In this article we will discuss about the Biosynthetic Pathway of Gluconeogenesis.
Biosynthetic Pathway from Pyruvate to Glucose:
Starting from glucose and ending with pyruvate, there are ten reaction steps in glycolysis. In gluconeogenesis direction, most of the reaction steps in the reverse sequence from pyruvate to glucose are catalysed by the enzymes of the glycolytic sequence, and thus proceed by the reversal of the steps employed in glycolysis.
There are however, three irreversible steps in the glycolytic pathway which cannot be utilized in the direction of gluconeogenesis, i.e., conversion of pyruvate to glucose.
As it happens, the reactions of glycolysis and gluconeogenesis differ in these steps catalysed by hexokinase, phosphofructokinase and pyruvate kinase. In fact, gluconeogenesis requires at these points along the reverse pathway three different enzymes, which are thermodynamically favourable in the direction of synthesis.
In germinating oil seeds, the glycolytic pathway can operate in the reverse direction with the formation of hexose sugar from PEP.
This metabolic process of converting lipid into carbohydrate is an essential feature in oil seed plants, which store a major part of the energy in the form of fat rather than as starch. It starts with the production of acetyl CoA through the fatty acid oxidation reactions. This acetyl CoA then enters a modified Krebs cycle (i.e., glyoxylate cycle) in mitochondria to yield oxaloacetate.
So, the first step in gluconeogenesis is the conversion of oxaloacetate into phosphoenolpyruvate by PEP carboxykinase according to the reaction:
Since the direct reversal of the pyruvate kinase reaction towards the conversion of pyruvate to PEP cannot occur in view of its irreversible nature, such an alternative reaction is employed. Indeed, PEP carboxykinase has a very low affinity for CO2, hence the enzyme is biologically active only in the direction of PEP formation.
Phosphoenolpyruvate generated from pyruvate by the above alternative reaction is converted to fructose-1,6-bisphosphate by following six enzymatic reactions of glycolysis in the reverse order, starting with enlace and ending with FBP aldolase, all of which function reversibly in both glycolysis and gluconeogenesis.
The subsequent eighth step in gluconeogenesis cannot operate with the glycolytic enzyme phosphofructokinase because it catalyzes an irreversible reaction:
This enzyme does not function in the reverse direction because of an un-favourable ΔG0. In gluconeogenesis, this step is catalyzed by another enzyme fructose-1,6-bisphosphatase.
This is an allosteric enzyme, which is inhibited by negative modulator amp and stimulated by positive modulators 3-PGA and citrate. In the next step, fructose-6-phosphate can be easily converted to glucose-6-phosphate by the enzyme glucose phosphate isomerase, which is essentially reversible and functions in both glycolysis and gluconeogenesis.
The last step involving dephosphorylation of glucose-6-phosphate to regenerate free glucose cannot operate by the reversal of hexokinase enzyme, which is irreversible. Instead this is brought about by the hydrolytic enzyme, glucose-6-phosphatase which is exergonic and irreversible.
Indeed, the question as to whether glucose will undergo degradation in glycolysis or it will be synthesized in gluconeogenesis obviously depends on the need of the plant at any particular stage of its life cycle. Since both the processes are confined to cytosol, a control mechanism must be employed so that the flow of carbon operates either in the downhill or uphill direction.