Metabolic Regulation 2: Study with Video Lessons, Practice Problems & Examples

Phosphofructokinase 1 (PFK1) is a key regulatory enzyme in glycolysis, catalyzing the commitment step by phosphorylating fructose 6-phosphate. It is allosterically regulated by ATP (inhibitory) and ADP/AMP (stimulating). Fructose 2,6-bisphosphate, produced by PFK2, is crucial for PFK1 activation. Insulin activates PFK2, while glucagon inactivates it, demonstrating feedback inhibition. Pyruvate kinase, another glycolytic enzyme, is inhibited by ATP and activated by fructose 1,6-bisphosphate. Acetyl CoA activates pyruvate carboxylase in gluconeogenesis, highlighting the interconnectedness of these metabolic pathways.

Metabolic Regulation 2

So after hexokinase, phosphofructokinase 1 is like our 2nd most influential enzyme on glycolysis. And it, you might remember, catalyzes that commitment step to glycolysis when you burn your second ATP to phosphorylate the molecule a second time, right? So just to, you know, be crystal clear here. I'm sorry if this is a review but you know, as soon as glucose enters the cell, it's turned into glucose 6 phosphate regardless of what it's going to go on to do. But if you're going to commit to glycolysis, that's where phosphofructokinase comes in, right? That's that commitment step to glycolysis. Glucose 6 Phosphate can do other things but once PFK is done, it’s glycolysis time. Anyways, so this, you know, it should make sense then that phosphofructokinase is like the 2nd most influential enzyme on glycolysis, on the rate of glycolysis. It actually has allosteric regulatory sites that bind ATP and hopefully, you know, you just guess that well, ATP is the product, right, the downstream product so I bet ATP is going to have an inhibitory effect and you betcha. High ATP concentrations lower enzyme activity. Citrate actually will also inhibit PFK 1. Citrate, you might recall the first molecule formed in the citric acid cycle. It is actually an important molecule for biosynthesis and it will be transported out into the cytoplasm, right? Citrate is going to be produced in the mitochondrial matrix but it's transported out into the cytoplasm to undergo biosynthesis And, if it is in excess concentrations, it'll inhibit PFK1 and and you know, let's think about this logically. If the citric acid cycle is going gangbusters, right? It's overproducing. It's going to filter off some citrate to do biosynthesis with. It's like we don't need all these, right? We're doing we're making too much so let's filter some off back in the cytosol, do biosynthesis but if you're filtering off a lot, you're going to really build up a lot of citrate

Here’s what students ask on this topic:

What is the role of phosphofructokinase 1 (PFK1) in glycolysis?

Phosphofructokinase 1 (PFK1) is a key regulatory enzyme in glycolysis. It catalyzes the commitment step by phosphorylating fructose 6-phosphate to fructose 1,6-bisphosphate, using ATP. This step is crucial because it commits the glucose molecule to the glycolytic pathway, meaning it will be broken down to produce energy. PFK1 is allosterically regulated by several molecules: ATP and citrate inhibit its activity, while ADP, AMP, and fructose 2,6-bisphosphate stimulate it. This regulation ensures that glycolysis proceeds when the cell needs energy and slows down when energy is abundant.

How does fructose 2,6-bisphosphate regulate PFK1 activity?

Fructose 2,6-bisphosphate is a potent allosteric activator of PFK1. It binds to PFK1 and significantly increases its activity, even at very low concentrations (around 1 micromolar). This molecule is produced by the enzyme phosphofructokinase 2 (PFK2) and plays a crucial role in regulating glycolysis. When insulin levels are high, PFK2 is activated, leading to an increase in fructose 2,6-bisphosphate levels, which in turn activates PFK1 and promotes glycolysis. Conversely, glucagon inactivates PFK2, reducing fructose 2,6-bisphosphate levels and thus inhibiting PFK1 activity.

What is the effect of ATP on phosphofructokinase 1 (PFK1) activity?

ATP acts as an allosteric inhibitor of phosphofructokinase 1 (PFK1). When ATP levels are high, it binds to specific regulatory sites on PFK1, reducing the enzyme's activity. This makes sense from a metabolic standpoint because high ATP levels indicate that the cell has sufficient energy, and thus, the need for further glycolysis is reduced. This feedback inhibition helps to prevent the unnecessary breakdown of glucose when energy is already abundant, thereby conserving cellular resources.

How does insulin affect the activity of phosphofructokinase 2 (PFK2)?

Insulin activates phosphofructokinase 2 (PFK2) by promoting the dephosphorylation of the enzyme. When insulin levels are high, it signals that blood glucose levels are also high. This leads to the activation of PFK2, which then increases the production of fructose 2,6-bisphosphate. The elevated levels of fructose 2,6-bisphosphate activate PFK1, thereby enhancing glycolysis. This process helps to lower blood glucose levels by promoting its breakdown and utilization for energy production.

What is the role of pyruvate kinase in glycolysis, and how is it regulated?

Pyruvate kinase is the enzyme that catalyzes the final step of glycolysis, converting phosphoenolpyruvate (PEP) to pyruvate while generating ATP. It is regulated by several mechanisms: it is activated by fructose 1,6-bisphosphate (a glycolytic intermediate) and inhibited by ATP, acetyl-CoA, and long-chain fatty acids. In the liver, pyruvate kinase is also regulated by glucagon, which activates protein kinase A to phosphorylate and inactivate pyruvate kinase when blood sugar is low. This regulation ensures that glycolysis is promoted when energy is needed and inhibited when energy is abundant or when glucose needs to be conserved.