Which reaction describes gluconeogenesis' energy cost in terms of GTP?

Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate substrates. This process is vital for maintaining blood glucose levels, especially during fasting or intense exercise.

The energy cost of gluconeogenesis can be quantified in terms of high-energy phosphate compounds that are utilized during the pathway. Specifically, for each molecule of glucose synthesized from pyruvate, the process consumes energy in the form of GTP and ATP.

In gluconeogenesis, there are two critical steps that require GTP. The conversion of pyruvate to phosphoenolpyruvate (PEP) involves pyruvate carboxylase and PEP carboxykinase. The first step, catalyzed by pyruvate carboxylase, requires one molecule of ATP to convert pyruvate to oxaloacetate. Subsequently, the decarboxylation of oxaloacetate to PEP requires one molecule of GTP. Thus, two molecules of GTP are utilized during gluconeogenesis.

This understanding of the energy cost related to GTP in gluconeogenesis is crucial for appreciating how the body manages its energy resources during the synthesis of glucose.