Glucose's Impact on Lac Operon - Online Tutor, Practice Problems & Exam Prep

Glucose serves as the primary energy source for prokaryotes, inhibiting the lac operon when present. Low glucose levels increase cyclic AMP (cAMP), which activates cyclic AMP receptor protein (CRP), enhancing lac operon transcription. This positive control mechanism ensures that lactose is utilized only when glucose is scarce. The relationship between glucose and cAMP is inversely proportional, affecting transcription rates. Understanding this regulation is crucial for grasping metabolic pathways and gene expression in prokaryotic cells.

GlucoseLevels, cAMP, & the Lac Operon

In this video, we're going to begin our lesson on glucose's impact on the lac operon. And so we're going to focus on glucose levels, a molecule called cAMP, and how they affect the LAC operon. And so what's important to note is that in most prokaryotes, glucose is actually the preferred energy source even in the presence of lactose. And so what this means is that if glucose is available to cells, then the cells are going to be using glucose as their primary energy source, not lactose, and therefore the lac operon should be turned off in the presence of glucose.

Now, it turns out that glucose levels are linked to cellular levels of a molecule that's called cyclic AMP or just cAMP for short, through a process that we're not going to get into in this video. However, what you should know is that when glucose concentrations are really really low or when glucose is just absent and glucose is not available for metabolism, then cellular levels of this molecule called cAMP are going to increase. And so high cellular cAMP levels will actually increase the rate of transcription of the lac operon. And so basically what we're seeing here is when glucose is low or absent cAMP levels increase absent. But if glucose is available then the lac operon will be turned off. And so this allows glucose to be used as the preferred and the primary energy source. And lactose will only be used as a secondary energy source when glucose is not available.

Now cAMP levels, they actually do not affect the repressor proteins activity. So they do not affect lac repressor's activity. They only increase transcription of the lac operon when glucose is absent. And as we move forward in our course, we'll be we'll be able to talk about exactly how it is that cAMP affects the lac operon's transcription. But for now, let's take a look at this image down below. And so what it's saying here in this example is that glucose levels control cAMP levels in the cell. And the cAMP levels in the cell are gonna control the rate of the lac operon transcription.

And so over here on the left-hand side, notice that we have a scenario. We're showing you a cell here. So here is our cell, and notice that there is a high glucose concentration, this little green hexagon represents glucose, and there is low lactose concentration. And so notice that on the inside of our cell because there is high glucose that actually translates to having low cAMP within the cell. And so notice that within our cell the cAMP molecule, which is represented by this little green circle that we see here, the cAMP levels inside the cell are relatively low when glucose concentrations are high. So there's an inverse relationship there between glucose concentrations and cAMP concentrations. When one is high, the other is low. And the low cAMP is actually going to translate to having low lac operon transcription. And so the lac operon will not be transcribed at a high rate. It will be a low lac operon transcription when glucose is high, and again, this allows glucose to be used as the primary energy source.

However, over here on the right-hand side, notice that we have a different scenario. Again, we still have our cell here in the middle, but notice that this time there's a low glucose concentration and a high lactose concentration. And so the low glucose concentration is going to have an inverse relationship with the amount of cAMP. And so there's going to be high levels of cAMP within the cell. And so you can see that within our cell down below here. All of these little green circles that you see are representing the cAMP molecules. And there are a lot of cAMP on over here on this side as compared to over here on this side. And so the high cAMP is going to translate to having high lac operon transcription. And so, what this allows for is the lactose is going to be used as an energy source when glucose is not available. And so again, this level of regulation allows glucose to be the primary energy source and lactose to be used only as a secondary energy source when glucose is not available.

Now as we move forward in our course, we're going to talk more details about exactly how is it that cAMP increases the transcription of the lac operon, and we'll be able to learn more as we move forward in our course. So I'll see you all in our next video.

Glucose's Impact on Lac Operon Example 1

So here we have an example problem that wants us to complete the table shown below, and notice that this table is demonstrating environmental and cellular levels of glucose and lactose concentrations, along with whether the lac operon is expressed under the stated conditions. We will focus on filling out one row at a time, ensuring all the blanks are completed.

Notice that the first row has high environmental levels of glucose and lactose. Consequently, cellular levels of glucose will also be high, as expected when environmental levels are the same. Remember that glucose has an inverse relationship with cAMP levels; thus, if glucose concentrations are high, then cAMP levels will be low. When lactose concentration levels in the environment are high, this will also lead to high cellular levels of lactose. Recall that the lac operon is expressed only when glucose is unavailable since it is the preferred energy source, and the lac operon should be off when glucose is available. Given the high glucose levels, the cell will use glucose as the primary source, even though lactose is available. Therefore, the lac operon will not be expressed.

The second example considers high environmental glucose levels and low lactose levels. This condition results in high cellular glucose and low cellular lactose. Recall the inverse relationship between cAMP and glucose: high glucose leads to low cAMP. Under these conditions, the lac operon would only be needed to break down lactose if lactose were present, which it is not. Furthermore, since glucose is the preferred source, the lac operon will not be expressed. Here, again, the lac operon will be off.

Now skipping to the fourth example, we observe low environmental glucose levels, which translate to low cellular glucose levels. cAMP has an inverse relationship with glucose, so low glucose results in high cAMP levels. With low lactose in the environment, this translates to low lactose at the cellular level as well. Considering that glucose is not available as the primary energy source but lactose is also low, the lac operon should not be expressed under these circumstances.

Finally, focusing on the third example, which is highlighted (this was initially skipped), we note that environmental glucose levels are low but lactose levels are high. This results in low glucose and high cAMP at the cellular level, along with high cellular lactose levels. Under these conditions, the lac operon should be expressed since glucose is not available as the energy source and lactose is present. High cellular cAMP levels will further support the expression of the lac operon.

This explanation concludes the completion of this table. We will further analyse and give more practice on how cAMP impacts the lac operon in our subsequent lesson. I look forward to seeing you all in the next video.

How does extracellular glucose inhibit transcription of the lac operon?

• A. By strengthening the binding of the repressor to the operator.
• B. By weakening the binding of the repressor to the operator.
• C. By inhibiting RNA polymerase from opening the strands of DNA to initiate transcription.
• D. By reducing the levels of intracellular cAMP.

Positive Control by cAMP & CRP

So from our previous lesson videos, we know that cyclic AMP or cAMP will increase the rate of lac operon transcription. But in this video, we're going to talk about exactly how that works by focusing on positive control by cAMP and a molecule that we're going to introduce called CRP. And so CRP is really just an abbreviation for cyclic AMP receptor protein or just CRP for short. And CRP is actually an activator protein, which recall from our previous lesson videos, activator proteins are regulatory proteins that will stimulate transcription. And so CRP is an activator protein that will stimulate the lac operon transcription when CRP is bound to cAMP. And so you can see that even embedded in CRP's full name here, it has cyclic AMP. And so it does rely on cAMP. And so recall from our previous lesson videos that low glucose levels are going to translate to high cellular cAMP levels. And high cellular cAMP levels are going to allow cAMP to bind and activate CRP. And an active CRP is going to act as an activator protein, and it is going to bind to a region of DNA upstream of the lac promoter, and it is going to stimulate transcription by helping to recruit RNA polymerase. And when the RNA polymerase is recruited and bound to the promoter, then transcription is stimulated and can proceed. And so essentially what we're saying here is that when glucose levels are low, if we have low glucose, that's going to translate to having high cAMP. And having high cAMP is going to allow cAMP to bind to and activate CRP. So we have an active CRP and having an active CRP is going to increase the rate of lac operon transcription. And so, really, this here this line here shows you the takeaway, that low glucose levels lead to high cAMP levels, high cAMP levels lead to an active CRP, and an active CRP increases the rate of lac operon transcription. And so let's take a look at our image down below to get a better understanding of how cAMP and CRP positively control the expression of the lac operon. And it's positive control because it is increasing and turning on the lac operon. And so that's what we're focusing on in this image is positive control of the lac operon mainly by an active CRP protein. And so if we take a look at our image down below, notice that we have it broken up into 2 halves. We have the top half here and then we have the bottom half down below. We'll focus on the top half first, And what you'll notice is that we're showing you the lac operon over here, but this time notice that the lac operon has a new region that we have not yet introduced, and we're showing it here for the first time, and that is the CRP binding site. And so the CRP binding site is the site that is upstream of the promoter, that is going to be where the active CRP will bind. And so notice that, in the top half of this image, we have some specific conditions indicated by this box over here. And so notice here you can see that there's glucose here, And so notice here you can see that there's glucose here in the cell, these little, green hexagons, and you can also see that there is lactose within the cell. And we know already that lactose, a derivative of lactose, will bind to the LAC repressor, LAC I, and inactivate the LAC repressor. But also when glucose is present glucose is, going to lead to having low levels of cAMP. And low levels of cAMP means that the inactive, we're going to have an inactive CRP protein. And the inactive CRP protein will not bind to the CRP binding site. And so all of these conditions mean that the RNA polymerase, even though it is not being blocked by the inactive lac repressor, it will not be able to bind to the promoter very effectively without the active CRP here. And so the inactive CRP is not going to bind, and we need this active CRP to bind in order for RNA polymerase to also bind. And so what we're seeing here is that transcription of the lac operon is off in this state here. So the lac operon is in an off position here, under these conditions. And again, this makes sense because we know that glucose is the preferred energy source. And so if there are high levels of glucose, then that means that the cell is going to be using glucose as the energy source and it should not waste energy transcribing the lac operon because even though lactose is high, it is not going to be using lactose as the energy source. It is going to be using glucose as the energy source here. Now notice down below in this image, we have some slightly different conditions. Notice that the lactose concentration levels are still high just as they were before. Okay. So lactose levels are still high. But this time, notice that the glucose concentration levels are low. And again, we know that cAMP has an inverse relationship with glucose levels. And so if glucose levels are low, that means that cAMP levels are going to be high. And so you can see here in this image that cAMP, this little green molecule, its concentration is high. And when it is high, cAMP is going to bind to CRP. And when cAMP binds to CRP, it activates CRP, so we have an active CRP. And the active CRP will bind to this CRP binding site. And the active CRP bound to the CRP binding site is going to help to recruit the RNA polymerase so that the RNA polymerase will actually bind. And if the RNA polymerase is bound, then it can proceed forward with transcription and activate all of these genes. And so what we have in here is a lac operon that is on and transcription and gene expression are on not only does it require lactose levels to be high to inactivate the lac repressor, but it also requires glucose levels to be low, so that cAMP can activate CRP and CRP can bind and recruit RNA polymerase and RNA polymerase can stimulate transcription. And so this here concludes our brief introduction to positive control by cAMP and CRP, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

When glucose is present:

• A. cAMP is high, CRP binds to the activator binding site, and transcription of the lac operon is turned off.
• B. cAMP is low, CRP binds to the site activator binding site, and transcription of the lac operon is turned on.
• C. cAMP is high, CRP does not bind to the activator binding site, and transcription of the lac operon is turned on.
• D. cAMP is low, CRP does not bind to the activator binding site, and transcription of the lac operon is turned off.

Here’s what students ask on this topic:

How does glucose affect the lac operon in prokaryotes?

In prokaryotes, glucose serves as the preferred energy source. When glucose is present, it inhibits the lac operon, preventing the transcription of genes involved in lactose metabolism. This inhibition occurs because high glucose levels lead to low levels of cyclic AMP (cAMP). Low cAMP levels mean that the cyclic AMP receptor protein (CRP) remains inactive. Without active CRP, RNA polymerase cannot effectively bind to the promoter region of the lac operon, thus reducing transcription. This regulatory mechanism ensures that the cell prioritizes glucose metabolism over lactose metabolism, conserving energy and resources.

What role does cAMP play in the regulation of the lac operon?

cAMP (cyclic AMP) plays a crucial role in the positive regulation of the lac operon. When glucose levels are low, cAMP levels increase. High cAMP levels allow cAMP to bind to the cyclic AMP receptor protein (CRP), activating it. The active CRP then binds to a specific site upstream of the lac operon promoter. This binding helps recruit RNA polymerase to the promoter, enhancing the transcription of the lac operon. Thus, cAMP acts as a signal that indicates low glucose availability, enabling the cell to switch to lactose metabolism by increasing the transcription of the lac operon genes.

Why is glucose considered the preferred energy source over lactose in prokaryotes?

Glucose is considered the preferred energy source over lactose in prokaryotes because it can be directly utilized in glycolysis, providing a more efficient and immediate source of energy. The metabolism of glucose requires fewer enzymatic steps compared to lactose, which must first be hydrolyzed into glucose and galactose by the enzyme β-galactosidase. This preference is reflected in the regulation of the lac operon, where the presence of glucose inhibits the transcription of genes involved in lactose metabolism. This ensures that the cell conserves energy and resources by using the most readily available and efficient energy source first.

How does CRP (cyclic AMP receptor protein) influence lac operon transcription?

CRP (cyclic AMP receptor protein) influences lac operon transcription through a mechanism known as positive control. When glucose levels are low, cAMP levels increase, allowing cAMP to bind to CRP and activate it. The active CRP then binds to a specific site upstream of the lac operon promoter. This binding facilitates the recruitment of RNA polymerase to the promoter, thereby enhancing the transcription of the lac operon. In essence, CRP acts as an activator protein that, when bound to cAMP, stimulates the transcription of genes necessary for lactose metabolism, ensuring that the cell can utilize lactose as an alternative energy source when glucose is scarce.

What is the relationship between glucose levels and cAMP levels in prokaryotic cells?

In prokaryotic cells, there is an inverse relationship between glucose levels and cAMP (cyclic AMP) levels. When glucose levels are high, cAMP levels are low. Conversely, when glucose levels are low or absent, cAMP levels increase. This inverse relationship is crucial for the regulation of the lac operon. High glucose levels result in low cAMP levels, leading to an inactive cyclic AMP receptor protein (CRP) and reduced transcription of the lac operon. Low glucose levels result in high cAMP levels, which activate CRP, enhancing the transcription of the lac operon. This regulatory mechanism ensures that the cell prioritizes glucose metabolism and only switches to lactose metabolism when glucose is not available.