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RESPIRATION Pentose Phosphate Pathway Or Hexose Monophosphate Pathway RESPIRATION Pentose Phosphate Pathway or Hexose Monophosphate Pathway Pentose phosphate pathway or hexose monophosphate pathway (HMP pathway) is the other common pathway to break down glucose to pyruvate and operates in both aerobic and anaerobic conditions. This pathway produces NADPH, which carries chemical energy in the form of reducing power and is used almost universally as the reductant in anabolic (energy utilization) pathways (e.g., fatty acid biosynthesis, cholesterol biosynthesis, nucleotide biosynthesis) and detoxification pathways (e.g., reduction of oxidized glutathione, cytochrome P450 monooxygenases). Also, the pentose phosphate pathway generates pentose sugar ribose and its derivatives, which are necessary for the biosynthesis of nucleic acids (DNA and RNA) as well as ATP, NADH, FAD, and coenzyme A. In this way, though the pentose phosphate pathway may be a source of energy in many microorganisms, it is more often of greater importance in various biosynthetic pathways. Pentose phosphate pathway consists of two phases: the oxidative phase and the non-oxidative phase. Oxidative Phase: The oxidative phase of the pentose phosphate pathway initiates with the conversion of glucose 6- phosphate to 6-Phosphogluconate. NADP+ is the electron acceptor yielding NADPH during this reaction. 6-Phosphogluconate, a six-carbon sugar, is then oxidativelydecarboxylated to yield ribulose 5-phosphate, a five-carbon sugar. NADP+ is again the electron acceptor yielding NADPH. In the final step of oxidative phase, there is isomerisation of ribulose 5-phosphatc to ribose 5- phosphate by phosphopentose isomerase and the conversion of ribulose 5-phosphate into its epimerxylulose 5-phosphate by phosphopentose epimerase for the transketolase reaction in non- oxidative phase. Nonoxidative Phase: In the non-oxidative phase, enzyme transketolasecatalyzes the transfer of a two carbon fragment of xylulose 5-phosphate to ribose 5-phosphate forming the seven-carbon sedoheptulose 7-phosphate and three-carbon glyceraldehyde 3-phosphate. Enzyme transaldolase then catalyses the transfer of a three-carbon fragment from sedoheptulose 7- phosphate to glyceraldehyde 3-phosphate resulting in six-carbon fructose 6-phosphate and four carbon erythrose 4-phosphate. Now transketolase acts again, forming fructose 6-phosphate and glyceraldehyde 3-phosphate from erythrose 4-phosphate and xylulose 5-phosphate. Two molecules of glyceraldehyde 3-phosphate formed by two interations of these reactions can be converted into a molecule of fructose 1, 6- bisphosphate. The overall result of pentose phosphate pathway is that 3 glucose 6-phosphates are converted to two fructose 6-phosphates, glyceraldehyde 3-phosphate, and three CO2molecules, as shown in the following equation: + 3 glucose 6-phosphate + 6 NADP + 3H2O → 2 fructose 6-phosphate + glyceraldehyde 3-phosphate + + 3CO2 + 6 NADPH + 6H Fructose 6-phosphate and glyceraldehyde 3-phosphate intermediates are used in two ways. The fructose 6-phosphate can be converted back to glucose 6-phosphate, while glyceraldehyde 3- phosphate is converted to pyruvate by glycolysis-enzymes. The glyceraldehyde 3-phosphate also may be returned to pentose phosphate pathway through glucose 6-phosphate formation. This results in the complete degradation of glucose 6-phosphate to CO2 and the production of great deal of NADPH. .
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