Hexose Monophosphate Shunt PDF

Hexose Monophosphate Shunt  Hexose Monophosphate Shunt The pentose phosphate pathway is an alternate route for the metabolism of glucose It has two other names; - Phosphogluconate pathway - Pentose Phosphate pathway  The pentose phosphate pathway (PPP) has...

Hexose Monophosphate Shunt  Hexose Monophosphate Shunt The pentose phosphate pathway is an alternate route for the metabolism of glucose It has two other names; - Phosphogluconate pathway - Pentose Phosphate pathway  The pentose phosphate pathway (PPP) has two main functions The PPP does not generate ATP but it has two major functions; i) Generation of NADPH which functions as a reductant in - anabolic (synthetic) pathways, e.g., - the production of reduced glutathione in RBCs & other cells ii) Production of ribose residues for nucleotide and nucleic acid synthesis.  The Pentose phosphate pathway has two phases The enzymes of the pentose phosphate pathway, as of glycolysis are cytosolic As in glycolysis oxidation is achieved by dehydrogenation; but NADP and not NAD+ is the hydrogen acceptor. The sequence of the pathway has two phases i) Oxidative non reversible phase – glucose undergoes dehydrogenation and decarboxylation to yield a pentose, ribulose 5- P ii) Non oxidative reversible phase – ribulose 5-P is converted back to glucose by a series of reactions involving mainly two enzymes: transketolase and transaldolase  The oxidative phase generates NAPH Glucose-6-phosphate Dehydrogenase catalyzes oxidation of the aldehyde, at C1 of glucose-6-phosphate, to a carboxylic acid, in ester linkage (lactone). NADP+ serves as electron acceptor.  Oxidative steps 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.  Last oxidative step 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 the oxidant.  The non oxidative phases generates ribose precursors This entails extensive carbon atom rearrangement Ribulose 5-P epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phospha Ribose 5-phosphate isomerase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5- phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis.  Transketolase transfers a 2-C fragment from Non oxidative phases xylulose-5-P to ribose- 5-P Transketolase utilizes as prosthetic group thiamine pyrophosphate (TPP), a derivative of vitamin B1. Transaldolase catalyzes transfer of a 3-C dihydroxyacetone moiety, from sedoheptulose-7- phosphate to glyceraldehyde-3- phosphate In a further reaction catalyzed by transketolase, xylulose 5-phosphate donates a two-carbon unit to erythrose 4- phosphate to form fructose 6-phosphate and glyceraldehyde 3- phosphate. The pentose phosphate pathway is a shunt The pathway begins with the glycolytic intermediate glucose 6-P. It reconnects with glycolysis because two of the end products of the pentose pathway are glyceraldehyde 3-P and fructose 6-P; two intermediates further down in the glycolytic pathway. It is for this reason that the pentose pathway is often referred to as a shunt. Net result: 3C5  2C6 + C3 Regulation of the Pentose Pathway Glucose 6-phosphate DH is the regulatory enzyme. NADPH is a potent competitive inhibitor of the enzyme. Usually the ratio NADPH/NADP+ is high so the enzyme is inhibited. But, with increased demand for NADPH, the ratio decreases and enzyme activity is stimulated.  NADPH NADPH is an electron carrier Two forms: - Reduced: NADPH - Oxidised: NADP+  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  Muscle has only low activity of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. it is capable of synthesizing ribose 5-phosphate by reversal of the nonoxidative phase of the pentose phosphate pathway utilizing fructose 6-phosphate. It is not necessary to have a completely functioning  Role of NADPH in neutralisation of H2O2 Reactive oxygen species (ROS) are by-products of aerobic metabolism Leads to oxidative stress when produced ROS may be involved in cancer, inflammation and aging Production of superoxide in red blood cells – Hb-Fe2+-O2 -> Hb-Fe3+ + O2-. Spontaneous rxn, 1% per hour O2-. + 2H2O -> 2H2O2 Both O2-. & H2O2 can produce reactive free radical species, damage cell membranes, and cause hemolysis  Dealing with oxidative stress Antioxidan t enzymes -Superoxide dismutase -Glutathione peroxidase -Glutathione reductase -Catalase  Other antioxidants Vitamins A, E and C Resveratrol Melatonin Coenzyme Q10 Selenium Bioflavonoids -3 fatty acids  Glutathione and erythrocytes O O H + H3N C CH2 CH2 C N CH C N CH2 COO H H 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  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. Regeneration of reduced glutathione requires NADPH, produced within erythrocytes in the Pentose Phosphate Pathway. Glutathione Reductase catalyzes: GSSG + NADPH + H+  2 GSH + NADP+ Glutathione and Erythrocytes GSH is extremely important particularly in the highly oxidizing environment of the red blood cell. Mature RBCs have no mitochondria and are totally dependent on NADPH from the pentose phosphate pathway to regenerate GSH from GSSG via glutathione reductase. In fact, as much as 10% of glucose consumption, by erythrocytes, is  Glutathione and Erythrocytes The reduced form of glutathione serves as a sulfhydryl buffer. It maintains cysteine residues in hemoglobin and other proteins in a reduced state. GSH is essential for normal RBC structure and keeping hemoglobin in Fe++ state.  G6PD deficiency hemolytic anemia related 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 eg; – Antimalarials - pamaquine – purine glycoside from fava beans Proteins become cross linked leading to Heinz body formation and cell lysis.  Glucose 6-phosphate DH deficiency and nonspherocytic hemolytic anemia. Over 300 genetic variants of the G6PD protein are known. Thus, there is a remarkable variation in the clinical spectrum. G6PD deficiency is an inheritable X- linked recessive disorder. Most common disease causing enzyme in humans Approximately 10-14% of the male African American population is affected. Glucose 6-phosphate Dehydrogenase deficiency. People with the disorder are not normally anemic and display no evidence of the disease until the red cells are exposed to an oxidant or stress. Drugs that can precipitate this reaction: antimalarial agents sulfonamides (antibiotic) aspirin nonsteroidal antiinflammatory drugs (NSAIDs) nitrofurantoin quinidine quinine  Favism Individuals with G6PD deficiency must not eat Fava beans. Pythagoras Erythrocytes lyse=dark or black urine. Interestingly – G6PD deficient individuals increased resistance to plasmodium falciparum (malaria parasite)  NADP and the Cytochrome P450 (CYPS) monooxygenase system CytP450 is involved in phase I drug metabolism by hydroxylating substances which makes them more soluble NADP provides the reducing equivalents for the enzyme cytochrome P450 reductase [Phase II of drug detox is to increase solubility even more by adding C-6 oxidized sugar (glucuronic acid); glucuronization, or sulfation]