Glycolysis in Introduction to Biochemistry - Lecture Slides | CHEM 3550, Study notes of Biochemistry

Download Glycolysis in Introduction to Biochemistry - Lecture Slides | CHEM 3550 and more Study notes Biochemistry in PDF only on Docsity! Chapter 17 Glycolysis Part I/II Mary K. Campbell Shawn O. Farrell http://academic.cengage.com/chemistry/campbell Spring 2011 HOMEWORK # 1, 2, 7- 9, 13 -17, 21, 30 – 32, 36, 37, 41, 42, 44, 46 Shutcss dm bhoodsbre an Anasrobic comcditbans Pyruvate | Lactate Aerobde comditions a, <Cheric 6 GLYCOLYSIS: KEY CONCEPTS • Glycolysis: Major pathway; 1st stage of glucose metabolism 1. Universal. 2. 10 enzyme-catalyzed reactions beginning with glucose (+ other hexoses). 3. Pathway Strategy: Glucose split in half, forming 2 molecules of ketoacid, Pyruvate. 7 GLYCOLYSIS: KEY CONCEPTS-2 Fates of Pyruvate 1. Aerobic Glycolysis: • Pyruvate converted to Acetyl-CoA  sent to CAC. 2. Anaerobic Glycolysis: • Pyruvate converted to lactic acid 3. Anaerobic alcoholic fermentation: • Pyruvate converted to ethanol. The 1st 5 Reactions of Glycolysis Step 1. Phosphorylation: Glucose converted to glucose-6-phosphate Step 2. Isomerization: glucose-6-phosphate converted to fructose-6-phosphate Step 3. Phosphorylation: fructose-6-phosphate to yield fructose-1,6-bisphosphate. Step 4. Cleavage: fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxy- acetone phosphate. Step 5. Isomerization: dihydroxyacetone phosphate converted to glyceraldehyde-3- phosphate 1st Phase: Glucose  GAP Step 1: Glucose phosphorylated (ATP) to give glucose-6-phosphate (Glu-6-P). • Reaction exergonic; coupled to ATP hydrolysis (Net ∆Go’ = -4.0 kcal/mol); • Enzyme: Hexokinase. Substrates: Glu, Fru and Man. • Chemical energy from ATP now in Glu-6-P**. • 12 Comments on the Hexokinase * • Hexokinase: exists in multiple forms (isozymes) in yeast and in mammalian cells. • Liver has ~ 3 different isoforms (hexokinases) plus a glucokinase form. • Glucokinase: most active at high blood glucose levels. • Has a higher Km value than “regular hexokinases”. • Allows liver to respond to large increases in blood sugar concentrations Fig 17.5 PFK exists in Many Isoforms** MsL Mog Heterogeneous forms M |M M |M M4 l4 Homogeneous forms @ Brooks/Cole, , Cengage Leaming Fig. 17-6, p. 501 Fig 17.6 Allosteric Effects on Phosphofructokinase 1st Phase: Glucose to GAP • Step 4: Fru-1,6-bisphosphate split into two 3-carbon fragments: 1) Dihydroxyacetone phosphate (DHAP) and 2) D-Glyceraldehyde-3-phosphate (GAP). • Enzyme: Aldolase 17.3 “Payoff phase” [Dividends Phase] of Glycolysis: Conversion Glyceraldehyde-3-P to Pyruvate Fig. 17-7, p. 503 Fig 17.7 2nd Half Glycolysis: Payoff (Dividends) Phase The 2nd five (5) Reactions of Glycolysis Step 6. Oxidation/phosphorylation of two (2) glyceraldehyde-3-phosphate to give two 1,3- bisphosphoglycerate. Step 7. Transfer of a phosphate group from 1,3- bisphosphoglycerate to ADP to give 3-phospho- glycerate (3-PGA). Step 8. Isomerization of 3-phosphoglycerate to give 2-phosphoglycerate (2-PGA). Step 9. Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate (PEP). Step 10. Transfer phosphate group from phospho- enolpyruvate to ADP to give Pyruvate. p. 504 Overall reaction: Electron transfer (oxidation) + phosphorylation; has slightly positive ∆G value (endergonic; ∆Go’ = 1.5 kcal/mol). Product: 1,3-bisPGA is a “high energy compound” Step 6: Oxidation & Phosphorylation Reactions 2nd Phase: GA3P  Pyruvate • Step 7: 1,3-bisphosphoglycerate converted to 3- phosphoglycerate (1st reaction producing ATP) • Two 1,3-bisphosphoglycerates transfer a phosphate group to 2 ADP  2 ATP. • Enzyme: phosphoglycerate kinase • Reaction exergonic overall: ∆Go’ = - 4.5 kcal/mol. • i.e., Sum of the endergonic phosphorylations of ADP with exergonic hydrolyses of phosphate anhydride • Substrate-level phosphorylation Step 7: 1,3-PGA > 3-PGA O O | C—O—P—O- | | lo we HCOH + ADP O Phosphoglycerate | kinase H,C -—O— | —=0° O- 1,3-bisphosphoglycerate 0 | C—O | HCOH + ATP O | H.C —O— 1 =) i. 3-Phosphoglycerate © Brooks/Cole, Cengage Leaming 2nd Phase (Cont.) • Step 10: Phosphenolpyruvates (PEP) transfers a phosphate group to ADP, producing 2 ATP and 2 Pyruvates. • Enzyme: Pyruvate kinase (PK) • Inhibited by ATP; if energy not needed, reaction is slowed/stopped. • Second substrate level phosporylation 31 Net 2 molecules of ATP and 2 NADH are produced for each Glucose split into 2 Pyruvates. Key Control Points in Glycolysis • Key Enzymes in pathway are sites of allosteric regulation/control: 1.Hexokinase 2.Phosphofructokinase 3.Pyruvate kinase • All 3 reactions exhibit large decreases in free energy (∆Go’). • More on regulation later! 35 Comments: Step #8 Step 8. Phosphoglycerate Mutase: 3-PGA 2-PGA • Mutase reaction also used to create the intermediate 2,3-Bisphophoglycerate (2,3-BPG) from 2-PGA. • In RBCs: one function of glycolysis is the production of 2,3-bisphosphoglycerate. • Recall: 2,3-BPG – an allosteric inhibitor of the oxygenation of hemoglobin. 17.4 METABOLISM OF PYRUVATE Cae nA NADH 1S higpbios pliers lvoe rane CEES) ese Siete + GlyocraldchydeS-phosphace A for enieys Fhosphogiveerae (3G) SuMesplregiveerace C2) Phrosphocnolpyrunae (PER) Fructose 6 plvosphiane 4 Preuctosc bt G-te gpeoaphcrne Dbydeoxyacetone phosphine (DHAPS yor rakbe harcieS-pieocpptiaa ce (Cry Pe cee ented baa Anmcrobic cemraclatices /\, givers inva Conversion of Biycorsbdclyd phosphate ve Pymuwate aac counted formarion Sf four mobecubes «af sor Fig. 17-2, p. 495 40 ANAEROBIC Pyruvate Metabolism • Anaerobic organisms/cells: grow in No/low O2 concentrations. • Fermentation: Extraction of energy from Pyruvate w/o O2 [electron acceptor]. • There are 2 different types of Fermentation: 1) Lactate Fermentation and 2) Alcoholic Fermentation. • Note: Other molecules act as the terminal electron acceptors to oxidize NADH back to NAD+ so metabolism can continue. 41 1. Lactate Fermentation • Lactate Fermentation: Pyruvate converted to Lactic acid • Enzyme = Lactate dehydrogenase. • 2 electrons plus 1 H+ transferred from NADH to the keto group on pyruvate. Pyruvate + NADH + H+ Lactic Acid + NAD+ • A pure Redox reaction. • Two NADH molecules (glycolysis) are oxidized back to NAD+ while reducing 2 Pyruvates to 2 lactates. (=0+ NADH + HY ——— [Ho—C—H MD) | Lactate CH, dehydrogenase CH, Pyruvate Lactate Brooks/Cole, Cengage Learning 45 *Lactate Fermentation – Muscle Fatigue* • During strenuous exercise, muscles using oxygen faster than blood can supply it [in oxygen debt (low intracellular O2 concen- tration)]. • Oxidative (aerobic) pyruvate metabolism is slowed, and Pyruvate is shunted to lactate formation. • Muscle cramps or burning are due to high [H+] from lactic acid in muscle cells. • Lactate is not a total “dead-end metabolite”: • Lactate can be used to make glucose via gluconeogenesis (liver; Chap 18). 3. Alcoholic Fermentation (Yeasts) • Two reactions convert Pyruvate to ethanol (Yeasts): 1. Decarboxylation of Pyruvate to Acetaldehyde 2. Reduction of Acetaldehyde to Ethanol • 1. Pyruvate decarboxylase: enzyme that catalyzes first reaction. • Requires Mg2+ and the cofactor, Thiamine pyro- phosphate (TPP) (Vitamin B1 = Thiamine) 3. Alcoholic Fermentation (Cont.) 2nd Reaction: Reduction of acetaldehyde to ethanol Acetaldehyde + NADH/H+  Ethanol + NAD+. • Enzyme: Alcohol dehydrogenase • Another NADH-linked dehydrogenase • Also a tetramer. • Note: NADH reoxidized back to NAD+; allows Glycolysis to proceed. 50 Ethanol Fermentation (Practical Usages) • Alcohol concentrations > 13% inhibit the growth of yeast; higher alcohol concentrations require distillation. • Baker’s yeast is used commercially to cause bread to rise [CO2] during fermentation of sucrose. 51 Human alcohol consumption (1) • Recall: Ingested alcohol is 1st oxidized to acetaldehyde by the enzyme, Alcohol Dehydrogenase (ADH) in liver. Ethanol + NAD+ Acetaldehyde + NADH + H+ • The 2nd oxidation step is catalyzed by Acetaldehyde DH to convert acetaldehyde to Acetic acid (acetate). Acetaldehyde + NAD+ Acetate + NADH/H+ 52 Human alcohol consumption (2)* • Acetic acid converted to Acetyl CoA; oxidized to CO2 and H2O by the CAC and respiration (~95% of ingested ETOH is oxidized by these steps; 5% is excreted [urine] or exhaled). • Normal humans metabolize ethanol at average rate of ~ 100 mg/kg of body weight per hour. • How fast one can metabolize alcohol depends on body weight, gender, race (??), variations in ADH activity (genotype / phenotype). • Genetic Variants of ADH exist (polymorphisms of very high **activity): ~85 % Asians; ~20 % Caucasians; ~10 % African-Americans; 0 % Native Americans. 55 Key Concepts: • Glycolysis: Overall process is exergonic • Some reactions are endergonic, including two phosphorylations of ADP. • Energy from exergonic reactions helps drive endergonic processes. • Standard free energy changes for Glycolysis Process (See Table 17.1) • Also: See added table below. Table 17.1 on ag ee Step Reaction Enzyme kJmol* ~— kcalmol-’ kJ mol" 1 Glucose + ATP > Glucose-6-phosphate + ADP Hexokinase/ Glucokinase -16.7 40 -33.9 Glucose-6-phosphate — Fructose-6-phosphate Glucose phosphate isomerase — +1.67 +04 -2.92 Fructose-6-phosphate + ATP — Fructose-1, Phosphofructokinase -14.2 3.4 -18.8 6-bisphosphate + ADP 4 Fructose-1,6-bisphosphate — Dihydroxyacetone Aldolase +23.9 +5.7 -0.23 phosphate + Glyceraldehyde-3-phosphate 5 Dihydroxyacetone phosphate + Glyceraldehyde- Triose phosphate isomerase 47.56 41.8 +4241 3-phosphate 6 2(Glyceraldehyde-3-phosphate + NAD* + P; > Glyceraldehyde-3-P 2(46.20) = 2(+1.5) 2(-1.29) 1,3-bisphosphoglycerate + NADH + H*) dehydrogenase 7 2(1,3-bisphosphoglycerate + ADP > Phosphoglycerate kinase 2(-18.8) 2(-4.5) 2(40.1) 3-Phosphoglycerate + ATP) 8 2(3-Phosphoglycerate — 2-Phosphoglycerate) Phosphoglyceromutase 2(+4.4) 2(+1.1) 2(+0.83) 9 2(2-Phosphoglycerate Enolase 2(+1.8) 2(+0.4) 2(+1.1) Phosphoenolpyruvate + H,O) 10 2(Phosphoenolpyruvate + ADP Pyruvate kinase 2(-31.4) 2(-7.5) 2(-23.0) Pyruvate + ATP) Overall Glucose + 2ADP + 2P; + NAD* > Lactate dehydrogenase -73.3 2 Pyruvate — 2ATP + NADH + H* 2(-25.1) 2(Pyruvate + NADH + H* — Lactate + NAD*) 193.5 Glucose + 2ADP + 2P, — 2 Lactate + 2ATP *AG” values are assumed to be the same at 25°C and 37°C and are calculated for standard-state conditions (1 M concentration of reactants and products pH 7.0). “AG values are calculated at 310 K (37°C) using steady-state concentrations of these metabolites found in erythrocytes. © Brooks/Cole, Cengage Leaming Table 17-1, p. 500 57 Summary: Glycolysis Energy Production Glucose + 2Pi + 2ADP + 2NAD +  2 Pyruvates + 2 ATP + 2 NADH + 2 H + + 2H2O Review: Steps in the digestion of carbohydrates. PM 0 Unnumbered figure pg Conceptsin Blacher le Dietary carbohydrates (starch, glycogen, sucrose, lactose) Mouth ¥ Salivary o-annlase Polysaccharides, sucrose, lactose, and maltose Stomach Small intestine Y ¥ Pancreatic o-amylase, maltase, sucrase, lactase Monosaccharides. Absorption through small intestine lining a Monosaccharides in bloodstream 60 61 Entry of Other Carbohydrates into Glycolysis Dietary Carbohydrates: ingested in three general forms: (1) Polysaccharides [starches/glycogen], (2) Dissacharides, and (3) Monosaccharides. Recall: • Starches [amylose/amylopectin] come from veggies and cereal grains. • Glycogen comes from animal muscle tissue. 62 Entry of Other Carbohydrates into Glycolysis* 1) Dietary Polysaccharides • Starches/glycogen broken down to Glucose (mouth) by Enzyme, Salivary Amylase. • Amylase: inactivated in acidic stomach [low pH]. • Acid hydrolysis (H+ catalyzed), of sugars continues in stomach. • Glucose: absorbed directly into the blood stream via the intestinal wall to be transported to skeletal muscle, heart and brain. 65 Entry of Other Carbohydrates into Glycolysis* 3) Ports of entry for other monosaccharides are shown in next Figure 1) Galactose at Glucose-6-PO4 2) Mannose at Fructose-6-PO4 3) Fructose at DHAP and Glycer- aldehye-3-Phosphate (GAP). • Note: only a couple of simple modification steps are required to get the precursors into the pathway [except for Galactose]. Fig. 18-23, p. 554 Entry of Mannose, Galactose & Fructose into Glycolysis Glycerol 67 Entry of non-Carbohydrates into Glycolysis Glycerol • Glycerol comes from the degradation of fats, TAGs and glycerophospholipids. • Not a carbohydrate. • Requires 2 simple phosphate transfers & an oxidative modification to convert its carbon skeleton into DHAP • Can enter 2nd half of Glycolysis.