Lehninger Chapter 14: Glycolysis, Gluconeogenesis, Pentose Phosphate Pathways PDF

Carbohydrate Metabolism Lehninger, Chapter 14  Fates of Glucose Glycogen, starch, sucrose Synthesis of structural polymers Storage Oxidation via pentose phosphate pathway...

Carbohydrate Metabolism Lehninger, Chapter 14  Fates of Glucose Glycogen, starch, sucrose Synthesis of structural polymers Storage Oxidation via pentose phosphate pathway Glycolysis - Generates energy via oxidation of glucose - Short-term energy needs  Glycolysis Phase 1: - Preparatory (Investment) Phase Glucose is phosphorylated and cleaved to form 2 molecules of glyceraldehyde-3-phosphate. Two ATP molecules invested. Phase 2: - Payoff Phase 2 Glyceraldehyde-3-phosphates are converted to 2 pyruvates. Payoff of 4 ATP and 2 NADH molecules  Fates of Pyruvate (More Later) Aerobic Conditions: pyruvate is oxidized to yield the acetyl group of acetyl-coenzyme A. That acetyl group is then oxidized completely to CO2 by the citric acid cycle Pyruvate → Acetate → CO2 + H2O NADH + O2 + 2H+ → NAD+ + H2O Anaerobic Conditions: Aerobic conditions Hypoxic or anaerobic conditions Animals and microbes convert pyruvate into lactate when O2 is lacking Through transforming pyruvate into lactate, NADH is be oxidized to replenish NAD+ Replenishing NAD+ allows glycolysis (ATP production) to continue for a short time Summary of Glycolysis and Energetics Summary of Glycolysis and Energetics Summary of Glycolysis and Energetics Glycolysis Phase 1: - Preparatory (Investment) Phase Glucose is phosphorylated and cleaved to form 2 molecules of glyceraldehyde-3-phosphate. Two ATP molecules invested. Glycolysis: Step 1 Glucose-6-phosphate is produced from blood glucose by hexokinase Glucose phosphorylated at C-6 Keeps glucose in the cell!! Charged. Exergonic → irreversible Uses/stores energy of ATP First ATP invested! Hexokinase allosterically regulated by glucose-6-phosphate Mg2+ lowers activation energy (Ch #6 Biochem I – Enzyme Mechs) Glycolysis: Step 2 Isomerase converts glucose-6- phosphate to fructose-6- phosphate 6-membered ring to 5 Reversible (small ∆G) Mg2+ lowers activation energy (Ch #6 Biochem I – Enzyme Mechs) Glycolysis: Step 3 Fructose-6-phosphate is phosphorylated by phosphofructokinase-1 (PFK-1) Fructose phosphorylated at C-1 Symmetrical ☺ now, 1,6-biphos. Keeps all later products in the cell!! Charged. Exergonic → irreversible Uses/stores energy of ATP Second ATP invested! Mg2+ lowers activation energy (Ch #6 Biochem I – Enzyme Mechs) Glycolysis: Step 4 Fructose-1,6-bisphosphate is cleaved into two, 3-C molecules by aldolase: 1. Glyceraldehyde-3-phosphate (G-3-P) ☺ 2. Dihydroxyacetone phosphate (DHAP)  Gotta change this! 3. FROM HERE ON, EACH REACTION IS HAPPENING TWICE! (except Step 5 – ha!) Glycolysis: Step 5 Dihydroxyacetone phosphate (DHAP) from Step 4 is converted to Glyceraldehyde-3-phosphate (G-3-P) ☺ through triose phosphate isomerase Now we have two G-3-Ps from 1 glucose! Prep. Phase Done! Glycolysis Phase 2: - Payoff Phase 2 molecules of glyceraldehyde- 3-phosphate are converted to two molecules of pyruvate. Four ATP molecules and 2 NADH molecule produced.  Glycolysis: Step 6 Glyceraldehyde 3-phosphate is phosphorylated/oxidized to 1,3-Bisphosphoglycerate NAD+ is reduced to NADH 1,3-Bisphosphoglycerate has a phosphate group we can cleave, release energy to drive reactions (remember reaction coupling?). Also storing energy in the NADH.  Glycolysis: Step 7 ATP is synthesized (2 ATPs come back → net = zero at this point) as phosphoglycerate kinase transfers the phosphate group from 1,3-Bisphosphoglycerate to ADP 1,3-Bisphosphoglycerate → 3-Phosphoglycerate + ATP  Glycolysis: Step 8 3-Phosphoglycerate → 2-Phosphoglycerate Just moving a phosphate group around  Glycolysis: Step 9 2-Phosphoglycerate doesn’t have a lot of energy compared to another compound: Phosphoenolpyruvate So let’s convert 2-Phosphoglycerate to Phosphoenolpyruvate (PEP) via enolase  Glycolysis: Step 10 Phosphoenolpyruvate → Pyruvate + ATP Via pyruvate kinase Here is where we gain 2 ATP and now net two ATP. THIS IS THE PAYOFF! Irreversible Allosterically Activated by: AMP and fructose-1,6-biphosphate Allosterically Inhibited by: ATP and acetyl-CoA Note the Mg2+ Summary of Glycolysis and Energetics  Summary of Glycolysis and Energetics Step Reactant Product(s) Enzyme In Out 1 1 Glucose Glucose 6-phosphate Hexokinase ATP 2 1 Glucose 6-phosphate Fructose 6-phosphate Phosphohexose Isomerase 3 1 Fructose 6-phosphate Fructose 1,6-bisphosphate Phosphfructokinase-1 (PFK-1) ATP 4 1 Fructose 1,6-bisphosphate DHAP + Glyceral. 3-Phos. Aldolase 5 1 Dihydroxyacetone phosphate Glyceraldehyde 3-phosphate Triose Phosphate Isomerase 6 2 Glyceraldehyde 3-phosphate 1,3-Bisphphosphoglycerate Glyceral. 3-phos. Dehydrogenase 2 NADH 7 2 1,3-Bisphphosphoglycerate 3-Phosphoglycerate Phosphoglycerate Kinase 2 ATP 8 2 3-Phosphoglycerate 2-Phosphoglycerate Phosphoglycerate Mutase 9 2 2-Phosphoglycerate Phosphoenolpyruvate Enolase 10 2 Phosphoenolpyruvate Pyruvate Pyruvate Kinase 2 ATP Reaction Type Standard Free Energy Note 1 1 ATP energy input Exergonic (& regulated) Induced fit 2 1 Isomerization Delta-G small 3 1 ATP energy input Exergonic (& regulated) PFK-1: regulatory enzyme 4 1 Cleavage Endergonic 5 1 Isomerization Delta-G small 6 2 Aldehyde oxidized Delta-G small Energy in P-group and NADH 7 2 P-bond cleavage Exergonic Energy into ATP 8 2 Isomerization Delta-G small 9 2 Dehydration Delta-G small 10 2 P-bond cleavage Exergonic (& regulated) Energy into ATP Glycolysis and Cancer Other Pathways Can Lead to the Glycolysis Pathway Multiple feeder pathways exist for glycolysis: Glucose cleaved from glycogen and starch (polysaccharides) to Step 1 give glucose-1- Step 2 phosphate Dietary disaccharides Step 3 and polysaccharides are hydrolyzed Step 5 Step 4  Polysaccharide Feed-in to Glycolysis Starch Glycogen Repeatedly! Hexokinase Glycolysis  Disaccharide Feed-in to Glycolysis trehalase Trehalose 2 glucose Monosaccharide Feed-in to Glycolysis  Fates of Pyruvate Aerobic Conditions: pyruvate is oxidized to yield the acetyl group of acetyl-coenzyme A. That acetyl group is then oxidized completely to CO2 by the citric acid cycle Pyruvate → Acetate → CO2 + H2O NADH + O2 + 2H+ → NAD+ + H2O Anaerobic Conditions: Aerobic conditions Hypoxic or anaerobic conditions Animals and microbes convert pyruvate into lactate when O2 is lacking Through transforming pyruvate into lactate, NADH is be oxidized to replenish NAD+ Replenishing NAD+ allows glycolysis (ATP production) to continue for a short time Molecule 1 Similar Precursors/Energy Molecule 1 Molecule 2 Molecule 2 Molecule 3 Molecule 3 Carbohydrate Biosynthesis Our Body Needs Glucose…all the time, constantly,…  And/or: oxaloacetate, glucogenic amino acids  Glycolysis and Gluconeogenesis Gluconeogenesis is not the reverse of glycolysis The three exergonic and highly regulated reactions in glycolysis are replaced by alternative reactions in the gluconeogenic pathway. The results of these reactions are similar intermediates  Gluconeogenesis Step 1: (Bypass of Glycolysis Step 10) Antiparallel to Glycolysis Step 10. Glycolysis: PEP → Pyruvate Gluconeo.: Pyruvate → oxaloacetate → PEP Complex and costly (1 ATP + 1 GTP) Exergonic (because of the above) NADH consumed in mitochondria, reformed in cytosol What do you think stimulates/inhibits these enzymes? Consider what we are doing… Gluconeogenesis Step 1: (Bypass of Glycolysis Step 10) Vitamin B7/H Heard of it? *Note: The malate-aspartate shuttle is a ridiculously complex way to transfer oxaloacetate out of the mitochondria and into the cytosol AND perform redox reactions of NADH → NAD+ / NAD+ → NADH The NAD+ in the cytosol can go to NADH during glycolysis and the NADH can donate its electrons in the ETC during the citric acid cycle. It’s all connected! *loses mind*  This just adds a step when we don’t have pyruvate. Lactate can oxidized to pyruvate. We just did Step 1 of gluconeogenesis (bypass of step 10 of glycolysis.) Steps 2-7 of gluconeogenesis are the reverse of steps 9-4 of glycolysis. We now move to step 8 of gluconeogenesis (Bypass of step 3 of glycolysis)  Simple hydrolytic reaction What does a phosphatase do? Fructose-1,6- biphosphate to fructose-6-phosphate What makes this exergonic? We just did Step 8 of gluconeogenesis (bypass of step 3 of glycolysis). Step 9 of gluconeogenesis is the reverse of step 2 of glycolysis. We now move to step 10 of gluconeogenesis (Bypass of step 1 of glycolysis)  Simple hydrolytic reaction What does a phosphatase do? Glucose-6-phosphate to glucose What makes this exergonic? Substrate is converted as it passes into the ER of liver and kidney. Later glucose released into blood. Notes on Precursors of Gluconeogenesis Molecule 1 Similar Precursors/Energy Molecule 1 Molecule 2 Molecule 2 Molecule 3 Molecule 3 Pentose Phosphate Pathway Parallel to glycolysis Anabolic (like gluconeogenesis) Occurs in the cytoplasm Generates NADPH (reducing agent/electron acceptor – 60% generated here) Generates pentoses as well as ribose 5-phosphate. A precursor for nucleotide synthesis → Ribonucleotides (RNA/DNA)  You will see this glutathione stuff again!! DRUG/TOXIN METABOLISM