Phosphofructokinase

Phosphofructokinase (PFK) is an important enzyme in the glycolysis pathway. It catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate using adenosine triphosphate (ATP) as the phosphate donor. This reaction is a key regulatory step in glycolysis and is highly regulated by various metabolic signals.

Structure

Phosphofructokinase is a tetramer composed of different subunits, which can vary depending on the tissue type. In humans, there are three main isoforms of PFK: PFK-M (muscle type), PFK-L (liver type), and PFK-P (platelet type). These isoforms are encoded by different genes and can form homo- or heterotetramers, leading to a variety of enzyme forms with different kinetic properties.

Function

The primary function of phosphofructokinase is to regulate the rate of glycolysis. By converting fructose-6-phosphate to fructose-1,6-bisphosphate, PFK controls the flow of glucose into the glycolytic pathway. This step is considered the first committed step of glycolysis, making PFK a crucial control point in cellular metabolism.

Regulation

Phosphofructokinase is allosterically regulated by several molecules:

• **ATP**: High levels of ATP inhibit PFK, indicating that the cell has sufficient energy.
• **AMP**: High levels of AMP activate PFK, signaling that the cell needs more ATP.
• **Citrate**: High levels of citrate inhibit PFK, linking glycolysis to the citric acid cycle.
• **Fructose-2,6-bisphosphate**: This molecule is a potent activator of PFK, enhancing its affinity for fructose-6-phosphate and reducing the inhibitory effect of ATP.

Clinical Significance

Mutations in the genes encoding PFK can lead to various metabolic disorders. For example, a deficiency in PFK-M can cause Glycogen storage disease type VII, also known as Tarui's disease, which is characterized by muscle weakness and exercise intolerance.

See Also

• Glycolysis
• Hexokinase
• Pyruvate kinase
• Citric acid cycle
• Adenosine triphosphate