Sequential (KNF) Model - Online Tutor, Practice Problems & Exam Prep

The sequential model, also known as the KNF model, describes allosteric enzyme behavior where subunits transition from the tense (T) state to the relaxed (R) state independently, influenced by substrate binding through the induced fit mechanism. This model allows for both positive and negative cooperativity, contrasting with the concerted model, which only permits positive cooperativity and simultaneous state transitions. Understanding these dynamics is crucial for grasping enzyme kinetics and the sigmoidal behavior of allosteric enzymes, highlighting the importance of substrate interactions in enzymatic regulation.

Sequential (KNF) Model

So now that we've covered the concerted model, in this video we're going to introduce the second model that explains the sigmoidal kinetics of allosteric enzymes. And that second model is the sequential model. And so the sequential model is also sometimes referred to as the KNF model. And again, KNF is just the abbreviations of the last names of the 3 scientists that discovered the sequential model. And so it turns out again that it's more commonly referred to as just the sequential model than the KNF model. And for that reason, moving forward in our course, I'm mainly going to be referring to it as just the sequential model.

When it comes to the sequential model, unlike the concerted model, an allosteric enzyme's subunits will actually undergo sequential conversions, from the T state to the R state. What this means is that the subunits of an allosteric enzyme do not simultaneously convert from T state to R state. Instead, the subunits will convert from the T state to R state independently of one another, and this is because the T state to R state conversions are actually induced via the substrate binding. So it occurs via the induced-fit model. Ultimately, again, what this means is that the subunits of an allosteric enzyme can be present in different states, which means that hybrids are indeed allowed, which is again very, very different from the concerted model where hybrids are not allowed.

In other words, with the sequential model, T state to R state transitions do not necessarily encompass the entire allosteric enzyme. Instead, the T state and R state transitions occur within each subunit of the allosteric enzyme sequentially. If we take a look down below at our image, notice what we're saying is that when it comes to the sequential model or the KNF model, the transition from the allosteric enzyme, where all of its subunits are in the T state, to the allosteric enzyme where all of the subunits are in the R state, is going to occur sequentially with each subunit. What this means is that it's possible for one subunit to be in the R state, whereas the other 3 subunits are in the T state. It's pretty much possible for every combination that you can think of. And so that's, again, because the transitions from T state to R state occur sequentially with each subunit, allowing for all of these hybrids that we see here. And again, this is very, very different from the concerted model.

So now that we understand the basics of the sequential model, in our next lesson video we're going to talk about how the sequential model actually explains positive and negative cooperativity, and therefore can explain the sigmoidal kinetics of allosteric enzymes. So I'll see you guys in our next video.

Sequential (KNF) Model

So now that we know the basics of the sequential model, in this video we're going to talk about how the sequential model allows for both positive and negative cooperativity, and that's actually very different than the concerted model. And so unlike the concerted model, which only allows for positive cooperativity, again, the sequential model allows for both positive and negative cooperativity. And so when it comes to cooperativity, this is just describing the idea that substrate binding to one of the subunits of an allosteric enzyme actually increases the likelihood that neighboring subunits are going to take on either the T state or the R state confirmation.

And so as we already know, from our previous lesson videos, positive cooperativity is essentially the idea that binding of the substrate to an allosteric enzyme actually makes it easier for other allosteric enzyme subunits to bind the substrate too. And so here we're saying that positive cooperativity is just this idea that binding of the substrate, to a subunit of an allosteric enzyme actually promotes neighboring subunits to take on the R state, which is the relaxed state and binds substrates more efficiently. And so, of course, negative cooperativity is going to be the exact opposite of positive cooperativity. And so negative cooperativity says that binding of the substrate to an allosteric enzyme subunit actually makes it harder for other allosteric enzyme subunits to bind the substrate. And so here we're saying that negative cooperativity is just this idea that binding of the substrate molecule to an allosteric enzyme subunit will actually promote neighboring allosteric enzyme subunits to take on the T state, which is the tense state, and binds substrates inefficiently.

And so down below in our example, we're going to distinguish the positive and negative cooperativity of the sequential model. And so, notice, at the top left, right here, essentially we're showing you an image that represents positive cooperativity of the sequential model. And so, of course, on the far left here, what we have is the allosteric enzyme with all 4 subunits in the T state. And when it comes to the sequential model, we know that when it converts from the T state to all 4 subunits in the R state, the subunits are going to sequentially convert and that is because the substrate is actually going to induce each individual subunit to convert from the T state to the R state. And so here we have some substrate binding to the top left subunit, converting it from the T state to the R state. And notice that with positive cooperativity, once substrate is bound to that, one subunit with the R state, it is going to influence or promote the neighboring subunits to also take on the R state. And so notice that, this R's R state subunit promotes these two neighboring subunits, to take on a state where it has an increased substrate binding affinity, making them more likely, increasing the likelihood that these neighboring subunits are going to take on the R state. And so notice that as soon as we add more substrate, we have these subunits here binding, and converting to the R state. And then, of course, now this subunit is being promoted, so that it increases its likelihood to bind substrate. And then, of course, as soon as we add substrate and it becomes available, all of these subunits are going to quickly, bind substrate and be in this R state. And so notice that positive cooperativity here, on this enzyme kinetics plot on the right corresponds with this green curve. And we already know that positive cooperativity allows for the sigmoidal kinetics that we see with allosteric enzymes. And so we can label this green curve, as showing positive cooperativity. Now, notice that the black curve here represents the curve where there is absolutely no cooperativity. And we know that it's, actually Michaelis-Menten enzymes that do not have any cooperativity because cooperativity is really just a feature of allosteric enzymes. And so, here with the no cooperativity, notice that it resembles a rectangular hyperbola, just like Michaelis-Menten enzymes.

Now, when it comes to negative cooperativity, again, this is going to be the opposite of positive cooperativity. And so, with the sequential model again, on the far left we have the allosteric enzyme and all 4 subunits are in the T state and then, of course, each subunit is going to sequentially convert from the T state to the R state. But notice here that upon substrate binding to this subunit up here, it does convert to the R state, but with negative cooperativity, it actually promotes the neighboring subunits to take on the T state and inefficient binding. And so notice that with the substrate bound to the subunit, the neighboring subunits here take on this decreased substrate binding affinity. And so, as we add more substrate, this subunit here can bind some substrate. But, again, these 2 subunits still have the decreased substrate binding affinity. And so in order to get these substrates to, essentially bind, in order to get these subunits to bind substrate, we have to increase the substrate concentration by a lot in order to get this subunit here to bind substrate. And then, of course, if we want this subunit to bind substrate, we're going to have to increase the substrate concentration a lot more as well. And so, ultimately, what this leads to is having a curve that looks like this blue one right here. And so notice that it takes quite a lot of substrate concentration in order to get this curve all the way up to the V_max. And so, that is a feature of negative cooperativity.

And so some allosteric enzymes actually do, show some negative cooperativity. And so the sequential model is, essentially a model that allows for both positive and negative cooperativity. And so for allosteric enzymes that display negative cooperativity, it's the sequential model that would be a better model. However, if positive cooperativity is displayed, then either the concerted or the sequential model could explain the positive cooperativity that we see. And so, in our next lesson video, we're going to directly compare and contrast the concerted as well as the sequential model. And so, this here concludes our lesson on how the sequential model allows for both positive and negative cooperativity. And so I'll see you guys in our next video.

True or False: Most allosteric enzymes behave according to the concerted model, not the sequential model.

• A. True.
• B. False.

Sequential (KNF) Model

In this video, we're going to compare and contrast what we already know about the concerted and sequential models. And so even though in our previous lesson videos, we talked about these 2 models as if they were completely separate models, it turns out that most allosteric enzymes actually behave according to some combination of both of these models. Essentially, a combination of the concerted and sequential models, which means that it's really important for us to understand the unique features that each of these models brings to the table. And notice down below we actually have a table, and on the left hand side, we have the concerted model, whereas on the right hand side, we have the sequential model. And so recall from our previous lesson videos that sometimes the concerted model is referred to as just the MWC model, and sometimes the sequential model is referred to as just the KNF model. Now recall that the unique feature of the concerted model is actually this word concerted, which we know from our previous organic chemistry course means jointly happening altogether at the same time. Essentially, it means simultaneously. And so we already know that with the concerted model, the T state to R state conversions occur simultaneously in all of the subunits of the allosteric enzyme so that the T state to R state conversions encompass the entire allosteric enzyme as a whole. So that all of the subunits are always going to be in the same exact state. Now this is not the case with the sequential model, and that's because the T state to R state conversions do not encompass the entire allosteric enzyme. Instead, these T state to R state conversions occur sequentially in each of the subunits of the allosteric enzyme independently of one another. Now, with the concerted model, the T state to R state conversions do not require any substrate whatsoever. So over here, you can see that the T state to the R state conversions will occur even in the absence of substrate, where that is not the case with the sequential model. And that's because with the sequential model, the T state to R state conversions only occur via substrate binding in the induced fit model. And so over here, you can see that the T state to the R state conversion is going to occur independently in each of the individual subunits sequentially, via substrate binding to that particular subunit. And this means that hybrids are definitely going to be allowed with the sequential model, as indicated in our table. But, of course, we know that with the concerted model, absolutely no hybrids are allowed whatsoever. And we also know that the concerted model does explain the sigmoidal kinetics of allosteric enzymes, which means that it does allow for positive cooperativity. But recall that it only allows for positive cooperativity, and it does not allow for negative cooperativity. Whereas with the sequential model, it actually allows for both positive and negative cooperativity. And so this here concludes our compare and contrast of the concerted model and the sequential model, and we'll be able to get some practice utilizing these concepts as we move forward in our course, so I'll see you guys in our next lesson video.

The Sequential model for allosteric enzyme behavior:

• A. Cannot account for the reactions that display negative cooperativity.
• B. Postulates binding of substrates & inhibitors by the induced-fit model.
• C. Requires that the conformation of all subunits change simultaneously.
• D. Is conceptually and mathematically simpler than the concerted model.

Which of the following best describes negative cooperativity?

• A. Binding of one substrate molecule stimulates binding of a second substrate.
• B. Binding of one substrate molecule inhibits binding of a second substrate.
• C. Binding of one substrate molecule leads to negative reaction rates.
• D. Binding of one substrate molecule causes a negative effect/result in the cell.

Here’s what students ask on this topic:

What is the sequential model (KNF model) in biochemistry?

The sequential model, also known as the KNF model, describes the behavior of allosteric enzymes where subunits transition from the tense (T) state to the relaxed (R) state independently. This transition is influenced by substrate binding through the induced fit mechanism. Unlike the concerted model, where all subunits change state simultaneously, the sequential model allows for subunits to be in different states at the same time. This model can explain both positive and negative cooperativity, making it a versatile framework for understanding the sigmoidal kinetics of allosteric enzymes.

How does the sequential model explain positive and negative cooperativity?

The sequential model explains positive and negative cooperativity through the independent state transitions of enzyme subunits. Positive cooperativity occurs when substrate binding to one subunit increases the likelihood that neighboring subunits will also transition to the R state, enhancing their substrate-binding affinity. Conversely, negative cooperativity happens when substrate binding to one subunit makes it harder for neighboring subunits to transition to the R state, reducing their substrate-binding affinity. This dual capability distinguishes the sequential model from the concerted model, which only allows for positive cooperativity.

What are the key differences between the concerted model and the sequential model?

The concerted model (MWC model) and the sequential model (KNF model) differ in several key ways. In the concerted model, all subunits of an allosteric enzyme transition from the T state to the R state simultaneously, and this transition can occur without substrate binding. The concerted model only allows for positive cooperativity. In contrast, the sequential model involves independent transitions of subunits from the T state to the R state, induced by substrate binding. This model allows for both positive and negative cooperativity. Additionally, hybrids of subunits in different states are possible in the sequential model but not in the concerted model.

Why is the sequential model important for understanding enzyme kinetics?

The sequential model is crucial for understanding enzyme kinetics because it provides a comprehensive framework for explaining the behavior of allosteric enzymes. By allowing for independent subunit transitions and both positive and negative cooperativity, the sequential model can account for the sigmoidal kinetics observed in many allosteric enzymes. This understanding helps in elucidating how substrate binding influences enzyme activity and regulation, which is essential for designing drugs and understanding metabolic pathways.

How does substrate binding influence subunit transitions in the sequential model?

In the sequential model, substrate binding influences subunit transitions through the induced fit mechanism. When a substrate binds to a subunit of an allosteric enzyme, it induces a conformational change from the T state to the R state in that specific subunit. This change can either promote (positive cooperativity) or inhibit (negative cooperativity) the transition of neighboring subunits to the R state. This mechanism allows for a more nuanced and flexible response to substrate binding compared to the concerted model, where all subunits transition simultaneously.