In the section about the induced fit model for enzyme substrate binding, my MCAT textbook claims that "The substrate has induced a change in the shape of the enzyme. This interaction requires energy, and therefore, this part of the reaction is endergonic."

As a chemist, I am a bit confused here because induced fit suggests that some kind of bonding must be happening between the enzyme and substrate (hydrogen, ionic, covalent, etc) that would induce a change in structure of both enzyme and substrate. Any kind of bond formation is STRICTLY enthalpically exothermic, so why is the formation of an enzyme-substrate complex endergonic?

Am I missing an entropic component here? Or is my assumption wrong, regarding how the induced fit chemistry operates?

I would assume that it would TAKE energy to then cleave/break all the bonds that induced structural change and release the catalyzed substrate...


Most reactions require an activation energy to cross an energy barrier (this you may already know) after which the reaction is downhill. Even though enzyme catalysed reactions have a relatively lower activation energy compared to that of an uncatalysed reaction, there is still a transition state that has a higher free energy compared to the products. This transition state corresponds to the enzyme-substrate complex.

               enter image description here
            Image Source: https://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme/Activation_energy

Free energy has enthalpic and entropic components (ΔG=ΔH-TΔS). Even though formation of bonds may reduce the enthalpy, the locking of the substrate and enzyme in a conformation also decreases the entropy. This could lead to overall increase in free energy (See here. For more details you can refer to this book- The Enzymes: Mechanisms of Catalysis by Paul Sigman. ISBN: 0121227200).


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