The Alternative Pathway of Glucose Catabolism – Explained!

The co-factors required are ATP, magnesium ions, NADP, manganese, and thiamine pyrophosphate, etc.

The important feature of this pathway is that it does not involve any ATP molecule for its operation, once glucose-6-phosphate has been formed.

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This means that the pathway may continue to function under relatively anaerobic conditions.

This pathway is found active in liver, adipose tissue, mammary gland, adrenal cortex and testis.

The glucose metabolized by this alternative pathway gives rise to im­portant intermediates, such as ribose which are required as building blocks for essential constituents in the cellular economy.

In the first reaction, gIucose-6-phosphate is formed and this, in turn, is oxidized in the presence of NADP forming gluconate-6-phos- phate.

The gluconate-6-phosphate undergoes an oxidative decarboxy­lation in the presence of NADP, yielding ribulose-5-phosphate.

The ribulose-5-phosphate may undergo one of two isomerizations to form either ribose-5-phosphate or to xylulose-5-phosphate in the presence of phosphoriboisomerase and phosphoketopentose epimerase, respec­tively.

A molecule of ribose-5-phosphate and a xylulose-5-phosphate react with each other, yielding a molecule of sedoheptulose-7-phosph- ate (a seven carbon sugar) and a molecule of triosephosphate.

This reaction is catalyzed by the enzyme transketolase which requires thiamine pyrophosphate as a co-factor.

These two sugars then parti­cipate in transaldolation reaction to give a fructose-6-phosphate and a molecule of erythrose-4-phosphate (a four carbon sugar).

The erythrose-4-phosphate and another molecule of xylulose-5-phosphate also undergo a transketolation reaction, forming fructose-6-phosphate and a triosephosphate.

The two molecules Of fructose-6-phosphate are isomerized and form glucose-6-phosphate which re-enters the metabolic pathways for glucose, while the triosephosphate formed is degraded to pyruvate and enters the tricarboxylic acid cycle.

Energetics of carbohydrate metabolism : It should be remembered that in anaerobic glycolysis, each glucose unit of glycogen forms two molecules of triose and subsequent products.

Thus the dissimilation of one glucose unit of glycogen to two molecules of lactic acid gene rates energy sufficient to be bound as four newly-formed ATP mole­cules.

But one high energy phosphate bond from ATP is used in phosphorylation reaction (fructose-6-phosphateafructose-l-6-di- phosphate).

Therefore, the net release of energy is equivalent to 3ATP—approx. 36 K call mole of glucose.

If the process starts from free blood glucose, the net release is only 2ATP molecules, since one high energy phosphate bond from ATP is used in the hexokinase reaction.

When pyruvate is transformed into acetyl CoA and CO2, the oxidation of the NADH provides 6 moles of ATP per glucose equivalent.

Since complete oxidation of each mole of acetyl CoA yields 12 moles of ATP, 24 miles of ATP are generated per glucose equivalent.

The complete aerobic dissimilation of two molecules of pyruvic acid (i.e., one glucose unit) thus results in the generation of 38ATP molecules which are equivalent to 266 kilo calories of utilizable energy. The following table is showing the details of production of ATP mole­cules.

ATP molecules are important to ensure a proper functioning Of the body, the body must maintain a satisfactory supply of ATP or means of ATP, because it is responsible for the chemical reactions.