Allosteric Kinetics: Videos & Practice Problems

Allosteric enzymes exhibit unique kinetics, characterized by a sigmoidal curve, unlike the rectangular hyperbola seen in Michaelis-Menten enzymes. The Michaelis constant (K_m) is replaced by K_0.5 for allosteric enzymes, indicating substrate concentration for half-maximal velocity (V_max). Additionally, allosteric enzymes display a threshold effect, remaining inactive below a certain substrate concentration, which acts as an on/off switch for enzyme activity. This regulatory mechanism allows for precise control in metabolic pathways, enhancing cellular responses to substrate availability.

Allosteric Kinetics

So now that we've introduced allosteric enzymes, in this video, we're going to talk about how allosteric enzymes display allosteric kinetics. And so these allosteric enzymes are actually pretty easy for biochemists to identify, and that's because allosteric enzymes behave very differently than the Michaelis-Menten enzymes that we covered in our previous lesson videos. This includes responding differently to changes in substrate concentration as well as to the presence of inhibitors.

It turns out that most allosteric enzymes display a sigmoidal curve, or an S-shaped curve, on a kinetics plot instead of a rectangular hyperbola like Michaelis-Menten enzymes do. If we take a look at our image down below, notice on the left, the enzyme kinetics plot, we're showing you a Michaelis-Menten enzyme, and it's forming the same shape that we've seen so many times before in our previous lesson videos, referred to as a rectangular hyperbola. Whereas, if we take a look at the enzyme kinetics plot on the right, notice that we're showing an allosteric enzyme instead of a Michaelis-Menten enzyme. And instead of showing a rectangular hyperbola, it's showing this S-shaped curve here which is referred to as a sigmoidal curve.

Now, recall that the Michaelis constant, \(K_m\), is the exact substrate concentration that allows for half of the \(v_{max}\). It turns out that the Michaelis constant variable, \(K_m\), only applies to Michaelis-Menten enzymes. The substrate concentration that allows for half of the \(v_{max}\) for allosteric enzymes is represented by the variable \(K_{0.5}\). \(K_{0.5}\) is pretty much the allosteric enzyme equivalent to the Michaelis constant \(K_m\) for the Michaelis-Menten enzymes. Also, on that note, the Michaelis-Menten equation only applies to Michaelis-Menten enzymes and does not apply for allosteric enzymes. So these are definitely some differences to take note of about allosteric kinetics.

This here concludes our introduction to allosteric kinetics, and we'll continue to learn more about allosteric kinetics as we move forward in our course. So I'll see you guys in our next video.

Which of the following statements regarding allosteric kinetics is false?

• A. The rate of an allosteric enzyme reaction is dependent on substrate concentration [S].
• B. The reaction velocity and substrate concentration always proportionally change in a Lineweaver-Burk plot.
• C. At saturating [S], the kinetics of an allosteric enzyme will follow the Michaelis-Menten model.
• D. Allosteric enzymes display second order kinetics leading to sigmoidal curvature on a kinetics plot.
• E. None are false because all of the above are true.

Answer: (B)

Allosteric Kinetics

So another way for biochemists to easily identify allosteric enzymes is by plotting the enzyme kinetic data onto a Lineweaver-Burk plot. And that's because allosteric enzyme kinetic data does not actually form a straight line on a Lineweaver-Burk plot like the enzyme kinetic data of Michaelis-Menten enzymes do. And so, notice down below over here on the left-hand side, we have a Lineweaver-Burk plot showing the enzyme kinetic data of a Michaelis-Menten Enzyme. And notice that the enzyme kinetic data of the Michaelis-Menten Enzyme forms a straight line on the Lineweaver-Burk plot. And so here we can write in straight line. And so, of course, what this means is that the Lineweaver-Burk equation is going to apply for Michaelis-Menten enzymes. However, if we take a look at the Lineweaver-Burk plot over here on the right-hand side, notice it's showing the enzyme kinetic data of the allosteric enzyme. And notice that the enzyme kinetic data of an allosteric enzyme plotted onto a Lineweaver-Burk plot does not form a straight line. And so this, of course, means that the Lineweaver-Burk equation does not apply to allosteric enzymes. And again, it's pretty easy for biochemists to identify these allosteric enzymes when they plot the enzyme kinetic data onto the Lineweaver-Burk plot. And so this here concludes our introduction to allosteric enzyme kinetics and we'll be able to get a little bit of practice in our next video. So I'll see you guys there.

Allosteric Kinetics

So another feature that allosteric enzymes display that Michaelis-Menten enzymes don't is this threshold effect. Let's explain this a bit further. At very, very low substrate concentrations, it turns out that Michaelis-Menten enzymes are actually more sensitive to changes in substrate concentration than allosteric enzymes. We can actually see that down below in our enzyme kinetics plot where this black curve here represents the Michaelis-Menten enzymes and the blue curve here represents allosteric enzymes. Notice here we're specifically focusing on low substrate concentration, somewhere around this range right here on the x-axis. At these low substrate concentrations, notice that the allosteric enzyme pretty much doesn't even respond to changes in substrate concentration, whereas the Michaelis-Menten enzyme definitely does respond to changes in concentration when it is low like this. That's exactly what we're mentioning up above in this line right here.

It turns out, however, that eventually a threshold substrate concentration is reached. And once this threshold substrate concentration is reached, this is where allosteric enzymes become much more sensitive to the changes in substrate concentration. Notice down below in our plot here that the threshold substrate concentration is reached, and the allosteric enzyme is much more sensitive to changes in substrate concentration. This ultimately leads to the initial reaction velocity or the \(v_0\) of the allosteric enzymes approaching \(v_{max}\) within a smaller and narrower range of substrate concentration. We can also see that down below, and the ranges are represented by these bars below the x-axis here: the blue bar corresponds with the allosteric enzyme and the gray bar corresponds with the Michaelis-Menten enzyme. You can see that once we have the threshold substrate concentration reached, the allosteric enzyme approaches \(v_{max}\) in a very narrow substrate concentration. However, for the Michaelis-Menten enzyme, it pretty much goes from 0 up to \(v_{max}\) within a much wider range. Again, that's pretty much what we were saying up above in this line right here.

All of this here actually creates what's known as the threshold effect. The threshold effect is something that allosteric enzymes display that Michaelis-Menten enzymes don't. Really, when it comes down to it, the threshold effect just says that below a certain substrate concentration, there's pretty much very little to no allosteric enzyme activity. Below the subthreshold substrate concentration, the allosteric enzyme has practically zero activity. But once the threshold substrate concentration is reached, the allosteric enzyme's activity gets turned on, and it can reach \(v_{max}\) very quickly. This threshold substrate concentration acts like an on and off switch for the allosteric enzyme. Whereas for the Michaelis-Menten enzyme, it's pretty much always on as long as there's some substrate concentration, the Michaelis-Menten enzyme is always going to be active. This threshold effect feature of allosteric enzymes gives them another form of regulation that cells can use to regulate their metabolic pathways.

This here concludes our introduction to the threshold effect of allosteric enzymes, and we'll be able to get a little bit of practice as we move forward in our course. So I'll see you guys in our next video.

Because of the _____________ substrate-binding-site(s) & conformation(s) on an allosteric enzyme, the range of [S] to reach the V_max is _______________ for allosteric enzymes than it is for Michaelis-Menten enzymes.

• A. Single, smaller.
• B. Single, greater.
• C. Multiple, narrower.
• D. Multiple, wider.

Answer: (C)

Here’s what students ask on this topic:

What is the difference between allosteric enzymes and Michaelis-Menten enzymes in terms of their kinetics?

Allosteric enzymes and Michaelis-Menten enzymes differ significantly in their kinetics. Allosteric enzymes exhibit a sigmoidal (S-shaped) curve on a kinetics plot, indicating cooperative binding of substrates. In contrast, Michaelis-Menten enzymes display a hyperbolic curve, reflecting a simple saturation mechanism. The Michaelis constant (K_m) is used for Michaelis-Menten enzymes to represent the substrate concentration at half-maximal velocity (V_max). For allosteric enzymes, this is replaced by K_0.5. Additionally, allosteric enzymes show a threshold effect, remaining inactive below a certain substrate concentration, which acts as an on/off switch for enzyme activity. This allows for more precise regulation in metabolic pathways.

How can you identify allosteric enzymes using a Lineweaver-Burk plot?

Allosteric enzymes can be identified using a Lineweaver-Burk plot by observing the shape of the plotted data. For Michaelis-Menten enzymes, the enzyme kinetic data forms a straight line on a Lineweaver-Burk plot. However, for allosteric enzymes, the data does not form a straight line. This deviation from linearity indicates the presence of allosteric regulation. The Lineweaver-Burk equation, which applies to Michaelis-Menten enzymes, does not apply to allosteric enzymes, making this plot a useful tool for distinguishing between the two types of enzymes.

What is the threshold effect in allosteric enzymes?

The threshold effect in allosteric enzymes refers to their unique response to substrate concentration. At very low substrate concentrations, allosteric enzymes exhibit little to no activity, unlike Michaelis-Menten enzymes, which are more sensitive to changes in substrate concentration. Once a specific threshold substrate concentration is reached, allosteric enzymes become highly sensitive to changes in substrate concentration, rapidly approaching their maximal velocity (V_max). This threshold acts as an on/off switch for enzyme activity, allowing for precise regulation in metabolic pathways.

Why does the Michaelis-Menten equation not apply to allosteric enzymes?

The Michaelis-Menten equation does not apply to allosteric enzymes because it is based on the assumption of a hyperbolic relationship between substrate concentration and reaction velocity, which is characteristic of Michaelis-Menten enzymes. Allosteric enzymes, however, exhibit a sigmoidal (S-shaped) curve due to cooperative binding of substrates. This cooperative behavior cannot be accurately described by the Michaelis-Menten equation. Instead, allosteric enzymes use the K_0.5 value to represent the substrate concentration at half-maximal velocity (V_max), reflecting their unique kinetic properties.

What is K_0.5 in the context of allosteric enzymes?

K_0.5 is a parameter used to describe the kinetics of allosteric enzymes. It represents the substrate concentration at which the reaction velocity is half of the maximal velocity (V_max). This is analogous to the Michaelis constant (K_m) used for Michaelis-Menten enzymes. However, K_0.5 is specific to allosteric enzymes and reflects their sigmoidal (S-shaped) kinetic behavior, which arises from cooperative substrate binding. Understanding K_0.5 is crucial for studying the regulatory mechanisms and kinetic properties of allosteric enzymes.