Enzymes do this when changing activation energy

[yestomeds]

Enzymes decrease the activation energy by:

A) modifying the local environment
B) Forming transient covalent bonds
C) acting as e- donors or acceptors
D) breaking bonds in the enzyme (itself) to provide energy

D) is false. A through C are correct.

Can you help me understand why each of the A, B, C are correct?

Like, how answer choice A might be correct, for example.

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[shefv]

Enzymes provide an ideal local environment inside of their active sites for the reaction to take place.
Enzymes may make temporary bonds with the reactants to facilitate the breakage of other bonds in the reactant molecules. This point also goes with the idea of accepting or donating electrons.

Breaking bonds in the enzymes to provide energy is false because enzymes are not consumed in the reaction and "provide energy" part is wrong since enzymes lower activation energy needed for a reaction to take place, not provide additional energy so that the reaction can take place.

 

[texasdevil]

I'll comment as well.

A) Enzymes can alter the pH and electrostatic potential of local environments, which can change the rate (and decrease the activation energy ceteris paribus)

B) Enzymes often act by holding the substrate in a specific orientation that allows specific molecules to interact. By holding the substrate in this orientation, the enzyme guarantees more reactions (or collisions / interactions) will occur per second by reducing the inherent randomness of the system

C) Enzymes can also temporarily alter the charge state of a molecule, which can make it more likely to interact with another charge state. Some reactions that involve two negatively charged anions to form one molecule can benefit from an enzyme that neutralizes their negative charges (which would of course be passed on to a cofactor / other molecule)

Hope this helps!

 

[MrRed]

Hey I just want to address one very blatant flaw in choice D that no one has spoken of yet.
Breaking bonds ALWAYS requires energy (it is always endothermic), it does not release energy. Energy is released when new bonds a formed that are of lower energy than the original bonds were.
It seems counter intuitive, but its all about the potential energy of the system -- in this case two atoms or molecules.
So that makes choice D automatically 100% wrong. Breaking bonds NEVER provides energy on its own.

 

[texasdevil]

Hey I just want to address one very blatant flaw in choice D that no one has spoken of yet.
Breaking bonds ALWAYS requires energy (it is always endothermic), it does not release energy. Energy is released when new bonds a formed that are of lower energy than the original bonds were.
It seems counter intuitive, but its all about the potential energy of the system -- in this case two atoms or molecules.
So that makes choice D automatically 100% wrong. Breaking bonds NEVER provides energy on its own.

This is not true for ATP hydrolysis. ATP is one of the few (I don't have any other examples) exceptions to this rule because of the close proximity of the negatively charged phosphate groups. These phosphate groups induce electrostatic repulsion, which is analogous to stretching a rubber band. Breaking the third phosphate group away from the molecule releases this pent up energy. Conversely, ATP requires energy to be formed from an inorganic phosphate and ADP, and therefore must release energy if this bond is broken (first law of thermodynamics). This is exactly how ATP provides energy. The only reason D is incorrect is because of the phrase in the enzyme (itself). This doesn't make any sense because an enzyme must be regenerated through the reaction, so those bonds would have to be reformed through some other energy-consuming reaction, which would provide the energy for the reaction as a whole.

 

[MrRed]

This is not true for ATP hydrolysis. ATP is one of the few (I don't have any other examples) exceptions to this rule because of the close proximity of the negatively charged phosphate groups. These phosphate groups induce electrostatic repulsion, which is analogous to stretching a rubber band. Breaking the third phosphate group away from the molecule releases this pent up energy. Conversely, ATP requires energy to be formed from an inorganic phosphate and ADP, and therefore must release energy if this bond is broken (first law of thermodynamics). This is exactly how ATP provides energy. The only reason D is incorrect is because of the phrase in the enzyme (itself). This doesn't make any sense because an enzyme must be regenerated through the reaction, so those bonds would have to be reformed through some other energy-consuming reaction, which would provide the energy for the reaction as a whole.

I'm sorry texasdevil but that's just not true. The reason ATP hydrolysis is exothermic is partially due to what you said:
1. negative charges on phosphate groups exert electrostatic repulsion
2. more resonance stabilization in ADP + Pi vs ATP
3. More effective solvation of ADP + Pi vs ATP alone

With that said; the actual act of the breaking of the bond in ATP is endothermic. This may as well be a law, there are no exceptions to this fact. Bonds ALWAYS require energy to break, and forming bonds always releases energy. The reason that any reaction is exothermic is due to stronger bonds, and more stable products. The reasons listed above are why the enthalpy change for the reaction of ATP hydrolysis is favorable under physiological conditions.
ATP has relatively high energy bonds; thus the amount of energy required to break them under catalyzed hydrolysis is relatively small compared to the amount of energy released due to the forming of ADP + Pi. Remember; the reaction in its completion is: ATP + H2O --> ADP + Pi
The ADP was formed due to the combination of the OH from water and the adenosine diphosphate that would be present in the transition state, while the Pi is actually a combination of the inorganic phosphate + a hydrogen from water. The bonds formed here are much more stable than the phosphoanhydride linkage broken in ATP.

If you need more convincing, think about a free energy diagram for ATP hydrolysis (or ANY chemical reaction for that matter). Where are the reactants starting? ALWAYS at a lower energy than the transition state. Whether the products have a higher/lower energy than the reactants in this case doesn't matter. You can see that in order to initially break bonds, you MUST put energy into the system. It is only after the bonds have been broken and the new bonds form that we observe a decrease in free energy (exothermic/exergonic reactions; although the two may be mutually exclusive depending on conditions) which is the release of energy into the system that is used to power other reactions in the cell in the case of ATP hydrolysis.

Again, breaking bonds is always, always, always a process that requires the addition of energy into the system. This is the activation energy characteristic of every chemical reaction.

If you would like another person to explain it -- as I'm not perfect at explaining things; check out the following link for a quick read:
http://www.masterorganicchemistry.c...may-be-bad-but-the-divorce-still-costs-money/

 

[texasdevil]

Ah yes, I agree. I was viewing it from an absolute perspective. SDN is lucky enough to have peer-review on esoteric MCAT questions 😀