In this video, we're going to begin our discussion on the different types of enzyme catalysis. So recall from our previous lesson videos that enzymes can actually lower the energy of activation required for a reaction in several different ways, including lowering the entropy of the system by bringing substrates closer together so that they can react faster, properly orienting the substrate so that their functional groups are also properly oriented and they can react easier, conformations easier, and desolvating a substrate so that it can achieve transition state confirmations easier, and desolvating a substrate so that the hydration shells do not interfere and slow down the reaction. And again, these are all different ways that enzymes can lower the energy of activation that we already covered in our previous lesson videos. And so between all of these different ways, we know that there is still a common theme to stabilize the transition state. Now what we haven't yet talked about are the different types of enzyme catalysis reaction mechanisms that enzymes use. And so, what are some of those different types of enzyme catalysis mechanisms? Well, it turns out there are 4 mechanisms that enzymes primarily use for catalysis, and so the first is acid base catalysis. The second is electrostatic catalysis. The third is metal ion catalysis. And the 4th is covalent catalysis. And so moving forward in our course, we're gonna talk about each of these different types of enzyme catalysis reaction mechanisms in their own separate videos. But in our next lesson video, we'll talk about the very first enzyme catalysis mechanism, which is acid base catalysis. And so I'll see you guys in that lesson video.
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concept
Catalysis
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Video transcript
So in our last lesson video, we said that there are 4 main types of enzyme catalysis. And in this lesson video, we're gonna talk about our very first one, which is general acid base catalysis. And so acid base catalysis is, of course, when either an acid or a base catalyzes a reaction. And of course, that's going to occur via a proton transfer, or an h plus transfer, since we already know that acids and bases are all about either donating or accepting protons. And so what's to know is that unstable charged intermediate molecules that form during a reaction can actually be stabilized with proton transfers. And that's exactly why acid base catalysis can be so critical to some reactions. And so let's take a look at our image down below in our example to clear up some of this. And so notice that what we have here is a reaction in this box, and down below, what we have is the free energy diagram for the same exact reaction that we have up above. And so notice that this reaction up above is actually a 2 step reaction shown by these two reaction arrows. And so in the first step of our reaction, notice that the electron density on this blue oxygen right here is attacking our carbonyl group, and essentially the electron density that's in the double bond of this carbonyl group is shifting up onto this red oxygen. And so, ultimately, what that leads to is the formation of this charged intermediate molecule here, where this red oxygen has a negative charge. And so recall from up above, we said that sometimes these charged intermediate molecules that form during a reaction can actually be unstable. And it turns out that's exactly the case with our charged intermediate molecule here. It's actually unstable. And we can tell from our free energy diagram down below, where if we take a look at our charge intermediate molecule, notice that its free energy is actually higher than the free energy of either the reactants or the free energy of the product, and so that's what makes our charge intermediate unstable. And so what I also want you guys to notice here is that, notice that the rate of the forward reaction here, or the, energy of activation for the forward reaction to convert the intermediate here into the final product, is actually much larger, significantly larger, than the energy of activation that's required for the reverse reaction to convert the intermediate backwards back into the reactants. And so recall that a smaller energy of activation means that the reaction occurs faster. So what this means is that the backwards reaction here, the reverse reaction, shown here, actually occurs much faster than the forward reaction because of the larger energy of activation. And so what this means is that when this intermediate forms, the tendency is for it to break down backwards, right back into the reactants, rather than to, essentially go forward and be converted into the final product. Which means that our final product is not going to be formed at a substantial rate if the energy of activation for this second step is so large. So we can actually get a more substantial, rate of formation for this product if we decrease the energy of activation for this second step of our reaction. And so notice that the second step of our reaction here, indicated by this second arrow, is just a proton transfer or an h plus transfer. And so essentially, what's happening is a proton is being transferred to this red oxygen to protonate it and stabilize our final product. As we can see down below, our final product is stabilized because it has such low free energy. And so, what's important to know is that this second step here, this proton transfer, can actually be catalyzed, by 2 main types of acid base catalysis. And so the first main type of acid base catalysis is specific acid base catalysis. And so just like it sounds with specific acid base catalysis, only one specific acid or base can serve as the proton transfer source, and that one specific acid or base is going to be the solvent. Only the solvent can serve as the proton transfer source with specific acid base catalysis. That's why it's so specific. And so, recall that with biological systems, the solvent is going to be water. And so with specific acid base catalysis, it's water that's going to always serve as the proton transfer source. And so as we can see down below with our non enzymatic reaction that occurs via specific acid base catalysis, which is the black curve in our diagram. Notice that with specific acid base catalysis, sometimes, this can actually just be too slow of a process. And it's slow because notice that this second step here, the energy of activation, again, is much larger than the energy of activation for the acid base slow via specific acid base catalysis, just like we're saying up above. And so, what we can do to speed up this second step of our reaction here is to, perform our is And so just like our subtopic title tells us, this is really the main type of acid base catalysis that we want to focus on in this video. And so general acid base catalysis, just like it sounds like, is actually pretty general. And that's because it doesn't require a specific acid or base. It actually can occur via any acid or base, and that's why it's so general because any acid or base can serve as the proton transfer source. However, with general acid base catalysis, the enzyme is actually involved in the process. And so, we can see that with general acid base catalysis, the enzyme's active site is really what's going to mediate the proton transfer source. And because the enzyme is, involved, that's going to speed up the reaction and lower the energy of activation. And so we can see that in our energy diagram as well. Notice that the enzyme catalyzed reaction via general acid base catalysis is shown by this green curve right here. And so we can see that the energy of activation here is, significantly lowered in comparison to, the energy of activation that it was with the non enzymatic reaction. And so, what this means is that the product is going to be formed at a more substantial rate. And so, essentially what we can see here with this second step here, this proton transfer, that if, the solvent serves as the proton transfer source, then that means it's going to be specific acid based catalysis because, the water can serve as the proton transfer source. And remember, water has the ability to autoion and form this H three O plus or this hydronium ion that can serve as the proton transfer source. And so what helps me remember that, specific acid base catalysis is specifically when the solvent serves as the proton transfer source, is that, I I see the s in specific, and that reminds me of the s for the solvent. And so, here we can see that the solvent can serve as the proton transfer source. That would be specific acid base catalysis. And so that's why it's black and color coded in black with the energy diagram below. But we know that if the enzyme is mediating the proton transfer source, then, it's going to be general acid base catalysis because any acid or base can serve as the proton transfer source. It's just that the enzyme is now mediating this proton transfer. And so that's why it's in green, and we can label it as general, for general acid base catalysis, and it's color coded with the green, curve down below. And so, this here concludes our lesson on the difference between specific and general acid base catalysis, and we'll be able to apply these concepts that we've learned in this video and our practice problems. So I'll see you guys there.
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Problem
Problem
Which of the following could not clearly be a contributor to general acid-base catalysis (circle all that apply)?
A
Lysine.
B
Glutamic acid.
C
Water.
D
Tyrosine.
E
Phenylalanine.
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Problem
Problem
The catalytic mechanism below is an example of:
A
General acid catalysis.
B
Specific acid catalysis.
C
General base catalysis.
D
Specific base catalysis.
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Problem
Problem
Which of the following best applies to general acid-base catalysis?
A
A proton is transferred between the enzyme and substrate.
B
Uses nucleophilic functional groups.
C
May take part in interactions involving Fe2+.
D
Catalyst retains its original form after reaction occurs.
E
Other than losing/gaining a H+, the catalyst retains its original form.