4. Regulation of Glycolysis in the Liver

PFK-1 and PFK-2

PFK-1 is inhibited by ATP. The logic being that when there’s an abundance of energy in the cell, there’s no need for glycolysis.

A velocity plot of [F6P] at low [ATP] shows a classic hyperbolic relationship (MM enzyme); whereas at higher [ATP], the curve becomes S-shaped, indicating inhibition.

PFK-1 ATP curve
Class slide from HarvardX. All rights are theirs.

PFK-1 is activated by AMP. A high AMP means that ATP production is down and so glycolysis needs to proceed. What about ADP?

ATP -> ADP -> AMP

but

ADP + ADP —(adenylate kinase)–> ATP + AMP

So part of ADP is converted back to ATP. This makes AMP a better indicator of a cell’s low energy state.

Also, AMP is a more sensitive measure of PFK-1 than ADP. In a cell, [AMP] << [ADP] < [ATP]. Thus, small changes in [ATP] cause large changes in [AMP].

Class slide from HarvardX. All rights are theirs.
Class slide from HarvardX. All rights are theirs.

Intermediates

The second function of glycolysis the formation of intermediates that can be routed into biosynthetic pathways–amino acids, lipids, etc. These intermediates also contribute to the modulation of glycolytic flux.

Citrate

Citrate from the CAC is another measure of a cell’s energy. It can be routed to lipid synthesis or it can circle back to inhibit PFK-1. Since Citrate is a good measure of cell’s energy state, a high [Citrate] signals glycolysis to slow down. It does this by inhibiting PFK-1. This enhances the inhibitory effect of ATP.

Special Case: the Liver

In the Liver, [ATP] is always high. So it’s inhibitory function on PFK-1 is limited. Instead, a related enzyme more important here. Welcome, Phosphofructokinase-2 or PFK-2 for short.

pfk-2

F6P is converted to Fructose 2,6-bisphosphate (F26P) via PFK-2. It has two domains, a phosphatase domain and a kinase domain. Where else did we see this?

Another HarvardX class lecture screen grab. All rights are theirs.
Another HarvardX class lecture screen grab. All rights are theirs.

Plotting the velocity curve of [F6P] for the liver there’s a S-curve, indicative of the high levels of ATP there (i.e., PFK-1 is inhibited). If the levels of PFK-2 rise, that curve will switch to a hyperbolic curve, indicating that PFK-2 is an allosteric activator of this reaction. Or put another way, F26P increase the affinity PFK-1 has for its substate F6P.

F26P itself is regulated by external factors.

The [Glu] in the liver tends to be roughly the same as the concentration in the blood. So if [Glu] in the blood is high, glucose wants to be metabolized in the liver. In fact, in this case [F6P] is high and thus PFK-2 is activated, which in turn accelerates the synthesis of F26P and from that, PFK-1 is activated and glycolysis proceeds.

PFK-2 is operating in its kinase domain in this case.

This is called Feed-Forward Stimulation.

Got it? No, then re-read it. It’s important.

Conversely, when [Glu] is low, we need to put a brake on Glycolysis. In this case, PFK-2 flips over into its phosphatase domain.

The switch between domains is under hormonal control.

Hormones

Low blood glucose triggers the secretion of Glucagon by the pancreas. Glucagon binds to glucagon receptors at the surface of liver cells. The resulting bond changes the conformation of those receptors, which triggers the activation of a small G protein. This activates an enzyme called Adealyl Cyclase. Adealyl Cyclase catalyzes the conversion of an ATP into cAMP. The [cAMP] then rises. cAMP interacts with Protein Kinase A or PKA (recall the role PKA played in Lipid Lypolysis).

PKA becomes activated and then phosphorylates PFK-2 on the kinase domain. This inhibition of the kinase domain leads to the corresponding activation of its phosphatase domain.

Therefore, PFK-2 will catalyze the conversion of F26P back into F6P.

As [F26P] decreases, PFK-1 activity decreases as well.

Class lecture slide
Class lecture slide

Thus, Glucose consumption will slow down in the liver, allowing other organs like the brain will have access to the Glucose it needs. When the [Glu] rises and Glucagon signals stops, PFK-2 will switch back to its Kinase domain, [F26P] will rise, and PFK-1 activity will resume.

 

 

 

 

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