When no glucose is consumed, the body has to rely on energy sources other than dietary glucoses. The brain itself uses approximately 120 grams of glucose a day! When both glycogen stores and dietary glucose run low, the liver (and to a lesser extent the kidneys) reverts to gluconeogenesis in order to maintain blood sugar levels. As mentioned in my notes of glycolysis, 7 of the glycolysis reactions are easily reversible under normal cellular conditions. In gluconeogenesis, these simply run in reverse. The three irreversible steps (ΔG <<0), hexokinase, phosphofructokinase (PFK-1), and pyruvate kinase are bypassed with bypass reactions. Looking at the big picture, gluconeogenesis converts non-carbohydrate precursor molecules such as lactate, pyruvate, TCA intermediates, and the carbon skeleton of most amino acids into glucose.
The bypass reactions of hexokinase and PFK-1 are conceptually straightforward. To overcome the thermodynamic barrier, the bypass reactions are paired with ATP hydrolysis, which is the driving force of these reactions. Note here that while ATP hydrolysis is always favorable, it’s extremely slow without an enzyme to catalyze the reaction. Importantly, ATP isn’t regenerated, and the phosphate is released as free phosphate. Accordingly, the G6P -> Glucose hexokinase bypass reaction uses the enzyme glucose-6-phosphatase and the F1,6BP -> F6BP PFK-1 bypass reaction uses the enzyme fructose-1,6-bisphosphatase. A phosphatase is an enzyme that removes a phosphate group from its substrate through hydrolysis.
The tricky part of gluconeogenesis is also the most energy intensive part. The conversion of pyruvate to phosphoenolpyruvate (PEP) is difficult since adding a free phosphate group to any molecule is thermodynamically unfavorable (ΔG >>0). The basic scheme is shown below.
First, pyruvate carboxylase “activates” pyruvate at the cost of one ATP, making oxaloacetate. Aside: In general, β-keto acids may be considered “high-energy” compounds because decarboxylation of the β-carboxyl group is strongly exergonic. This same process is also used in fatty-acid synthesis. Then, decarboxylation of oxaloacetate provides the driving force for the second reaction. The phosphate comes from GTP. GTP and ATP are equivalent in energy, since nucleotide diphosphate kinase freely converts ATP and GDP to ADP and GTP under typical cellular conditions. Overall, gluconeogenesis requires six high-energy phosphate bonds (4 ATP and 2 GTP) as well as 2 reduced electron carriers (2 NADH). It is important to note that while the majority of intermediates of cellular respiration can undergo gluconeogenesis, acetyl-CoA cannot. This is why free fatty acids cannot be converted to glucose. It is also important to note that the conversion of pyruvate to PEP involves an intermediate of malate. Pyruvate -> oxaloacetate -> malate occurs in the matrix of the mitochondria. In the matrix, NADH is reduced to NAD+ when oxaloacetate is converted to malate. The inner mitochondrial membrane has transporters for pyruvate and malate but not for oxaloacetate or NAD+/NADH. Malate is transported into the cytosol and converted into oxaloacetate -> PEP. The conversion of malate to oxaloacetate involves the oxidation of NAD+ to NADH in the cytosol. This convoluted pathway has the advantage of “transporting” one NADH out of the mitochondrion, where it’s plentiful, in exchange for one NAD+. NADH in the cytosol is necessary for gluconeogenesis.