Based on the Induced-Fit model of enzyme action, enzymes catalyse a reaction by lowering the activation energy of a single forward reaction over and over. But I read that enzymes don't change the overall energy change of the reaction $(\Delta H_{solution})$ they just speed up the reaction.

How can enzymes reduce the activation energy of every forward reaction but still not affect the $\Delta H$? $\Delta H$ is sometimes called change in heat because it is measured in Joules and it's either endothermic or exothermic (this bit is chemistry).

I'm a highschool student and I'm just trying to find out how the required activation energy doesn't affect the energy change for the whole reaction.

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    $\begingroup$ Welcome to SE Biology. Please finish the Tour and learn how to ask questions here. One of the first things to do is to search for previous postings of the same or related questions of which there are many on the topic. You are incorrect in your recall of what you read. It is the Gibbs Free Energy of a reaction, ΔG, that enzymes do not affect. This is definitely not the same as ΔH: ΔG = ΔH − TΔS. So your question is invalid as it stands. I would advise you to read (this section of Berg et al.) [ncbi.nlm.nih.gov/books/NBK22584/] which addresses this topic. $\endgroup$ – David Mar 8 at 17:42
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    $\begingroup$ I’m voting to close this question because it is based on incorrect premises. $\endgroup$ – David Mar 8 at 17:44

You're on a valley on one side of a mountain (representing some fairly stable state of your reactants), and you'd like to go to a town on the other side at a lower elevation (representing another stable state for your products, at lower energy than your current state: this is an energetically favorable reaction).

You can go over the mountain: high activation energy to climb up to the top, then you go down the slope to eventually get down to the town.

You can go through a pass between mountains instead: much lower activation energy to get over the pass, then you eventually get down to the town.

Whichever path you choose, you're at the same elevation (energy, $\Delta H$ compared to where you started), with the same products (your location in space), but if you took the pass you've gotten there via a different chemical route that was easier to achieve energetically at the peak.

Enzymes let you take the path through the pass rather than over the top of the mountain. They don't change where you started from or where you're going, so they don't change the overall energy of the reaction.


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