Question

In the first by-pass step of gluconeogenesis, the conversion of pyruvate to phosphoenolpyruvate, pyruvate is carboxylated by pyruvate carboxylase to oxaloacetate, which is subsequently decarboxylated by PEP carboxykinase to yield phosphoenolpyruvate. Explain using thermodynamic terms why this by-pass step is necessary. Why couldn't the glycolytic enzyme responsible for interconverting phosphoenolpyruvate and pyruvate also participate in the gluconeogenesis pathway?

Answer 1

Gluconeogenesis is a metabolic pathway that leads to the synthesis of glucose from pyruvate and other non-carbohydrate precursors, even in non-photosynthetic organisms.

Bypass 1, Pyruvate to Phosphoenolpyruvate

Conversion of pyruvate to phosphoenolpyruvate (PEP) requires the action of 2 enzymes. The first is an ATP-requiring reaction catalyzed by pyruvate carboxylase (PC), which catalyzes the carboxylation of pyruvate to the TCA cycle intermediate, oxaloacetic acid (OAA). The second enzyme of bypass 1 is the GTP-dependent PEP carboxykinase (PEPCK), which converts OAA to PEP. Since PC incorporated CO2 into pyruvate and it is subsequently released in the PEPCK reaction, no net fixation of carbon occurs.

Thermodynamics of Conversion of pyruvate to PEP

This is much tricker. PEP has a very large free energy of hydrolysis ( = -61.9 kJ/mol), so simple phosphate transfer from ATP is ruled out. Essentially, synthesis of the PEP phosphate is worth two ATP high energy phosphates, and one way or another 2 ATP are consumed per PEP made. pyruvate dikinase catalyzes this reaction.

pyruvate + ATP + Pi PEP + AMP + PPi = +28.7 kJ/mol

This is supported by the action of pyrophosphatase ,

PPi + H₂O 2 Pi = - 33.4 kJ/mol.

both reactions sum gives = - 4.7 kJ/mol.