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The Phosphogluconate Pathway, also called The Pentose Phosphate Pathway has three distinct phases namely Oxidation Phase,Isomerization Phase and Rearrangement Phase.
Oxidation Phase
The beginning molecule for the PPP is glucose-6-P which is the second intermediate metabolite in glycolysis. Glucose-6-P is oxidized in the presence of glucose-6-P dehydrogenase and NADP+. This step is irreversible and is highly regulated. NADPH and fatty acyl-CoA are strong negative inhibitors to this enzyme. The purpose of this is to decrease production of NADPH when concentrations are high or the synthesis of fatty acids is no longer necessary.
The metabolic product of this step is gluconolactone which is hydrolytically unstable. Gluconolactonase causes gluconolactone to undergo a ring opening hydrolysis. The product of this reaction is the more stable sugar acid, 6-phospho-D-gluconate.
6-phospho-D-gluconate is oxidized by NADP+ in the presence of 6-phosphogluconate dehydrogenase which yields ribulose-5-phosphate.
The oxidation phase of the PPP is solely responsible for the production of the NADPH to be used in anabolic processes.
Isomerization Phase
Ribulose-5-phosphate can then be isomerized by phosphopentose isomerase to produce ribose-5-phosphate. Ribose-5-phosphate is one of the main building blocks of nucleic acids and the PPP is the primary source of production of ribose-5-phosphate.
If production of ribose-5-phosphate exceeds the needs of required ribose-5-phosphate in the organism, then phosphopentose epimerase catalyzes a chirality rearrangement about the center carbon creating xylulose-5-phosphate.
The products of these two reactions can then be rearranged to produce many different length carbon chains. These different length carbon chains have a variety of metabolic fates.
Rearrangement Phase
There are two main classes of enzymes responsible for the rearrangement and synthesis of the different length carbon chain molecules. These are transketolase and transaldolase.
Transketolase is responsible for the cleaving of a two carbon unit from xylulose-5-P and adding that two carbon unit to ribose-5-P thus resulting in glyceraldehyde-3-P and sedoheptulose-7-P.
Transketolase is also responsible for the cleaving of a two carbon unit from xylulose-5-P and adding that two carbon unit to erythrose-4-P resulting in glyceraldehyde-3-P and fructose-6-P.
Transaldolase is responsible for cleaving the three carbon unit from sedoheptulose-7-P and adding that three carbon unit to glyceraldehyde-3-P thus resulting in erythrose-4-P and fructose-6-P.
The end results of the rearrangement phase is a variety of different length sugars which can be fed into many other metabolic processes. For example, fructose-6-P is a key intermediate of glycolysis as well as glyceraldehyde-3-P.
otienosam answered the question on April 6, 2020 at 14:16
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