With help of diagram explain the pentose phosphate pathway or hexose monophosphate shunt,
The pentose phosphate pathway, is a unique pathway used to create products vital in the body for various reasons. The HMP shunt is an alternative pathway to glycolysis and is used to produce ribose-5-phosphate and nicotinamide adenine dinucleotide phosphate (NADPH). This pathway occurs in the oxidative and non-oxidative phases, each comprising a series of reactions.
The oxidative stage
The first reaction involves conversion of glucose to glucose-6 phosphate and is catalyzed by the enzyme hexokinase and a molecule of ATP is also utilized.The second reaction involves conversion of glucose-6 phosphate to 6-phosphogluconolactone.Its catalyzed by an enzyme glucose-6 phosphate dehydrogenase (G6PD) in the presence of Mg++ ion. A molecule of NADPH is produced as a result. The third reaction involves conversion of 6-phosphogluconolactone to 6-phosphogluconate and is catalyzed by hydrolase enzyme. The fourth reaction involves conversion of 6-phosphogluconate to ribose-5 phosphate is catalyzed by the enzyme 6-phosphogluconate dehydrogenase to produce 3-keto-6-phosphogluconate which undergoes decarboxylation to produce ribulose-5 phosphate.A molecule of NADPH is generated as a result.
The non oxidative stage
A series of reversible sugar phosphate inter-conversion reaction follows the oxidative reaction where Ribulose-5-phosphate is epimerized to produce xylulose 5-phosphate in the presence of enzyme phosphor pentose epimerase. Ribulose-5-phosphate is also keto-isomerized into ribose 5-phosphate.
Xylulose-5-phosphate transfers two carbon moiety to ribose 5-phospahate in the presence of enzyme transketolase to form sedoheptulose-7-phosphate and glyceraldehyde 3—phosphate. Similarly,Sedoheptulose -7-phosphate transfers three carbon moiety to glyceraldehyde -3-phosphate to form fructose 6-phopsphate and erythrose 4-phosphate in the presence of enzyme transaldolase.
Transketolase enzyme catalyses the transfer of two carbon moiety from Xylulose-5-phsphate to erythrose-4- phosphate to form fructose-6-phosphate and glyceraldehyde-3-phosphate.
The resulting fructose-6-phosphate and glyceraldehyde-3-phosphate later enter into glycolysis and kreb’s cycle.
The rate and direction of reversible reaction depends upon the cell needs where if a cell needs only NADPH, then fructose-phosphate and glyceraldehyde-3-phosphate are converted back to glucose by reverse glycolysis, otherwise converted to pyruvate and enter TCA cycle generating ATPs.
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