the V Apr 6 '15 at 6:16
• @LtotheV Exactly! Although the role of water molecules isn't necessarily an active preventing by blocking the active site (there will always be some water molecules there, e.g. due to hydrogen bonding), It really is more about finding less catalase and $\ce{H2O2}$ molecules in the same volume element. Assuming a constant movement of molecules in solution, it just takes longer for the reactants to form a collision complex while making their way through the water. And since rate is conversion over time... – Klaus-Dieter Warzecha Apr 6 '15 at 6:31

If you diluted the catalase, while keeping the peroxide concentration fixed (by adding both water and peroxide to the solution), then you're right, the total reaction rate (measured in reactions catalyzed per second, within the whole solution) would not change.

However, by adding just water to the solution, you're diluting both the enzyme and the peroxide. The lower the peroxide concentration is, the longer it takes for each catalase enzyme unit to bump into another peroxide molecule after reacting with the previous one.