Energetics and Substrates of the Gluconeogenesis Pathway

Gluconeogenesis is characterized as a metabolic pathway that is energetically very expensive; however, it remains essential for the formation of new glucose, particularly when cells are lacking in energy.
The conversion of pyruvate into a single molecule of glucose requires a substantial investment of high-energy phosphate groups. Specifically, the process utilizes:
- $4$ molecules of ATP (Adenosine Triphosphate).
- $2$ molecules of GTP (Guanosine Triphosphate).
In total, $6$ high-energy phosphate groups are consumed for every molecule of glucose synthesized from pyruvate.
The pathway also involves a reduction-oxidation step where $2$ molecules of NADH are converted to $2$ molecules of NAD plus. This conversion is specifically required for the stage of the metabolic pathway involving the conversion of $2$ molecules of $13\text{ phosphoglycerate}$ .

The free energy change ( $\Delta G$ ) for the gluconeogenesis pathway (converting pyruvate to glucose) is approximately $-16\,kJ\,mol^{-1}$ .
This contrasts with the process of glycolysis, where glucose is converted into pyruvate. In glycolysis:
- The process actually generates energy, specifically producing $2\text{ ATP}$ and $2\text{ NADH}$ .
- The free energy change ( $\Delta G$ ) for glycolysis is significantly more exergonic, at approximately $-63\,kJ\,mol^{-1}$ .
The distinct difference in free energy changes ( $-16\,kJ\,mol^{-1}$ versus $-63\,kJ\,mol^{-1}$ ) demonstrates that gluconeogenesis is not simply the biological reverse of glycolysis.
The high energy cost associated with gluconeogenesis is a physiological necessity to ensure the entire process remains irreversible.
The pathway utilizes three specific bypass steps to overcome the irreversible reactions of glycolysis, and these bypass steps are themselves irreversible within the cell.
Energy Investment Logic: The reason cells must invest such a high amount of energy to convert pyruvate or lactate to glucose is to prevent the loss of potential energy. If pyruvate were simply excreted, the cell would lose the "considerable potential" of that pyruvate to be converted into ATP through aerobic oxidation. Thus, gluconeogenesis allows the cell to recover a high-energy substrate that can later be used for ATP production when demand is high.

A critical metabolic limitation exists in mammalian cells regarding fatty acids: they cannot be converted into glucose.
The metabolic breakdown of fatty acids exclusively yields Acetyl coenzyme A.
In mammalian metabolic pathways, there is no existing mechanism to convert Acetyl coenzyme A back into pyruvate, which is the primary substrate needed to enter the gluconeogenesis pathway.
However, triacylglycerols (fats) can provide a minor contribution to gluconeogenesis via glycerol:
- When fats are broken down, a small amount of glycerol is produced.
- This glycerol can be utilized for gluconeogenesis, though the amount of new glucose created this way is described as "very tiny."

Unlike mammals, other organisms such as plants, yeast, and many bacteria possess a specialized pathway known as the glyoxylate cycle.
The glyoxylate cycle allows these organisms to convert Acetyl coenzyme A into oxaloacetate.
Consequently, these organisms can utilize fatty acids as the starting material for the synthesis of new glucose.
In plants, this pathway is particularly essential during the germination of seedlings. Before the development of leaves and the onset of photosynthesis, the seedling relies on energy and carbohydrates provided by this pathway.
The growing seedling converts energy from stored seed oils into both ATP and the carbohydrates needed for cell wall biosynthesis.

Glucose can be synthesized from various substrates, including lactate, pyruvate, or oxaloacetate.
Generally, any compound that serves as an intermediate in the Citric Acid Cycle can potentially be converted to glucose.
Structurally, gluconeogenesis shares seven steps with the glycolytic pathway; these seven steps are the exact reversal of glycolysis reactions.
The three irreversible steps of glycolysis must be bypassed using exergonic reactions. These are identified as the three bypasses:
- Bypass 1: The conversion of pyruvate to phosphoenolpyruvate (PEP). This is a multi-step process where pyruvate enters the mitochondria to be converted to oxaloacetate. In the cytosol, oxaloacetate is converted to phosphoenolpyruvate by the action of the enzyme PEP carboxykinase. This bypass involves the utilization of ATP and GTP.
- Bypass 2: The conversion of Fructose 1 6 bisphosphate to Fructose six phosphate. The enzyme Fructose bisphosphatase one catalyzes this reaction by removing a phosphate group.
- Bypass 3: The conversion of glucose six phosphate to synthesis the final product, new glucose. This reaction is catalyzed by the enzyme glucose six phosphatase, which removes the phosphate group.

In mammals, the primary sites for gluconeogenesis are the liver, kidneys, and the small intestine.
The glucose produced in these organs is vital for providing energy to specific consumers that rely heavily on glucose, including:
- The brain.
- Skeletal muscles.
- Erythrocytes (red blood cells).
This production is especially critical when cells are under stress and have an immediate requirement for energy.

The lecture concludes with a problem regarding the energy cost of the combined cycle of glycolysis and gluconeogenesis:
Problem Statement: What is the total cost in ATP equivalents of transforming glucose to pyruvate via glycolysis (considering how many ATP are consumed and generated) and then transforming that pyruvate back into glucose via gluconeogenesis? This calculation requires evaluating the net ATP/GTP consumption and generation across both processes.