Phosphofructokinase 1 (PFK1) plays a crucial role in glycolysis, acting as the second most influential enzyme after hexokinase. It catalyzes a key commitment step in glycolysis by phosphorylating fructose 6-phosphate to fructose 1,6-bisphosphate, utilizing ATP in the process. This step is critical because, while glucose 6-phosphate can be directed towards various metabolic pathways, the action of PFK1 commits the molecule to glycolysis.
PFK1 is subject to allosteric regulation, meaning its activity can be modulated by the binding of specific molecules. High concentrations of ATP, the end product of glycolysis, inhibit PFK1, signaling that energy levels are sufficient and glycolysis can slow down. Similarly, citrate, a key metabolite from the citric acid cycle, also inhibits PFK1 when present in excess, indicating that the cell has enough building blocks for biosynthesis. Conversely, low energy states represented by ADP and AMP stimulate PFK1 activity, promoting glycolysis when energy is scarce.
Fructose 2,6-bisphosphate is a potent allosteric activator of PFK1, significantly enhancing its activity even at low concentrations (around 1 micromolar). This molecule is produced by phosphofructokinase 2 (PFK2), which is regulated by insulin and glucagon. Insulin promotes the dephosphorylation and activation of PFK2, leading to increased levels of fructose 2,6-bisphosphate and enhanced glycolysis. In contrast, glucagon phosphorylates and inactivates PFK2, reducing fructose 2,6-bisphosphate levels and thus inhibiting glycolysis.
PFK2 and fructose 2,6-bisphosphatase are interconnected; when one is active, the other is inactive. This dual regulation allows for a coordinated response to metabolic needs. For instance, xylulose-5-phosphate from the pentose phosphate pathway stimulates protein phosphatase, which activates PFK2, further promoting glycolysis when biosynthetic precursors are abundant.
Pyruvate kinase is another key enzyme in glycolysis, particularly in the liver, where it is inactivated by glucagon signaling during low blood sugar states. This mechanism prevents the liver from consuming glucose when it needs to release it into the bloodstream. Pyruvate kinase is activated by fructose 1,6-bisphosphate, linking it to the upstream glycolytic pathway, and is inhibited by ATP, acetyl-CoA, and alanine, which signal sufficient energy and substrate availability.
In gluconeogenesis, pyruvate carboxylase serves as the first enzyme, reversing the action of pyruvate kinase. It is activated by acetyl-CoA, indicating that when energy substrates are plentiful, the cell may switch from glycolysis to gluconeogenesis to synthesize glucose. This interplay between glycolysis and gluconeogenesis is essential for maintaining energy homeostasis in the body.