In terms of glucose explain why biosynthetic anabolic pathwa

In terms of glucose, explain why biosynthetic anabolic pathways are not just the reversal of the corresponding catabolic pathways.

Solution

Catabolic reactions usually release energy that is used to drive chemical reactions. Anabolism refers to chemical reactions in which simpler substances are combined to form more complex molecules.Catabolic reactions are used to capture and save energy from nutrients, as well as to degrade larger molecules into smaller,molecular raw materials for reuse by the cell.The energy is stored in the form of energy-rich ATP,which powers the reactions of anabolism.

Anabolic pathways are usually not the simple reversal of the catabolic pathway.Enzymes speed up reactions but do not alter the thermodynamic equilibrium.If a common set of enzymes were used for an anabolic and catabolic pathway,inhibition of one of the enzymes would inhibit both anabolism and catabolism,and independent regulation of the pathways would not be possible.Independent regulation of complementary anabolic and catabolic pathways is achieved by having completely separate enzyme pathways,or by having separate enzymes at a key point in a shared pathway.

The opposing pathways of anabolism and catabolism may also use different reaction intermediates or different enzymatic reactions in some of the steps.For example,there are 11 enzymatic steps in the breakdown of glucose into pyruvic acid in the liver.But the liver uses only nine of those same steps in the synthesis of glucose, replacing the other two steps with a different set of enzyme-catalyzed reactions.This occurs because the pathway to degradation of glucose releases energy,while the anabolic process of glucose synthesis requires energy.The two different reactions of anabolism are required to overcome the energy barrier that would otherwise prevent the synthesis of glucose.

Gluconeogenesis which is a anabolic pathway consists of a series of eleven reactions forming the entire pathway (pyruvate to glucose).Seven reactions of gluconeogenic pathway are just reverse of the corresponding reactions of glycolysis.In each of these seven reactions the same enzyme catalyzes the reactions in both directions.However,four steps are unique to gluconeogenesis and the four enzymes catalyzing these steps belong to only gluconeogenesis.These are:Pyruvate carboxylase,Phosphoenolpyruvate carboxykinase (PEPCK),Fructose bis-phosphatase (FBPase),Glucose-6-phosphatase.