HMP Shunt

HMP Shunt

The pentose phosphate pathway is an alternative route for the metabolism of glucose. The two major functions of this pathway are:

• The formation of NADPH for synthesis of fatty acids and steroids, and maintaining reduced glutathione for antioxidant activity.
• The synthesis of ribose for nucleotide and nucleic acid formation.

Three molecules of glucose 6-phosphate give rise to three molecules of CO2 and three 5-carbon sugars

Two Phases

• Oxidative – Non Reversible
• Non Oxidative -Reversible
• No ATP is produced or consumed in HMP Shunt
• Rate Limiting Step glucose-6-phosphate is oxidized to 6-phosphogluconolactone by glucose-6-phosphatase.

Difference Between Glycolysis and HMP Shunt

• Oxidation utilizes NADP rather than NAD,
• Co2 is not produced in Glycolysis
• No ATP is generated in HMP Shunt
• Both are Cystolic Cycles
• Mg2+ ions are required for both glycolysis (for the phosphorylation of glucose by glucokinase) and the pentose phosphate pathway (for the functioning of transketolase and several other enzymes).

Erythrocytes are the specialized oxygen transporting cells of the body.

• As such, they are continuously exposed to an oxidizing environment.
• These conditions can have profound effects on membrane lipids as a consequence of their attack by peroxides (particularly hydrogen peroxide) prevalent in this environment.
• During the reduction of hydrogen peroxide catalyzed by glutathione peroxidase, glutathione acts as a donor of the necessary reducing equivalent generating oxidized glutathione.
  
• Glutathione (GSH) is a tripeptide of the structure gamma glutamyl cysteinyl-glycine, where the cysteine sulfhydryl side chains can form a disulfide bond between two molecules (designated as GSSG). To again perform the role of cofactor for glutathione peroxidase, the disulfide bond of GSSG must be reduced.
• This reaction is catalyzed by glutathione reductase which requires NADPH as a cofactor.
• Therefore, in order to maintain a red cell membrane structure, erythrocytes utilize the pentose phosphate pathway for the generation of large amounts of NADPH.