The three irreversible steps of glycolysis are catalyzed by the enzymes hexokinase (or glucokinase), phosphofructokinase-1 (PFK-1), and pyruvate kinase. These steps are crucial regulatory points in the glycolytic pathway and help prevent the backward flow of the glycolytic process during gluconeogenesis, the metabolic pathway that generates glucose from non-carbohydrate substrates.
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Hexokinase/Glucokinase: This enzyme catalyzes the phosphorylation of glucose to glucose-6-phosphate (G6P) using ATP. This step prevents free glucose from being produced during gluconeogenesis since G6P is not converted back to glucose under normal physiological conditions.
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Phosphofructokinase-1 (PFK-1): PFK-1 catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate and is a major regulatory step in glycolysis. This reaction is highly favorable and is effectively irreversible under cellular conditions. This regulation prevents the transformation of fructose-1,6-bisphosphate back to fructose-6-phosphate during gluconeogenesis.
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Pyruvate Kinase: This enzyme catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, also using ATP. This reaction is also considered irreversible, and the production of pyruvate during glycolysis prevents its conversion back to PEP in gluconeogenesis without employing a different set of enzymes.
In gluconeogenesis, to circumvent these irreversible steps of glycolysis, specific enzymes such as glucose-6-phosphatase, fructose-1,6-bisphosphatase, and pyruvate carboxylase (along with phosphoenolpyruvate carboxykinase) are used. By having these alternate pathways and regulatory mechanisms, the cell ensures that there is a proper balance between glycolysis and gluconeogenesis, thus preventing the futile cycle of simultaneous glucose synthesis and breakdown. This regulation is critical for maintaining glucose homeostasis in the body.