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Barriers of Gluconeogenesis
Substrates of Gluconeogenesis
The major substrates are-
a) The glucogenic amino acids,
c) Glycerol, and
These noncarbohydrate precursors of glucose are first converted into pyruvate or enter the pathway at later intermediates such as oxaloacetate and Dihydroxyacetone phosphate.
Liver and kidney are the major gluconeogenic tissues.
Gluconeogenesis meets the needs of the body for glucose when sufficient carbohydrate is not available from the diet or glycogen reserves. A supply of glucose is necessary, especially for the nervous system and erythrocytes. Failure of gluconeogenesis is usually fatal.
Reactions of Gluconeogenesis
In glycolysis, glucose is converted into pyruvate; in gluconeogenesis, pyruvate is converted into glucose. However, gluconeogenesis is not a reversal of glycolysis. Three nonequilibrium reactions in glycolysis catalyzed by hexokinase, phosphofructokinase and pyruvate kinase are considered thermodynamic barriers which prevent simple reversal of glycolysis for glucose synthesis (Figure-1)
Figure-1- Barriers of gluconeogenesis. Three irreversible reactions of glycolysis are substituted by alternative reactions. Pyruvate carboxylase, Phospho enol pyruvate carboxy kinase, Fructose 1,6 bisphosphatase and glucose-6-Phosphatase enzymes are unique to the pathway of gluconeogenesis. Lactate enters as pyruvate, glycerol as Dihydroxy acetone-phosphate and propionate as Succinyl co A. Acetyl Co A is not glucogenic but it is a positive modulator of pyruvate carboxylase enzyme.
Details of reactions
In gluconeogenesis, the following new steps bypass these virtually irreversible reactions of glycolysis:
1. First bypass (Formation of Phosphoenolpyruvate from pyruvate)
Reversal of the reaction catalyzed by pyruvate kinase in glycolysis involves two endothermic reactions. Phosphoenolpyruvate is formed from pyruvate by way of oxaloacetate through the action of pyruvate carboxylase and phosphoenolpyruvate carboxy kinase.
Mitochondrial pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate, an ATP-requiring reaction in which the vitamin biotin is the coenzyme. Biotin binds CO2 from bicarbonate as carboxybiotin prior to the addition of the CO2 to pyruvate.
Pyruvate carboxylase is a mitochondrial enzyme, whereas the other enzymes of gluconeogenesis are cytoplasmic. Oxaloacetate, the product of the pyruvate carboxylase reaction, is reduced to malate inside the mitochondrion for transport to the cytosol (Figure-2). The reduction is accomplished by an NADH-linked malate dehydrogenase. When malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD+-linked malate dehydrogenase in the cytosol.
Figure-2- showing the transportation of oxaloacetate outside the mitochondrion in the form of Malate
A second enzyme, phosphoenolpyruvate carboxykinase, catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate using GTP as the phosphate donor. In liver and kidney, the reaction of succinate thiokinase in the citric acid cycle produces GTP (rather than ATP as in other tissues), and this GTP is used for the reaction of phosphoenolpyruvate carboxykinase, thus providing a link between citric acid cycle activity and gluconeogenesis, to prevent excessive removal of oxaloacetate for gluconeogenesis, which would impair citric acid cycle activity.
2. Second bypass (Formation of Fructose 6-phosphate is formed from fructose 1,6-bisphosphate)
On formation, phosphoenolpyruvate is metabolized by the enzymes of glycolysis but in the reverse direction. These reactions are near equilibrium under intracellular conditions; so, when conditions favor gluconeogenesis, the reverse reactions will take place until the next irreversible step is reached. This step is the hydrolysis of fructose 1,6- bisphosphate to fructose 6-phosphate and Pi.
Fructose 1,6-bisphosphatase catalyzes this exergonic hydrolysis.
Its presence determines whether a tissue is capable of synthesizing glucose (or glycogen) not only from pyruvate, but also from triose phosphates. It is present in liver, kidney, and skeletal muscle, but is probably absent from heart and smooth muscle. Like its glycolytic counterpart, it is an allosteric enzyme that participates in the regulation of gluconeogenesis.
3. Third bypass (Formation of Glucose by hydrolysis of glucose 6-phosphate)
The fructose 6-phosphate generated by fructose 1,6-bisphosphatase is readily converted into glucose 6-phosphate (Figure-3). In most tissues, gluconeogenesis ends here. Free glucose is not generated; rather, the glucose 6-phosphate is processed in some other fashion, notably to form glycogen. One advantage to ending gluconeogenesis at glucose 6-phosphate is that, unlike free glucose, the molecule cannot diffuse out of the cell. To keep glucose inside the cell, the generation of free glucose is controlled in two ways. First, the enzyme responsible for the conversion of glucose 6-phosphate into glucose, glucose 6-phosphatase, is regulated.
Second, the enzyme is present only in tissues whose metabolic duty is to maintain blood-glucose homeostasis, tissues that release glucose into the blood. These tissues are the liver and to a lesser extent the kidney the enzyme is absent in muscle and adipose tissue, which therefore, cannot export glucose into the bloodstream.
Figure- The glucose 6-phosphatase enzyme is stabilized by the Ca2+-binding protein SP.After the cleavage, a set of transporters, T2 and T3, return the products orthophosphate and glucose back into the cytosol.
This final step in the generation of glucose does not take place in the cytosol. Rather, glucose 6-phosphate is transported into the lumen of the endoplasmic reticulum, where it is hydrolyzed to glucose by glucose 6-phosphatase, which is bound to the membrane (Figure-3) .An associated Ca2+-binding stabilizing protein is essential for phosphatase activity. Glucose and Pi are then shuttled back to the cytosol by a pair of transporters. The glucose transporter in the endoplasmic reticulum membrane is like those found in the plasma membrane. It is striking that five proteins are needed to transform cytosolic glucose 6-phosphate into glucose.
In the kidney, muscle and especially the liver, G6P be shunted toward glycogen if blood glucose levels are adequate.
To be continued in the next post
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