Glycolysis is primarily limited by the activity of three key regulatory enzymes: hexokinase (HK), phosphofructokinase-1 (PFK-1), and pyruvate kinase (PK). These enzymes catalyze irreversible reactions within the pathway and are subject to allosteric regulation and, in some cases, hormonal control.
Key Regulatory Enzymes in Glycolysis
These three enzymes act as control points to regulate the rate of glycolysis based on cellular energy needs and the availability of substrates.
1. Hexokinase (HK)
- Function: Hexokinase catalyzes the first step of glycolysis: the phosphorylation of glucose to glucose-6-phosphate (G6P).
- Regulation:
- Product Inhibition: HK is inhibited by its product, glucose-6-phosphate (G6P). This prevents the accumulation of G6P and slows down glycolysis when downstream steps are inhibited.
- Isozymes: Different hexokinase isozymes exist (HK I-IV), each with varying affinities for glucose and different regulatory properties. For example, glucokinase (HK IV), primarily found in the liver and pancreatic β-cells, has a lower affinity for glucose and is not inhibited by G6P. This allows the liver to continue phosphorylating glucose even when G6P levels are high.
2. Phosphofructokinase-1 (PFK-1)
- Function: PFK-1 catalyzes the committed step of glycolysis: the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate.
- Regulation: PFK-1 is the most important control point in glycolysis.
- Allosteric Activation:
- AMP and ADP: Indicate low energy charge; therefore, these molecules activate PFK-1, stimulating glycolysis.
- Fructose-2,6-bisphosphate (F2,6BP): A potent activator of PFK-1, especially in the liver. F2,6BP levels are regulated by the bifunctional enzyme phosphofructokinase-2/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) and are influenced by hormonal signals (insulin and glucagon).
- Allosteric Inhibition:
- ATP: High ATP levels indicate high energy charge, inhibiting PFK-1.
- Citrate: High citrate levels, an intermediate in the citric acid cycle, also signal high energy levels and inhibit PFK-1.
- Allosteric Activation:
3. Pyruvate Kinase (PK)
- Function: Pyruvate kinase catalyzes the final step of glycolysis: the transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, forming pyruvate and ATP.
- Regulation:
- Allosteric Activation:
- Fructose-1,6-bisphosphate (F1,6BP): The product of the PFK-1 reaction acts as a feed-forward activator of pyruvate kinase.
- Allosteric Inhibition:
- ATP: High ATP levels inhibit pyruvate kinase.
- Alanine: High alanine levels (produced from pyruvate) inhibit pyruvate kinase, indicating that the cell has sufficient building blocks.
- Covalent Modification: In the liver, pyruvate kinase is regulated by phosphorylation. Glucagon stimulates protein kinase A (PKA) which phosphorylates and inactivates pyruvate kinase, reducing glycolysis when blood glucose levels are low. Insulin reverses this effect.
- Allosteric Activation:
Hormonal Control
Hormones such as insulin and glucagon also play a crucial role in regulating glycolysis, particularly in the liver, by influencing the expression and activity of glycolytic enzymes.
Summary
The rate of glycolysis is tightly controlled by three key enzymes (hexokinase, phosphofructokinase-1, and pyruvate kinase) through allosteric regulation and, in some tissues, hormonal control. This ensures that glucose is metabolized according to the cell's energy needs and the body's overall metabolic state.
