Pentose phosphate pathway 1458783 223765784 2008-07-05T18:12:08Z Thijs!bot 1392310 robot Adding: [[cs:Pentosofosfátový cyklus]] The '''pentose phosphate pathway''' (also called Phosphogluconate Pathway, or Hexose Monophosphate Shunt [HMP shunt]) is a cytosolic process that serves to generate [[NADPH]] and the synthesis of pentose (5-[[carbon]]) [[sugar]]s. There are two distinct phases in the pathway. The first is the [[oxidation|oxidative]] phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5-carbon sugars. This pathway is an alternative to glycolysis. While it does involve oxidation of [[glucose]], its primary role is anabolic rather than catabolic. ==Uses== The primary functions of the pathway are: *To generate reducing equivalents, in the form of NADPH, for reductive biosynthesis reactions within cells. *To provide the cell with ribose-5-phosphate (R5P) for the synthesis of the nucleotides and nucleic acids. *Although not a significant function of the PPP, it can operate to metabolize dietary pentose sugars derived from the digestion of nucleic acids as well as to rearrange the carbon skeletons of dietary carbohydrates into glycolytic/gluconeogenic intermediates. Located exclusively in the cytoplasm, the pathway is one of the three main ways the body creates molecules with [[reduction (chemistry)|reducing]] power, accounting for approximately 60% of NADPH production in humans. One of the uses of NADPH in the cell is to prevent [[oxidative stress]]. It reduces the coenzyme [[glutathione]], which converts reactive H<sub>2</sub>O<sub>2</sub> into H<sub>2</sub>O. If absent, the H<sub>2</sub>O<sub>2</sub> would be converted to hydroxyl free radicals, which can attack the cell. Significantly, erythrocytes utilize the reactions of the PPP to generate large amounts of NADPH used in the reduction of glutathione It is also used to generate [[hydrogen peroxide]] for [[phagocytes]].<ref>{{GeorgiaImmunology|1/cytotox}}</ref> ==Phases== ===Oxidative phase=== In this phase, two molecules of [[NADP]]<sup>+</sup> are reduced to [[NADPH]], utilizing the energy from the conversion of [[glucose-6-phosphate]] into [[ribulose 5-phosphate]]. [[Image:PentosePhosphatePathway-OxidativeNADPH.png|center|800px|Oxidative phase of pentose phosphate pathway]] The entire set of reactions can be summarized as follows: {| class="wikitable" | '''Reactants''' || '''Products''' || '''Enzyme''' || '''Description''' |- | [[Glucose 6-phosphate]] + NADP+ || &rarr; [[6-phosphoglucono-&delta;-lactone]] + '''NADPH''' || [[glucose 6-phosphate dehydrogenase]] || [[Dehydrogenation]]. The hemiacetal hydroxyl group located on carbon 1 of glucose 6-phosphate is converted into a carbonyl group, generating a lactone, and, in the process, [[NADPH]] is generated. |- | [[6-phosphoglucono-&delta;-lactone]] + H<sub>2</sub>O || &rarr; [[6-phosphogluconate]] + H<sup>+</sup> || [[6-phosphoglucolactonase]] || [[Hydrolysis]] |- | [[6-phosphogluconate]] + NADP<sup>+</sup> || &rarr; [[ribulose 5-phosphate]] + '''NADPH''' + CO<sub>2</sub> || [[6-phosphogluconate dehydrogenase]] || Oxidative [[decarboxylation]]. NADP<sup>+</sup> is the electron acceptor, generating another molecule of [[NADPH]], a CO<sub>2</sub>, and [[ribulose 5-phosphate]]. |- | [[ribulose 5-phosphate]] || [[ribose 5-phosphate]] || [[Phosphopentose isomerase]] || [[Isomerization]]. (Can also be considered part of nonoxidative phase) |} The overall reaction for this process is: :Glucose 6-phosphate + 2 NADP<sup>+</sup> + H<sub>2</sub>O &rarr; ribulose 5-phosphate + 2 NADPH + 2 H<sup>+</sup> + CO<sub>2</sub><br> ===Non-oxidative phase=== [[Image:Ppp.svg|thumb|center|300px|The pentose phosphate pathway's Nonoxidative phase]] {| class="wikitable" | '''Reactants''' || '''Products''' || '''Enzymes''' |- | [[ribulose 5-phosphate]] || &rarr; [[ribose 5-phosphate]] || [[phosphopentose isomerase]] |- | [[ribulose 5-phosphate]] || &rarr; [[xylulose 5-phosphate]] || [[phosphopentose epimerase]] |- | [[xylulose 5-phosphate]] + [[ribose 5-phosphate]] || &rarr; [[glyceraldehyde 3-phosphate]] + [[sedoheptulose 7-phosphate]] || [[transketolase]] |- | [[sedoheptulose 7-phosphate]] + [[glyceraldehyde 3-phosphate]] || &rarr; [[erythrose 4-phosphate]] + [[fructose 6-phosphate]] || [[transaldolase]] |- | [[xylulose 5-phosphate]] + [[erythrose 4-phosphate]] || &rarr; [[glyceraldehyde 3-phosphate]] + [[fructose 6-phosphate]] || [[transketolase]] |} ===Regulation=== [[Glucose-6-phosphate dehydrogenase]] is the rate-controlling enzyme of this pathway. It is allosterically stimulated by NADP<sup>+</sup>. The ratio of NADPH:NADP<sup>+</sup> is normally about 100:1 in liver cytosol. This makes the cytosol a highly-reducing environment. Formation of NADP<sup>+</sup> by a NADPH-utilizing pathway, thus, stimulates production of more NADPH. ==See also== * [[Glucose-6-phosphate dehydrogenase deficiency|G6PDH deficiency]] - A hereditary disease that disrupts the pentose phosphate pathway * [[NADPH]] * [[RNA]] * [[thiamine deficiency]] ==Erythrocytes and the Pentose Phosphate Pathway== The predominant pathways of carbohydrate metabolism in the red blood cell (RBC) are glycolysis, the PPP and 2,3-bisphosphoglycerate (2,3-BPG) metabolism (refer to discussion of hemoglobin for review of role of 2,3-BPG). Glycolysis provides ATP for membrane ion pumps and NADH for re-oxidation of methemoglobin. The PPP supplies the RBC with NADPH to maintain the reduced state of glutathione. The inability to maintain reduced glutathione in RBCs leads to increased accumulation of peroxides, predominantly H<sub>2</sub>O<sub>2</sub>, that in turn results in a weakening of the cell membrane and concomitant hemolysis. Accumulation of H<sub>2</sub>O<sub>2</sub> also leads to increased rates of oxidation of hemoglobin to methemoglobin that also weakens the cell wall. Glutathione removes peroxides via the action of glutathione peroxidase. The PPP in erythrocytes is essentially the only pathway for these cells to produce NADPH. Any defect in the production of NADPH could, therefore, have profound effects on erythrocyte survival. Several deficiencies in the level of activity (not function) of glucose-6-phosphate dehydrogenase have been observed to be associated with resistance to the malarial parasite, Plasmodium falciparum, among individuals of Mediterranean and African descent. The basis for this resistance is the weakening of the red cell membrane (the erythrocyte is the host cell for the parasite) such that it cannot sustain the parasitic life cycle long enough for productive growth.<ref>[http://www.bioon.com/book/biology/mwking/pentose-phosphate-pathway.html Pentose Phosphate Pathway<!-- Bot generated title -->]</ref> ==References== <references/> ==External links== *[http://www2.ufp.pt/~pedros/bq/ppp.htm The chemical logic behind the pentose phosphate pathway] *{{MeshName|Pentose+Phosphate+Pathway}} *http://www.bioon.com/book/biology/mwking/pentose-phosphate-pathway.html {{Carbohydrate metabolism}} {{MetabolismMap}} {{Pentose phosphate pathway}} {{Pentose phosphate pathway intermediates}} [[Category:Inborn errors of metabolism]] [[Category:Genetic disorders]] [[Category:Metabolic disorders]] [[Category:Metabolic pathways]] [[Category:Phosphorus]] [[cs:Pentosofosfátový cyklus]] [[de:Pentosephosphatweg]] [[es:Ruta de la pentosa fosfato]] [[fr:Voie des pentoses phosphates]] [[it:Via dei pentoso fosfati]] [[he:מסלול הפנטוז פוספט]] [[ja:ペントースリン酸経路]] [[no:Pentosefosfatshunten]] [[pt:Via das pentoses-fosfato]] [[sv:Pentosfosfatvägen]] [[zh:磷酸戊糖途径]]