Textbooks commonly state that the role of enzymes is to speed up a chemical rxn by lowering its activation energy.

However, I'm unsure what enzymes like helicase, DNA/RNA polymerase, and restriction enzymes have to do with lowering the activation energy. Isn't helicase's primary role, for instance, to unwind double-stranded DNA?

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    $\begingroup$ What is the problem with the enzymes you mention? They all catalyse chemical reactions which have activation energies. Those for polymerases should be self-evident. If you are unaware of the reaction catalysed by helicases, consult the relevant Wikipedia page, for example. $\endgroup$ – David Jun 22 at 18:35

All reactions require an amount of energy to proceed called the activation energy. Enzymes lower that activation energy through various means. An example here is the unwinding of the double-stranded DNA requires breaking hydrogen bonds. Energy needs to be supplied for the reaction to proceed. The rate of the reaction can be described by Arrhenius' equation

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You can increase the rate of the reaction, or decreasing the activation energy. Enzymes are required to do that at physiological temperatures by lowering the activation energy, because increasing the temperature is usually not an option.

The enzymes you mentioned are no different.

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  • $\begingroup$ Helicase also hydrolyzes ATP. $\endgroup$ – canadianer Jun 22 at 19:19
  • $\begingroup$ Your answer seems to me misleading. Whether a chemical reaction requires energy or releases it is irrelevant to the requirement for an enzyme. Both types of reaction have an activation energy barrier to overcome. What is pertinent is that it is a chemical reaction which helicase catalyses — as has been mentioned, one involving the hydrolysis of ATP. $\endgroup$ – David Jun 22 at 21:50
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    $\begingroup$ Yes, an amount of energy is needed to overcome the activation barrier. Mentioning that helicase requiring ATP doesn't contribute anything to the answer. $\endgroup$ – user60545 Jun 22 at 22:58
  • $\begingroup$ (+1). For your second last sentence I would have said something like 'The Arrhenius equation predicts that "you can increase the rate of reaction by decreasing the activation energy or by increasing the temperature. Enzymes ..." ' Also, perhaps the more encompassing transition-state theory deserves a mention? $\endgroup$ – user1136 Jun 23 at 12:18
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    $\begingroup$ Since some might not consider the breaking of hydrogen bonds in DNA to be a chemical reaction, it might not then be immediately obvious why helicase is considered an enzyme without accounting for its hydrolytic activity. $\endgroup$ – canadianer Jun 23 at 17:03

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