4 - Lecture Lecture 4 CARBOHYDRATE METABOLISM (Part2)...

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Unformatted text preview: Lecture Lecture 4 CARBOHYDRATE METABOLISM (Part2) Fermentation Pentose Phosphate Pathway Chapter 17 (sections 1,2,3,5 and 23 (section 4) in Voet & Voet) BCH3120 General Intermediary Metabolism General Lecturer: Alfred Aziz, PhD Nutrition Research Division, Health Canada e-mail: [email protected] alfred January 12, 2011 Outline Outline 1. 2. 3. 4. Review of Glycolysis of Glycolysis Fermentation Pentose Phosphate Pathway Ph Conclusion 1. Review of Glycolysis 1. Review of Glycolysis 1. 2. 3. 4. Glycolysis is a series of reactions in which one molecule of glucose is degraded to two molecules of pyruvate, pyruvate, yielding biologically useful energy in the form of ATP and NADH Glycolysis is regulated at the level of the three irreversible reactions catalyzed by: hexokinase, by: phosphofructokinase and pyruvate kinase phosphofructokinase-1, and pyruvate kinase Carbohydrates other than glucose also feed into the glycolytic pathway Rapidly proliferating cells have an alternative glycolytic pathway that generates pyruvate through a reaction not catalyzed by pyruvate kinase The Enzymatic Reactions of Glycolysis Preparatory Phase Pay-off Phase Catabolic Catabolic Fates of Pyruvate 2 Pyruvate Ways to regenerate NAD+ But this requires requires oxygen! 2. Fermentation (A (Anaerobic Metabolism) What is FERMENTATION? Fermentation: Energy-yielding anaerobic breakdown of a nutrient molecule, such as glucose, without the net oxidation yields lactate the net oxidation; yields lactate, ethanol, or some other simple product Lactate Lactate dehydrogenase ΔG’˚ = -25.1 kJ/mol Fig. 17-24 Review [NAD+] can limit the rate of the rate of glycolysis glycolysis • Net yield of ATP per molecule of glucose is 4-2 = 2 4• NADH yield is 2 Fermentation? Fermentation? - - IN MUSCLE?? Lactate dehydrogenase Yes! Glucose breakdown with no net oxidation no What happens to the lactate exported from hypoxic muscle? • Hypoxia in muscles happens during high intensity supply activity, such as sprinting, when O2 supply can’t keep with the exercising muscle • Lactate travels in the blood to the liver where it converted to glucose through gluconeogenesis converted to glucose through gluconeogenesis. • The glucose is exported from the liver back into the blood blood to the muscle where it can support the anaerobic glycolysis. glycolysis. • This is referred to as the ‘Cori cycle’. ‘Cori The Cori cycle: Lactate produced by muscle glycolysis is transported by by the bloodstream to the liver, where it is converted to glucose to co to by gluconeogenesis. The bloodstream carries the glucose back to the muscles where it can be oxidized or stored as glycogen. Ethanol, rather than lactate, is the reduced product in is the reduced product in Ethanol Fermentation Yeast and other microorganisms polarizes the carbonyl oxygen of acetaldehyde, allowing transfer of hydride ion (red) from the of a hydride ion (red) from the reduced cofactor, NADH. Fig. 17-30 Before Before the biochemistry was understood, anaerobic metabolism was exploited in anaerobic metabolism was exploited in societies societies worldwide… • Leavened bread: CO2 from CO Brewer’s Yeast pyruvate decarboxylase causes the bread to rise bread to rise. * • Beer • Wines including champagne • Ethanol as fuel * Note that pyruvate decarboxylase is absent from vertebrate tissues, and and in other organisms that carry out lactic acid fermentation. Review Catabolic Fates of Pyruvate, the product of Glycolysis: Catabolic 3. The Pentose Phosphate Pathway The Pentose Phosphate Pathway • A pathway that serves to pathway that serves to interconvert interconvert hexoses and pentoses • Is a source of reducing equivalents and pentoses for biosynthetic processes and redox reactions • Is present in most organisms • Also called the phosphogluconate phosphogluconate pathway and the hexose monophosphate hexose shunt Oxidative Phase 6CH OPO 2 2 3 5 H 4 OH O H OH 3 H 6-Phosphogluconolactonase Glucose-6-phosphate Dehydrogenase 6 CH OPO 2 2 3 + NADPH + H 5 + O OH NADP H 2 OH H2 O H+ H 1 4 H OH H OH O H 1 3 H 2 OH O O 1C HC 2 OH HO 3CH HC O H 4 HC 5 OH CH2OPO32 6 glucose-6-phosphate h 6-phoshogluconolactone h 6-phosphogluconate h Glucose-6-phosphate Dehydrogenase catal catalyzes oxidation of the aldehyde (hemiacetal), the aldeh (hemiacetal) at C1 of glucose-6-phosphate, to a carboxylic acid in ester linkage (lactone) acid, in ester linkage (lactone). NADP+ serves as electron acceptor. Oxidative Phase (cont’d) O O 1C HC 2 OH Phosphogluconate Dehydrogenase NADP+ HO 3CH HC O H 4 HC OH CH OH NADPH + H+ 1 2 CO 2 HC CO2 OH HC OH 3 5 4 6 5 CH2OPO32 6-phosphogluconate CH2OPO32 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. NAD+ & NADP+ differ only Nicotinamide in the presence of an extra Adenine Dinucleotide phosphate on the O adenosine ribose of adenosine ribose of NADP+. O P O CH2 NH2 + N O H OH OH NH2 O N N O P nicotinamide H H O C H 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. H O O H N N O esterified to Pi in NADP+ H H OH adenine H CH2 OH Epimerase inter-converts stereoisomers ribulose-5P and xylulose-5-P. Ribulose Ribulose 5-phosphate Epimerase 3-epimerase HO Isomerase Isomerase converts the 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 product. Both reactions are reversible. H CH2OH C C C O C H C OH xylulose-5phosphate OH H C CH2OPO32 O H CH2OH OH HC CH2OPO32 O H C OH ribulose-5H phosphate Isomerase phosphate Ribose C OH C OH 5-phosphate H isomerase CH2OPO32 ribose-5ib phosphate NONNON-OXIDATIVE PHASE recycles pentose phosphates recycles pentose phosphates to to G6P First step: Non-oxidative phase occurs when there is no great need for ribose, but still need for NADPH NADPH The nonThe non-oxidative reactions of pentose phosphate pathway 1st Transketolase Reaction CH2OH Transketolase C CH2OH HC HO C O H C HO C H H C OH H C OH H C OH + CH2OPO32 CH2OPO32 HC H C O OH + CH2OPO32 H C OH H OH C H O O C OH H C OH CH2OPO32 xyluloseriboseglyceraldehyde- sedoheptulose5-phosphate 5-phosphate 3-phosphate 7-phosphate Transketolase transfers a 2-C fragment from xylulose-5-P to either ribose-5-P or erythrose-4-P. Transfer of the 2-C fragment to the 5-C aldose ribose-5phosphate yields sedoheptulose-7-phosphate CH2OH C HO Transaldolase O H2C CH C HC OH HC OH HC OH H2C HC O HC + OPO32 O HC OH HC OH HC OH HO H2C OPO32 H2C OH O CH HC + OPO32 OH HC OH H2C OPO32 sedoheptulose- glyceraldehyde- erythrose- fructose3-phosphate 4-phosphate 6-phosphate 7-phosphate Transaldolase catalyzes transfer of a 3-C dihydroxyacetone moiety, from sedoheptulose-7-phosphate to glyceraldehyde-3phosphate. . 2nd Transketolase Reaction Transfer of the 2-C fragment to the 4-C aldose erythrose4-phosphate yields fructose-6-phosphate. The The importance of Thiamine Pyrophosphate as a carrier of active acetaldehyde groups carrier of active acetaldehyde groups TCA TCA cycle (Citric acid, or Krebs cycle) 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) 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. Thi This H+ transfer is promoted by a Glu residue Gl adjacent to the pyrimidine ring. Transketolase Reaction Transketolase Reaction 2-carbon fragment Bound to TPP (carbanion) Transketolase Reaction (cont’d) Transaldolase Reaction Transaldolase Reaction Transaldolase Reaction (cont Transaldolase Reaction (cont’d) The nonThe non-oxidative reactions of pentose phosphate pathway The balance sheet below summarizes flow of 15 C atoms through Pentose Phosphate Pathway reactions by which 5-C sugars are converted to 3C 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. The nonThe non-oxidative reactions of pentose phosphate pathway Balanced Equation for PPP 3 Glucose 6-phosphate + 6 NADP+ + 3 H2O 2 Fructose 6-phosphate + Glyceraldehyde 3-phosphate + 6 NADPH + 3 CO2 + 6 H+ Fructose Glyceraldehyde NADPH CO Regulation Regulation of PPP NADPH regulates partitioning of G6-P between glycolysis and PPP G6- Take Home messages Take Home messages 1. 2. 3. 4. 4. 5. Fermentation is an anaerobic process that produces either lactate or ethanol and regenerates NAD+ ith an NAD necessary PPP consists of an oxidative phase and non-oxidative nonphase. The oxidative phase uses G6P to produce NADPH required for biosynthetic processes and protection against reactive oxygen species against reactive oxygen species The non-oxidative phase converts most of the pentose nonphosphate formed in the oxidative phase into the glycolytic intermediates F6P and G3P glycolytic intermediates F6P and G3P. The rest of the pentose phosphate is used in the synthesis of DNA, RNA, coenzymes and nucleotides) Metabolism and Medicine: A Clinical Case-Study Clinical Case A 10 yr old child of Middle-Eastern descent arrives in your medical clinic Middlecomplaining of fatigue and weakness of days duration complaining of fatigue and weakness of 2 days duration. On physical exam he appears jaundiced and has an enlarged spleen. spleen. He He has no recent medical history of illness and he hasn’t taken any medicine for the last 3 months. You order serum analyses, which show low hemoglobin, low hematocrit, You order serum analyses, which show low hemoglobin, low hematocrit, and and elevated indirect bilirubin. bilirubin. You ask for his recent diet history. You begin to suspect that this patient is suffering from an X-linked recessive disorder triggered by the consumption of fava beans in falafel , and you co you decide to consult dietitian about alternative regimens that will not cause his current symptoms. Read Read pages 867-68: 867Why wouldn’t Pythagoras eat Falafel? Answer: • Due to glucose-6-phosphate dehydrogenase deficiency • Affects up to 25% of people in tropical Africa, parts of the middle east and SE Asia • Fava beans contain divicine, which causes increased ROS production hemolysis kidney production, hemolysis, kidney failure… and sometimes death. Review Review questions – Applying what you know 1. What are the control points of glycolysis? Give an inhibitor and/or activator of t particular reaction. 2. Pyruvate and ATP are the end products of glycolysis. In active muscle cells Pyruvate and ATP are the end products of glycolysis In active muscle cells pyruvate is converted to lactate. Lactate is transported in the blood to the liver where it is recycled by gluconeogenesis to glucose, which is transported back to muscle for additional ATP production. Why don’t active muscle cells export pyruvate, which can also be converted to glucose by gluconeogenesis? 3. Which enzyme is an acceptor of the phosphate group from phosphoenolpyruvate Which enzyme is an acceptor of the phosphate group from phosphoenolpyruvate in cancerous cells: a. Pyruvate kinase M1 b. Pyruvate kinase M2 c. Enolase d. Phosphoglycerate mutase e. None of the above 4. True or false? The pentose phosphate pathway generates NADH for reductive biosyntheses. The pentose phosphate pathway interconverts Trioses, tetroses, pentoses, hexoses and heptoses. Thank you for your attention ...
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This note was uploaded on 12/18/2011 for the course BCH 3120 taught by Professor Mary-ellenharper during the Winter '11 term at University of Ottawa.

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