How Enzymes Speed Up Reactions?

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The effect of an enzyme on activation energy. Without affecting the free-energy change (∆G) for a reaction, an enzyme speeds the reaction by reducing its activation energy (EA).
Source: Urry, Lisa A.. Campbell Biology (p. 155). Pearson Education. Kindle Edition.

Campbell Biology

Proteins, DNA, and other complex cellular molecules are rich in free energy and have the potential to decompose spontaneously; that is, the laws of thermodynamics favor their breakdown. These molecules only persist because at temperatures typical for cells, few molecules can make it over the hump of activation energy. The barriers for selected reactions must occasionally be surmounted, however, for cells to carry out the processes needed for life. Heat can increase the rate of a reaction by allowing reactants to attain the transition state more often, but this would not work well in biological systems. First, high temperature denatures proteins and kills cells. Second, heat would speed up all reactions, not just those that are needed. Instead of heat, organisms carry out catalysis, a process by which a catalyst (for example, an enzyme) selectively speeds up a reaction without itself being consumed.

An enzyme catalyzes a reaction by lowering the EA barrier, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures, as we’ll discuss shortly. It is crucial to note that an enzyme cannot change the ∆G for a reaction; it cannot make an endergonic reaction exergonic. Enzymes can only hasten reactions that would eventually occur anyway, but this enables the cell to have a dynamic metabolism, routing chemicals smoothly through metabolic pathways. Also, enzymes are very specific for the reactions they catalyze, so they determine which chemical processes will be going on in the cell at any given time.

Source:

Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition. https://www.pearson.com/us/higher-education/series/Campbell-Biology-Series/2244849.html


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