Enzymes: Activation Energy and Mechanisms — Enzyme Catalysis

by Kevin Ahern, PhD

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    00:00 We have seen how enzymes flexibility enables enzymes to accomplish what they accomplish.

    00:06 But enzymes do have constraints that they have to work in.

    00:09 I have mentioned in this presentation numerous times now that enzymes and cells all are governed by their rules of the universe. That is they can't change the energies of reactions and so those are true for cells and those are also true for enzymes.

    00:23 Now enzymes, as we will see, are tricky little things. I have mentioned how enzymes cheat and enzymes are going to cheat with respect to energy as well.

    00:33 So let's consider a reaction of A going to B.

    00:36 In A going to B, this is plotted from an energy perspective on the screen what you see here.

    00:41 On the left side of the screen, we see a dot placed on the graph showing free energy.

    00:44 That is the energy that is associated with molecule A.

    00:50 In the process of going from A to B we see that there is a change in the energy that the energy is actually increased and we call this increase in energy, the activation energy. This necessary to get a reaction going.

    01:02 The reaction proceeds and as the reaction proceeds we an see that the free energy falls then we make a product B, that down by the end has a lower free energy than A had. That meant that energy was released in the process of going from A to B and this makes this reaction process favorable.

    01:24 Now it's important to note that this change in free energy, that's shown right here, this change in free energy can not be changed by an enzyme.

    01:34 That is there is no change between the starting and ending points of the enzyme.

    01:38 The enzymes does some other things, however.

    01:42 It's also important to note here that this height of the peak is really a critical place.

    01:46 The height of this peak is the place where the reaction can reverse and go backwards from where it came that is A can start and then go back.

    01:54 Or B if it got enough energy could climbed that curve and then go back to A.

    01:58 Otherwise A is gonna go forward to B and the reaction is going to be occurring.

    02:08 Now enzymes cheat, okay? Enzymes can change the activation energy There are no rules about activation energy, okay? There are rules about beginning and ending energies.

    02:19 But what activation energy changes do is they enable an enzyme to make more molecules more easily go through that transition.

    02:32 That is the magic of enzymes. How do they accomplish that? Well, they accomplish this in a couple of ways.

    02:36 One of the ways that they do is by the fact that they have binding sites that are very precisely oriented. So that the molecules are placed in the close proximity that they randomly would not be into close proximity so easily, right? And that means that it takes less energy for them to go through the next step in the process.

    02:54 By doing this enzymes are able to lower the transition energy.

    03:01 Well, if the transition energy is lower it makes it much more easy for A to go to B, as we have seen.

    03:07 You notice again, enzymes have had no change with overall free energy the energy of A is still A. The energy of B is still B, okay? Only that transitional state has made a difference.

    03:20 Now I wanna go through and spend some time talking about the mechanism of an enzymatic reaction.

    03:27 Mechanism is important to consider; because, with mechanism we can begin to see how enzymes are facilitating electronic changes necessary for a chemical reaction to occur.

    03:39 The example I will use is an example of a serine protease.

    03:41 A serine proteases are class of enzymes that cut proteins. They break peptide bonds.

    03:50 That's what they do. And they break not every peptide bond they see.

    03:54 But they break specific peptide bonds at specific places within the proteins that they bind to.

    03:59 Alright. So that means that they have binding specificity. They don't cut everything that they see.

    04:05 Serine proteases have flexibility so we saw in the initial illustration the flexibility of an enzyme and we are going to see occurring again here as we talked about the mechanism of the serine protease.

    04:19 The electronic environment is very critical for a reaction.

    04:23 In a chemical reaction, electrons are being manipulated, electrons are being moved around.

    04:26 And to be able to do that, one must have the environment for those electrons to readily be able move around.

    04:33 And we will see that happening in the active site of the serine protease.

    11:14 Enzymes also use coenzymes.

    04:40 Now in this example I won't show a coenzyme. But I will say that the coenzymes actually help an enzyme to accomplish what it accomplishes.

    04:51 Okay. Now, serine proteases, as I said, cleave peptide bonds, that's the catalytic thing that they do.

    About the Lecture

    The lecture Enzymes: Activation Energy and Mechanisms — Enzyme Catalysis by Kevin Ahern, PhD is from the course Enzymes and Enzyme Kinetics.

    Included Quiz Questions

    1. Catalysis only occurs in reactions with more than one substrate
    2. The enzyme in the ES complex has a different structure than the enzyme alone
    3. Binding of substrate induces a change in enzyme structure
    4. When product is released, the enzyme reverts to its original structure
    1. By lowering the activation energy of the reaction
    2. By lowering the free energy of the products
    3. By lowering the free energy of the reactants
    4. By increasing the activation energy of the reaction
    5. By increasing the free energy of the reactants

    Author of lecture Enzymes: Activation Energy and Mechanisms — Enzyme Catalysis

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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