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 of the allosteric enzymes approaching 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 in a very narrow substrate concentration. However, for the Michaelis-Menten enzyme, it pretty much goes from 0 up to 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 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.