Pentose Phosphate Pathway

Glucose in most animal tissues is catabolized into two molecules of pyruvate. The process takes place through the glycolytic pathway. Later, pyruvate is oxidized through the citric acid cycle to produce ATP. However, another metabolic fate is present for glucose. It generates NADPH as well as some specialized products essential for the cells.

What Is Pentose Phosphate Pathway?

Also called the hexose monophosphate shunt, it is a metabolic pathway that generates pentoses (5-carbon sugar) and NADPH. Interestingly, the pathway involves oxidation of glucose, but its main role in cells is anabolic, not catabolic. It makes use of NADP+ to regenerate NADPH through oxidation/reduction reaction. The reaction also includes formation of ribose 5-phosphate obtained mainly from glucose 6-phosphate.

NADPH is essential for fatty acid synthesis and plays an important role in reductive reactions in anabolism. As far as red blood cells are concerned, NADPH works to reduce the disulfide form of glutathione–it changes it into the sulfhydryl form. The reduced glutathione helps maintain the normal structure of red blood cells. It also helps keep hemoglobin in the ferrous state.

The process occurs in the cytosol in most organisms, but most of the process takes place in plastids in plants. The non-oxidative portion of pentose phosphate pathway creates carbon chain molecule, each with 3-7 carbons. These compounds serve as intermediates in gluconeogenesis and glycolysis or other biosynthetic processes.

What Is the Process of Pentose Phosphate Pathway?

It has two specific phases–the non-oxidative phase and oxidative phase. The oxidative phase takes place first and converts glucose 6-phosphate into ribulose-5-phosphate. Two moles of NADP+ are reduced to NADPH through the process. Here is how the overall process takes place.

Glucose 6-phosphate + 2 NADP++ H2O → ribulose-5-phosphate + 2 NADPH+ 2 H+ + CO2

The non-oxidative synthesis of 5-carbon sugars takes place after the oxidative phase. In this phase, ribulose-5-phosphate sometimes isomerizes to ribose-5-phosphate. This usually depends on the state of the body. Ribulose-5-phosphate can also undergo isomerizations as well as transketolations and transaldolations. The process produces pentose phosphates such as erythrose-4-phosphate, fructose-6-phosphate, and glyceraldehydes-3-phosphate. All these compounds are essential for a variety of biological processes, including synthesis of aromatic amino acids, and production of nucleic acids and nucleotides.

Glucose-6-phosphate dehydrogenase, which is stimulated by NADP+, works as the rate-control enzyme in pentose phosphate pathway. The process produces NADPH-utilizing pathways, which in turn generates NADP+. This goes on to stimulate glucose-6-phosphate dehydrogenase to continue with the production of NADPH. In mammals, the pathway occurs in the cytoplasm only and is most active in the mammary gland, liver and adrenal cortex. There is always a ration between NADPH and NADP+, which stays at 100:1 for NADPH: NADP+.

The Pentose Phosphate Pathway is among the ways your body works to create molecules with reducing power. The pathway produces up to 60% of NADPH required for healthy functioning of your body. It involves oxidation of glucose but it makes use of energy stored in NADPH to synthesize complex molecules. It is therefore considered anabolic, not catabolic.

What's more, cells in your body also make use of NADPH to deal with oxidative stress. Glutathione is reduced by NADPH through glutathione reductase. The process converts reactive H2O2 into H2O. It means that erythrocytes work through the pentose phosphate pathway in order to get the required amount of NADPH for glutathione reduction.

Reference:

http://www.cliffsnotes.com/study-guides/biology/biochemistry-i/carbohydrate-metabolism-ii/the-pentose-phosphate-pathway

https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/pentose.htm

 
 
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