Gluconeogenesis

Hello :) In this post we'll be discussing gluconeogenesis. I'll describe the function of gluconeogenesis and the locations of this pathway. I'll also list the irreversible reactions of glycolysis and the obligatory quartet of enzymes that regulate gluconeogenesis. I'll also talk about the regulation of glycolysis and gluconeogenesis.

Gluconeogenesis

Gluconeogenesis is the conversion of non-carbohydrates to glucose or glycogen. It is located only in the liver and the kidney (but to a lesser extent). It occurs in the cytosol, mitochondria and smooth endoplasmic reticulum of these cells. The function of gluconeogenesis is to maintain glucose levels when dietary carbohydrates are in low supply. Gluconeogenesis also maintains the TCA cycle intermediates and clears metabolites from the blood. It is essential for the long term maintenance of glucose homeostasis and commences after twelve hours of no or little glucose being present in the diet. Gluconeogenesis provides a major source of blood glucose in all starving animals.

Irreversible Reactions

You may think that to convert non-carbohydrates to glucose you could simply just reverse glycolysis. However, this cannot be done because some reactions in glycolysis are irreversible, particularly:

• The conversion of glucose to glucose-6-phosphate which is catalysed by hexokinase and involves the conversion of ATP to ADP (the first reaction). 
• The conversion of Fructose-6-Bisphosphate to Fructose-1,6-Bisphosphate which is catalysed by phosphofructokinase and involves the conversion of one ATP to ADP (the third reaction).
• The conversion of phosphoenolpyruvate to pyruvate which is catalysed by pyruvate kinase and involves the conversion of one ADP to ATP (the ninth step).

However, there are some enzymes which can be used to overcome these "irreversible" reactions.

The Obligatory Quartet of Gluconeogenesis

Four new enzymes are needed to overcome the three irreversible reactions of glycolysis. These are:

1. Pyruvate Carboxylase: an allosteric enzyme which is stimulated by AcetylCoA. Starvation and fat mobilisation stimulates the production of AcetylCoA and thus stimulates this enzyme.
2. Phosphoenolpyruvate CarboxyKinase: this is induced by glucocorticoids
3. Fructose-1,6-bisphosphatase: gluconeogenesis is mainly controlled from this enzyme. It is allosterically regulated by Fructose-2,6-bisphosphate.
4. Glucose-6-phosphatase: this is essential for the release of glucose from the liver cell into the hepatic vein.

Without these enzymes gluconeogenesis cannot occur. The liver and kidney are the only organs which express all four enzymes, thus they are the only places where gluconeogensis can occur. 

The Regulation of Gluconeogenesis

It is important that gluconeogenesis and glycolysis do not occur at the same time, thus reciprocal and co-ordinated regulation of gluconeogenesis and glycolysis is needed. A bifunctional protein, phosphofructokinase-2 (PFK2), as well as the cAMP dependent pathway are important in regulation. 

cAMP-dependent pathway:

When glucagon or adrenaline bind to their receptors on the surface of a cell they stimulate adenyl cyclase to produce cAMP. This causes cAMP Dependent Protein Kinase to be stimulated which phosphorylates several different metabolic enzymes. However, if insulin binds to a cell instead of glucagon or adrenaline, cAMP will not stimulate cAMP Dependent Protein Kinase to phosphorylate other enzymes. 

When glucagon causes cAMP Dependent Protein Kinase to be activated it phosphorylates pyruvate kinase which inhibits the final step of glycolysis. Thus, glycolysis is inhibited which prevents the glycolysis and gluconeogenesis from occuring at the same time.

Phosphofructokinase 2-The Bifunctional Enzyme

The concentration of fructose-2,6-bisphosphatase (F26BP) is critical to the direction of glycolysis or gluconeogenesis. The amount of F26BP is closely regulated through the activity of phosphofructokinase-2. This enzyme has a phosphofructokinase subunit, which adds phosphates to molecules, and a fructose-2,6-bisphosphatase (F26 bis Pase) subunit, which removes the phosphate group from fructose-2,6-bisphosphate. This type of enzyme is known as a bifunctional enzyme because it has two subunits which perform different functions. Which function is performed depends on whether or not the enzyme is phosphorylated. If it is not phosphorylated, it uses its phosphofructokinase activity to convert fructose-6-phosphate to F26BP. This stimulates glycolysis and inhibits gluconeogenesis. When the enzyme is phosphorylated, it uses its F26 bis Pase activity to destroy F26BP so that gluconeogenesis is no longer inhibited at fructose-1,6-bisphosphatase which is the major regulatory enzyme of the gluconeogenic obligatory quartet.

The major influence on the phosphorylation state of phosphofructokinase 2 is the presence of the hormone glucagon. As mentioned before, when glucagon binds to the surface of a cell it stimulates the production of cAMP inside the cell. This increases the activity of cAMP dependent protein kinase which phosphorylates the bifunctional enzyme so that the phosphofructokinase activity is inhibited and the F26 bis Pase activity is stimulated. The net result of the presence of glucagon is a drop in the concentration of fructose-2,6-bisphosphate and therefore an increase in the activity of fructose-1,6-phosphatase so gluconeogenesis is stimulated. Thus it is evident why glucagon is released when an animal is starving, it is because it stimulates gluconeogenesis. 

In a well fed animal, an intermediate of glycolysis, fructose-6-phosphate, is present at high concentrations. Fructose-6-phosphate stimulates the phosphofructokinase activity of phosphofructokinase 2 and inhibits the fructose-2,6-bisphosphatase activity. This results in an increase in the amount of fructose-2,6-bisphosphate present which increases the activity of phosphofructokinase. Thus, glycolysis is stimulated.