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Molecular Biochemistry II Pentose Phosphate Pathway Copyright © 1999-2007 by Joyce J. Diwan. All rights reserved. Pentose Phosphate Pathway Pentose Phosphate Pathway Other names: Phosphogluconate Pathway Hexose Monophosphate Shunt The linear part of the pathway carries out oxidation and decarboxylation of the 6-C sugar glucose-6-P, producing the 5-C sugar ribulose-5-P. 6CH 2OPO 32 5 H 4 OH O H OH 3 H 6 CH OPO 2 2 3 + NADPH + H 5 + O OH NADP 1 H OH OH H2O H+ H 4 H 2 6-Phosphogluconolactonase Glucose-6-phosphate Dehydrogenase H OH O H 1 3 H 2 OH O O 1C HC 2 OH HO 3CH HC OH 4 HC 5 OH CH 2OPO 32 6 glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate Glucose-6-phosphate Dehydrogenase catalyzes oxidation of the aldehyde (hemiacetal), at C1 of glucose-6-phosphate, to a carboxylic acid, in ester linkage (lactone). NADP+ serves as electron acceptor. 6CH 2OPO 32 5 H 4 OH O H OH 3 H 6 CH OPO 2 2 3 + NADPH + H 5 + O OH NADP 1 H 2 6-Phosphogluconolactonase Glucose-6-phosphate Dehydrogenase H OH H2O H+ H 4 OH H OH O H 1 3 H 2 OH O O 1C HC 2 OH HO 3CH HC OH 4 HC 5 OH CH 2OPO 32 6 glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate 6-Phosphogluconolactonase catalyzes hydrolysis of the ester linkage, resulting in ring opening. The product is 6-phosphogluconate. Although ring opening occurs in the absence of a catalyst, 6-Phosphogluconolactonase speeds up the reaction, decreasing the lifetime of the highly reactive, and thus potentially toxic, 6-phosphogluconolactone. O O 1C HC 2 OH Phosphogluconate Dehydrogenase NADP + HO 3CH HC OH 4 HC OH CH OH NADPH + H+ 1 2 C O 2 CO 2 HC OH HC OH 3 5 4 6 5 CH 2OPO 32 6-phosphogluconate CH 2OPO 32 ribulose-5-phosphate Phosphogluconate Dehydrogenase catalyzes oxidative decarboxylation of 6-phosphogluconate, to yield the 5-C ketose ribulose-5-phosphate. The OH at C3 (C2 of product) is oxidized to a ketone. This promotes loss of the carboxyl at C1 as CO2. NADP+ serves as oxidant. H Reduction of NADP+ (as with NAD+) involves transfer of 2e and 1H+ to the nicotinamide moiety. O H H C C NH2 + N O NH2 2e + H + N R R NADP+ NADPH NADPH, a product of the Pentose Phosphate Pathway, functions as a reductant in anabolic (synthetic) pathways, e.g., fatty acid synthesis. NAD+ serves as electron acceptor in catabolic pathways, in which metabolites are oxidized. The resultant NADH is reoxidized by the respiratory chain, producing ATP. NAD+ & NADP+ differ only in the presence of an extra phosphate on the adenosine ribose of NADP+. This difference has little to do with redox activity, but is recognized by substrate-binding sites of enzymes. It is a mechanism for separation of catabolic and synthetic pathways. Nicotinamide Adenine Dinucleotide H C NH 2 O O P O + N CH 2 O H H OH OH NH 2 O N N O P nicotinamide H H O O CH 2 O H N O adenine H esterified to Pi in NADP+ H H OH N OH Regulation of Glucose-6-phosphate Dehydrogenase: Glucose-6-phosphate Dehydrogenase is the committed step of the Pentose Phosphate Pathway. This enzyme is regulated by availability of the substrate NADP+. As NADPH is utilized in reductive synthetic pathways, the increasing concentration of NADP+ stimulates the Pentose Phosphate Pathway, to replenish NADPH. The rest of the pathway converts ribulose-5-P to the 5-C product ribose-5-P, or to 3-C glyceraldehyde-3-P & 6-C fructose-6-P. Additional enzymes include an Isomerase, Epimerase, Transketolase, and Transaldolase. Epimerase interconverts stereoisomers ribulose-5-P and xylulose-5-P. Isomerase converts the ketose ribulose-5-P to the aldose ribose-5-P. Both reactions involve deprotonation to an endiolate intermediate followed by specific reprotonation to yield the product. Both reactions are reversible. CH2OH Epimerase CH2OH C C O HO C H H C OH CH2OPO32 O H C OH H C OH CH2OPO32 xylulose-5phosphate HC O H C OH ribulose-5H phosphate Isomerase C OH H C OH CH2OPO32 ribose-5phosphate Transketolase & Transaldolase catalyze transfer of 2-C or 3-C molecular fragments respectively, in each case from a ketose donor to an aldose acceptor. D. E. Nicholson has suggested that the names of these enzymes should be changed, since Transketolase actually transfers an aldol moiety (glycoaldehyde), and Transaldolase actually transfers a ketol moiety (dihydroxyacetone). However the traditional enzyme names are used here. CH2OH Transketolase CH2OH HC O C O H C OH HO C H H C OH H C OH H C OH + CH2OPO32 xylulose5-phosphate CH2OPO32 HC H C O OH + CH2OPO32 C O HO C H H C OH H C OH H C OH CH2OPO32 riboseglyceraldehyde- sedoheptulose5-phosphate 3-phosphate 7-phosphate Transketolase transfers a 2-C fragment from xylulose5-P to either ribose-5-P or erythrose-4-P. Transketolase utilizes as prosthetic group thiamine pyrophosphate (TPP), a derivative of vitamin B1. Pyruvate Dehydrogenase of Krebs Cycle also utilizes TPP as prosthetic group. aminopyrimidine moiety CH2 N H3C thiazolium H3C ring CH2 O P O O C N CH2 + N H O O P O O S acidic H+ NH2 thiamine pyrophosphate (TPP) TPP binds at the active site in a “V” conformation. H+ dissociates from the C between N & S in the thiazolium ring. The aminopyrimidine amino group is near the dissociable H+, & serves as H+ acceptor. This H+ transfer is promoted by a Glu residue adjacent to the pyrimidine ring. H3C TPP The thiazolium carbanion reacts with the carbonyl C of xylulose-5-P to form an addition compound. N+ in the thiazole ring acts as an e sink, promoting C-C bond cleavage. H3C + N CH 2 N CH 2 C N xylulose-5-P S CH 2OH HO C H H C OH CH 2 C NH 2 2 CH 2OPO2OPO 3 + N CH 2 N O CH 2OPO32 H3C H3C C NH 2 Transketolase N 2 CH 2OPO2OPO 3 S HO C CH 2OH HO C H C OH active site intermediate H CH 2OPO32 subsequent cleavage H3C 2 CH 2 CH 2OPO2OPO 3 The 3-C aldose TPP + glyceraldehyde-3-P CH 2 N xylulose-5-P N S is released. C CH 2OH A 2-C fragment C O H3C N NH 2 remains on TPP. HO C H Completion is by H C OH reversal of these CH 2OPO32 Transketolase steps. H3C 2 CH 2 CH 2OPO2OPO 3 The 2-C fragment + CH 2 N condenses with N S C one of the aldoses HO C CH 2OH subsequent H3C N NH 2 erythrose-4-P (4-C) cleavage HO C H or ribose-5-P (5-C) active site H C OH to form a ketose-P intermediate 2 CH OPO 2 3 product. CH2OH Transketolase CH2OH HC O C O H C OH HO C H H C OH H C OH H C OH + CH2OPO32 xylulose5-phosphate CH2OPO32 HC H C O OH + CH2OPO32 C O HO C H H C OH H C OH H C OH CH2OPO32 riboseglyceraldehyde- sedoheptulose5-phosphate 3-phosphate 7-phosphate Transfer of the 2-C fragment to the 5-C aldose ribose-5-phosphate yields sedoheptulose-7-phosphate. Transfer of the 2-C fragment instead to the 4-C aldose erythrose-4-phosphate yields fructose-6-phosphate. CH2OH C HO Transaldolase O H2C CH C HC OH HC OH HC OH H2C + OPO32 HC O HC O HC OH HC OH HC OH H2C OPO32 H2C HO + OPO32 OH O CH HC OH HC OH H2C OPO32 sedoheptulose- glyceraldehyde- erythrose- fructose7-phosphate 3-phosphate 4-phosphate 6-phosphate Transaldolase catalyzes transfer of a 3-C dihydroxyacetone moiety, from sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate. Transaldolase has an a,b barrel structure. Enz-Lys NH2 CH2OH C HO CH2OH H Enz-Lys N + HO O CH HC OH HC OH sedoheptulose7-phosphate HC H2C OH OPO 32 erythrose-4phosphate Transaldolase OH O HC OH HC OH Schiff base intermediate CH HC OH HC OH HC OH H2C HC H2C C OPO 32 CH2OH Enz-Lys H N + HO + H+ C C H OPO 32 In Transaldolase, the e-amino group of a lysine residue reacts with the carbonyl C of sedoheptulose-7-P to form a protonated Schiff base intermediate. Enz-Lys NH2 CH2OH C Aldol cleavage releases erythrose-4phosphate. HO CH2OH H Enz-Lys N + HO O CH HC OH HC OH sedoheptulose7-phosphate HC H2C OH OH OPO 32 The Schiff HC base erythrose-4phosphate HC stabilizes the HC carbanion H2C on C3. Transaldolase C CH HC OH HC OH HC OH H2C O OH OH Schiff base intermediate OPO 32 CH2OH Enz-Lys H N + HO + H+ C C H OPO 32 Completion of the reaction is by reversal, as the carbanion attacks instead the aldehyde carbon of the 3-C aldose glyceraldehyde-3-P to yield the 6-C fructose-6-P. The diagram at right summarizes flow of 15 C atoms through Pentose Phosphate Pathway reactions by which 5-C sugars are converted to 3-C and 6-C sugars. IS = Isomerase EP = Epimerase TK = Transketolase TA = Transaldolase (3) ribulose-5-P EP IS ribose-5-P (2) xylulose-5-P TK glyceraldehyde-3-P sedoheptulose 7 P fructose-6- P TA erythrose-4-P TK fructose-6-P glyceraldehyde-3-P The balance sheet below summarizes flow of 15 C atoms through Pentose Phosphate Pathway reactions by which 5-C sugars are converted to 3-C and 6-C sugars. C5 + C5 C3 + C7 (Transketolase) C3 + C7 C6 + C4 (Transaldolase) C5 + C4 C6 + C3 (Transketolase) ____________________________ 3 C5 2 C6 + C3 (Overall) Glucose-6-phosphate may be regenerated from either the 3-C glyceraldehyde-3-phosphate or the 6-C fructose-6-phosphate, via enzymes of Gluconeogenesis. Depending on needs of a cell for ribose-5-phosphate, NADPH, and ATP, the Pentose Phosphate Pathway can operate in various modes, to maximize different products. There are three major scenarios: 2 NADP+ 2 NADPH + CO2 glucose-6-P ribulose-5-P ribose-5-P Pentose Phosphate Pathway producing NADPH and ribose-5-phosphate Ribulose-5-P may be converted to ribose-5-phosphate, a substrate for synthesis of nucleotides and nucleic acids. The pathway also produces some NADPH. 2 NADP+ 2 NADPH + CO2 glucose-6-P ribulose-5-P ribose-5-P fructose-6-P, & glyceraldehyde-3-P Pentose Phosphate Pathway producing maximum NADPH Glyceraldehyde-3-P and fructose-6-P may be converted to glucose-6-P for reentry to the linear portion of the Pentose Phosphate Pathway, maximizing formation of NADPH. 2 NADP+ 2 NADPH + CO2 glucose-6-P ribulose-5-P ribose-5-P fructose-6-P, & glyceraldehyde-3-P to Glycolysis for production of ATP Pentose Phosphate Pathway producing NADPH and ATP Glyceraldehyde-3-P and fructose-6-P, formed from 5-C sugar phosphates, may enter Glycolysis for ATP synthesis. The pathway also produces some NADPH. 2 NADP+ 2 NADPH + CO2 glucose-6-P ribulose-5-P ribose-5-P fructose-6-P, & glyceraldehyde-3-P to Glycolysis for production of ATP Pentose Phosphate Pathway producing NADPH and ATP Ribose-1-phosphate generated during catabolism of nucleosides also enters Glycolysis in this way, after first being converted to ribose-5-phosphate. Thus the Pentose Phosphate Pathway serves as an entry into Glycolysis for both 5-carbon & 6-carbon sugars. O + H3N H C CH2 COO CH2 C O N H CH C N H CH2 COO CH2 SH -glutamyl-cysteinyl-glycine Glutathione Glutathione is a tripeptide that includes a Glu linked by an isopeptide bond involving the side-chain carbonyl group. Its functional group is a cysteine thiol. One role of glutathione is degradation of hydroperoxides, that arise spontaneously in the oxygen-rich environment in red blood cells. Hydroperoxides can react with double bonds in fatty acids of membrane lipids, making membranes leaky. O + H3N H C CH2 COO CH2 C O N H CH C N H CH2 COO CH2 SH -glutamyl-cysteinyl-glycine Glutathione Glutathione Peroxidase catalyzes degradation of organic hydroperoxides by reduction, as two glutathione molecules (represented as GSH) are oxidized to a disulfide. 2 GSH + ROOH GSSG + ROH + H2O Glutathione Peroxidase uses the trace element selenium as functional group. The enzyme's primary structure includes an analog of cysteine, selenocysteine, with Se replacing S. Regeneration of reduced glutathione requires NADPH, produced within erythrocytes in the Pentose Phosphate Pathway. Glutathione Reductase catalyzes: GSSG + NADPH + H+ 2 GSH + NADP+ Genetic deficiency of Glucose-6-P Dehydrogenase can lead to hemolytic anemia, due to inadequate [NADPH] within red blood cells. The effect of partial deficiency of Glucose-6-phosphate Dehydrogenase is exacerbated by substances that lead to increased production of peroxides (e.g., the antimalarial primaquine).