23. Pentose Phosphate & Interconversion of Sugars (1).pptx

Pentose Phosphate Pathway and The Inter-conversion of Sugars MED 7125: Lecture 23 Annie N. Kirby, PhD, RD, CCMS [email protected] Office 241  Lecture Objectives a. Relate the goal of the pentose phosphate pathw...

Pentose Phosphate Pathway and The Inter-conversion of Sugars MED 7125: Lecture 23 Annie N. Kirby, PhD, RD, CCMS [email protected] Office 241  Lecture Objectives a. Relate the goal of the pentose phosphate pathway, identify the two phases, including which is reversible and which is irreversible, and identify the final products of the pathway besides NADPH. b. Recall metabolic pathways that depend on the NADPH produced by the pentose phosphate pathway. c. Interpret the importance of the first step of the pentose phosphate pathway (glucose-6- phosphate dehydrogenase) being the rate-limiting step of the pathway. How is this step regulated? d. Interpret the relationship between glucose-6-phosphate dehydrogenase deficiency and drug- induced hemolytic anemia. e. Recall the three most common monosaccharides found in the human diet and identify them in their most common dietary forms. f. Relate in general terms how the body can interconvert carbohydrate molecules to make any carbohydrate it needs (esp., glucose or fructose or galactose). g. Recall the metabolism of fructose in the liver, and relate the key role Aldolase B plays in this process. Interpret how defects in Aldolase B can affect fructose metabolism. Relate how fructose is metabolized by non-liver cells, such as muscle. h. Relate how galactose is processed and converted to UDP-glucose upon entering the cell. Identify the structural relationship between glucose and galactose, and their inter-conversion. i. Recall some of the cellular uses for UDP-galactose and UDP-glucose. j. Identify glucuronate and glucuronides and relate their functions in cells.  Pentose phosphate pathway An alternative pathway for G-6-P utilization: a shunt In the irreversible oxidative reactions of the pathway, one carbon of G6P is released as CO2; NADPH is generated; and ribulose-5- phosphate is produced. NADPH is used for reductive biosynthesis (particularly of fatty acids) and for protection against oxidative damage (e.g., by reduction of glutathione). Ribulose-5-phosphate provides ribose-5- Fig. 27.1 phosphate for nucleotide biosynthesis or  Two phases of Pentose Phosphate: Oxidative and Non-oxidative Oxidative phase: Consists of three irreversible oxidation reactions converting G-6-P  ketopentose + CO2 NADPH is formed during this phase Non-oxidative phase: Consists of 5 freely reversible reactions Produces ribose 5-P and glycolysis/gluconeogenesis intermediates Overall Reaction: 3 G-6-P + 6 NADP+  3 CO2 + 6 NADPH + 6 H+ + 2 fructose-6-P + glyceraldehyde-3-P  The Pentose Phosphate Pathway: The Oxidative Phase Step 1: G-6-P is oxidized to 6-phosphogluconolactone NADP+ is reduced to NADPH + H+ Enzyme: glucose-6-phosphate dehydrogenase Rate-limiting step for the pathway Clinical gem: a deficiency of G-6-P dehydrogenase causes insufficient NADPH production; generates many downstream  The Pentose Phosphate Pathway: The Oxidative Phase Step 2 6-phosphogluconolactone is hydrolyzed to 6- phosphogluconate Enzyme: Gluconolactonase  The Pentose Phosphate Pathway The Oxidative Phase Step 3 6-phosphogluconate undergoes an oxidation, followed by a decarboxylation CO2 is released, and a second NADPH + H+ is generated from NADP+ The remaining carbons form ribulose 5- phosphate Enzyme: 6-phosphogluconate  Pentose Phosphate Pathway The Oxidative Phase Net 2 NADPH produced per 1 G-6-P Important! These three reactions are irreversible due to the very large ΔG values. Important! The 1st and 3rd steps are inhibited by NADPH: cell doesn’t want to run this pathway if NADPH levels are sufficient  Pentose Phosphate Pathway Regulation When NADPH levels are normal, Phase 1 is shut down, but Phase 2 operates as needed The reactions of Phase 1 only occur if NADP+ is available Phase 1 Inhibits the pathway; Shuts down phase 1 ↑ NADPH Phase 2 ↓NADPH Stimulates the pathway /↑ NADP  The Pentose Phosphate Pathway The Non-oxidative Phase 5 rearrangement and transfer reactions that occur in 2 parts Freely reversible The Pentose Phosphate Pathway The Non-Oxidative Phase Part 1 (left figure): Ribulose 5-phosphate is isomerized to ribose 5- phosphate or epimerized to xylulose 5-phosphate Part 2 (right figure): Ribose 5-phosphate and xylulose 5-phosphate undergo reactions, catalyzed by transketolase and transaldolase, that transfer carbon units, ultimately forming fructose 6-phosphate and glyceraldehyde 3-phosphate Transketolase, requires thiamin pyrophosphate, transfers two-carbon units Transaldolase transfers three-carbon units  The Pentose Phosphate Pathway The Non-Oxidative Phase The reactions of the non-oxidative phase are all reversible Can function independently to meet metabolic needs for ribose 5-P or for glucose metabolism intermediates Ex: Ribose 5-P can be used for nucleotide biosynthesis, unless the need for glycolysis is greater…ribulose-5-P will be diverted back into the glycolytic pathway. These reactions follow cell needs  Functions of NADPH The pentose phosphate pathway produces NADPH for fatty acid synthesis. Under these conditions, the fructose-6-phosphate and glyceraldehyde-3-phosphate generated in the pathway reenter glycolysis. NADPH is also used to reduce glutathione (γ-glutamylcysteinylglycine). Glutathione helps to prevent oxidative damage to cells by reducing hydrogen peroxide (H2O2) Glutathione is also used to transport amino acids across the membranes of certain cells by the γ-glutamyl cycle Other uses of NADPH are: Cholesterol synthesis Neurotransmitter synthesis Nucleotide synthesis  Functions of NADPH NADPH is used by phagocytic cells: NADPH oxidase uses NADPH to form super oxide from O2 in the mechanism for killing microorganisms taken up by phagocytic cells NADPH is central player in the defense against ROS, especially in red blood cells NADPH helps to maintain glutathione in an active form. Glutathione peroxidase : 2GSH + ROOH  GSSG + ROH + H2O Glutathione reductase : GSSG + NADPH + H+  2GSH + NADP+ RBC are especially prone to oxidative stress H2O2 causes lipid peroxidation making the plasma membrane lipid bilayer fragile. Passage though narrow capillaries may make such RBC to rupture causing hemolytic anemia  Reactive Oxygen Species and Hemolysis G-6-P Dehydrogenase deficiency and its effect on RBC vitality is also considered a rare form of drug-induced hemolytic anemia. Not caused by a drug – but the effect on the RBC is the same so it is classified the same.  The Interconversion of Sugars: Fructose and Galactose Metabolism  Introduction Glucose is the most abundant sugar in the diet Fructose and galactose come from: Sucrose (fructose-glucose disaccharide) Lactose (galactose-glucose disaccharide) Fructose and galactose are transported into cells and phosphorylated on carbon 1, similar to glucose keeps them inside the cell for processing Once phosphorylated, fructose 1-P and galactose 1-P are metabolized to intermediates of the glucose metabolism  The Interconversion of Sugars Fructose All the sugars we need for various uses can be synthesized from dietary glucose. The availability of other sugars in our diet provides key starting material for the synthesis of carbohydrate derivatives in glycolipids, glycoproteins, proteoglycans (GAGs), etc. Today, we will focus on the three major dietary sugars: fructose, galactose, glucose  Essential Sugars Our bodies can synthesize any needed sugar from glucose. However, new research shows that getting certain carbohydrates in the diet greatly enhances many of our body systems: Immune function, wound healing, anti-cancer, anti-inflammatory systems, and many more… There are eight carbohydrates that are considered to be very important. Mannose, galactose, glucose, fucose, xylose, N-acetylneuraminic acid, N- acetylglucosamine and N-acetylgalactosamine Are they essential? By definition, no, since our bodies can make them. But our nutritional status improves dramatically when we have these in adequate supply in out diet. They are conditionally essential.  Fructose *Reminder: Aldolase also cleaves fructose 1,6 bisphosphate in glycolysis Metabolism Fructose is metabolized mainly in the liver Upon entering the cell, fructose is phosphorylated At carbon 1 by fructokinase, utilizing one ATP Fructose 1-P is cleaved by *aldolase B Forms dihydroxyacetone and glyceraldehyde Key point for disease: Aldolase B Aldolase B defect Accumulation of F-1-P, which is bad These patients need to avoid fructose! Glyceraldehyde is phosphorylated By triose kinase Forms Glyceraldehyde 3-P, using one ATP Fructose Metabolism In non-liver tissues: Fructose is phosphorylated at carbon 6 By hexokinase to form fructose 6-P Which enters glycolysis Hexokinase phosphorylates glucose preferentially But will work on fructose Two moles of ATP are used per mole of fructose Similar to glucose Subsequent yield of ATP is same as glycolysis  Fructose from Glucose (the polyol pathway) Fructose can also be produced from glucose: Glucose is reduced to sorbitol by aldose reductase Which reduces the aldehyde group to an alcohol Sorbitol is oxidized at C-2 to form fructose Fructose as a major energy source for sperm cells While in seminal fluid Use glucose when in the female reproductive tract  Two disorders associated with fructose metabolism Essential fructosuria Caused by a deficiency of fructokinase Fructose cannot be metabolized as rapidly as normal Blood fructose levels rise, and fructose appears in the urine Benign disorder Hereditary fructose intolerance (HFI) Caused by a defective aldolase B Aldolase B still functions normal in glycolysis, but not fructose metabolism Fructose 1-phosphate accumulates and inhibits glucose production Leads to severe hypoglycemia if fructose is ingested Treatment: avoidance of dietary fructose  Galactose Metabolism Chapter 30 Figure 30.7 Upon entering the cell, galactose is phosphorylated at carbon 1 By galactose kinase Uses one ATP to form galactose 1-P Galactose 1-P reacts with UDP-glucose By galactose 1-phosphate uridylyl transferase Forms glucose 1-P and UDP-galactose UDP-galactose is converted to UDP-glucose By UDP-glucose epimerase Net result: galactose is converted to a glucose metabolism intermediate  Diseases Associated with Faulty Galactose Interconversion Non-Classical Galactosemia Galactokinase is deficient Galactose can’t be processed Classical Galactosemia Can’t form UDP-galactose or make molecules dependent on UDP-galactose Galactose-1-P accumulates in liver Bad for liver cells  Other Fates of UDP-galactose UDP-galactose and UDP-glucose are used in the synthesis of: Glycoproteins Glycolipids Proteoglycans UDP-galactose is also used to form the milk sugar lactose in the mammary gland Chapter 30 Figure 30.2  Other Fates of UDP-glucose UDP-glucose used directly, or modified and used, for a variety of needs Note the important interrelationship of UDP-glucose and UDP- galactose  UDP-Glucuronate and Glucuronides UDP-glucose is an important starting material for the synthesis of other special sugars found in glycoproteins, proteoglycans, etc. First step is the production of UDP-glucuronate (oxidation of UDP- glucose) Next is the transfer of the glucuronate moiety onto another compound (“R”), which can be a protein, another sugar, etc. The glucuronate moiety is then modified to a final form, such as the glucosaminoglycans (GAGs), amino sugars, etc. NOTE: many non-water soluble compounds are converted to water soluble compound by adding glucuronides onto them. Best example is bilirubin, which is insoluble, but becomes soluble by the addition of 2 glucuronides. Same strategy used in phase 2 of liver detox system, when similar  Other Fates of UDP-glucuronate Key feature of glucuronate is its stable negative charge Increases the solubility of molecule to which it is attached Ex. Proteoglycans (recall the highly hydrated nature) Chapter 30 Ex. Steroids, drugs and xenobiotics to be eliminated Figure 30.3  Glucuronate functions to aid the excretion of non-polar substances Glucuronate is often attached to –OH containing compounds that are insoluble or only slightly soluble This adds negative charge and increases water solubility so the compounds can be moved and excreted Bilirubin, steroid hormones, drugs and drug products  Summary Pentose Phosphate Pathway Consists of both oxidative and nonoxidative reactions Oxidative steps of the pentose phosphate pathway generate reduced nicotinamide adenine dinucleotide phosphate (NADPH) and ribulose 5-phosphate from glucose 6- phosphate Ribulose 5-phosphate is converted to ribose 5-phosphate for nucleotide biosynthesis. NADPH is used as reducing power for biosynthetic pathways. The nonoxidative steps of the pentose phosphate pathway reversibly convert five- carbon sugars to fructose 6-phosphate and glyceraldehyde 3-phosphate. The needs of the cell dictate whether the cell will use the oxidative reactions of the pentose phosphate pathway, the non-oxidative reactions, or both sets of reactions. Reactions between sugars or the formation of sugar derivatives use sugars activated by attachment to nucleotides (a nucleotide sugar) Uridine diphosphate (UDP)-glucose and UDP-galactose are substrates for many glycosyltransferase reactions UDP-glucose is oxidized to UDP-glucuronate, which forms glucuronide derivatives of various hydrophobic compounds, making them more readily excreted in urine or bile  Question The pentose phosphate pathway generates which one of the following? a. NADH, which may be used for fatty acid synthesis. b. Ribose-5-phosphate, which may be used for the biosynthesis of ATP. c. Pyruvate and fructose 1,6-bisphosphate by the direct action of transaldolase and transketolase. d. Xylulose-5-phosphate by one of the oxidative reactions. e. Glucose from ribose-5-phosphate and CO.  Question Which one of the following metabolites is used by all cells for glycolysis, glycogen synthesis, and the hexose monophosphate shunt pathway? a. Glucose-1-phosphate b. Glucose-6-phosphate c. UDP-glucose d. Fructose-6-phosphate e. Phosphoenolpyruvate  Question A pregnant woman who has a lactase deficiency and cannot tolerate milk in her diet is concerned that she will not be able to produce milk of sufficient caloric value to nourish her baby. The best advice to her is which one of the following? a. She must consume pure galactose in order to produce the galactose moiety of lactose. b. She will not be able to breastfeed her baby because she cannot produce lactose. c. The production of lactose by the mammary gland does not require the ingestion of milk or milk products. d. She can produce lactose directly by degrading α-lactalbumin. e. A diet rich in saturated fats will enable her to produce lactose.  Question A long-distance runner who is planning to run a marathon decides to add fructose to the replacement fluid she will be using during the race. Which of the following statements regarding fructose best describes the fate of this sugar? A. It enters the glycolytic pathway as fructose 6-phosphate in the liver B. It is converted to uridine diphosphate (UPD)-fructose and then epimerized to UDP-glucose C. It is metabolized by a pathway other than the glycolytic pathway D. It is metabolized in the liver by an aldolase that recognizes fructose-1- phosphate E. It is phosphorylated by phosphofructokinase  Question A chronic alcoholic has recently had trouble with their ability to balance, becomes easily confused, and displays nystagmus. An assay of which of the following enzymes can determine a biochemical reason for these symptoms? a. Isocitrate dehydrogenase b. Transaldolase c. Glyceraldehyde-3-phosphate dehydrogenase d. Transketolase e. Glucose-6-phosphate dehydrogenase