Topic 10: CELLULAR RESPIRATION (lectures 14-16)OBJECTIVES:1. Know the basic reactions that form glycogen from glucose and degrade glycogen toglucose.2. You do not need to memorize every step of the glycolytic and glycogenolyticpathways. However, you should be able to describe the processes taking placeduring the activation, oxidative and energy yielding steps of glycolysis and howglycogenolysis differs from glycolysis in this regard.3. Know what is the fate of glycolytically produced NADH under aerobic vs. anaerobicconditions.4. Understand the structure of the mitochondrion in relation to the processes that aretaking place during the Krebs cycle, electron transport and oxidativephosphorylation.5. You do not need to memorize every step of the Krebs cycle. However, in terms of theKrebs cycle you should know what is ultimate yield fate of pyruvate carbons, the ATPyield by substrate level phosphorylation per pyruvate molecule and the yield ofreduced coenzyme per molecule of pyruvate.6. Understand how and where electrons enter the electron transport chain, where H+’sare translocated and how the flow of H+’s down their chemical concentrationgradient produces ATP (this should include an understanding of the fate of NADHand FADH2).7. Be able to compare the ATP yields of anaerobic vs. aerobic glucose breakdown.Overview- cellular respiration (aka cellular energy metabolism) takes place in twointracellular compartments, fig. 9.6; in the cytoplasm (glycolysis and glycogenolysis)and in the mitochondrion (Krebs Cycle).Because glycolysis does not involve oxygen it is often referred to as anaerobic (nooxygen) energy metabolism whereas the Krebs Cycle (and its associated electrontransport chain) are linked to oxygen utilization and therefore constitute aerobic energymetabolism. Both anaerobic and aerobic pathways result in ATP production bysubstrate level phosphorylation and/or chemiosmotic (oxidative) posphorylation.Glycolysis- the breakdown of glucose into two molecules of the 3-carbon compoundpyruvate with the resulting production of ATP as a result of substrate-levelphosphorylation (2 ATP’s/ molecule of glucose degraded). Net reaction:Glucose + 2 ADP + 2 NAD+ 2 pyruvate + 2 ATP + 2 NADH + 2H+Glycogenolysis- the breakdown of glucose units derived from glycogen into twomolecules of the 3-carbon compound pyruvate with the resulting production of ATP as aresult of substrate-level phosphorylation ( 3 ATP’s/ molecule of glucose degraded). Netreaction:1Glycogen(n glucose units) + 3 ADP + 2 NAD+ Glycogen(n glucose units-1) + 2 pyruvate + 3 ATP + 2 NADH + 2H+Digression on the biosynthesis and degradation of glycogen: glycogen is stored attimes of energy surplus. During the day it accumulates in muscle and liver primarily. Atnight glycogen is burned as a fuel. Biosynthesis-Glucose + ATP Glucose-6-P + ADP Glucose-6-P Glucose-1-PGlucose-1-P + UTP UDPGlucoseUDPGlucose + Glycogen(n glucose units) Glycogen(n glucose units + 1) + PiBreakdown-Glycogen(n glucose units) Glycogen(n glucose units-1) + Glucose-1-PThe basic pathway of glycolysis is present in prokaryotic cells and in fungi, plant andanimal cells. It can be divided into three basic phases (note your text only mentions twophases; I prefer to divide the so-called “energy pay-off phase into two phases):Fig. 9.8(1) ATP investment phase- ATP is used to “activate” glucose and a glucosederivative(2) Oxidative phase- one of the glucose breakdown products is oxidized by NAD+yielding NADH(3) ATP yielding phase- some of the energy associated with broken covalent bondsis trapped in the form of ATPGlycolysis- basic enzymatic steps (Fig. 9.9)(1) Hexokinase-activation(2) Phosphoglucoisomerase- (3) Phosphofructokinase- continued activation(4) Aldolase- cleavage of 6-carbon cmpound into two 3 carbon compounds(5) Isomerase-(6) Triose phosphate dehydrogenase- oxidative step(7) Phosphoglycerokinase- ATP yielding by substrate level phosphorylation(8) Phosphoglyceromutase-(9) Enolase-(10) Pyruvate kinase- ATP yielding by substrate level phosphorylation(NOTE: you are required to know reactions 1, 3, 6, 7 and 10)2Glycogenolysis - same basic steps except (1) hexokinase not necessary and (2)glucose-1-P is converted to glucose-6-phosphateGlycogen(n glucose units) Glycogen(n glucose units-1) + Glucose-1-PGlucose-1-P Glucose-6-P Fructose-6-P Pyruvate (phosphoglucomutase)Glycolysis viewed in phases (again):Activation:Hexokinase- glucose + ATP glucose-6-P + ADP - ATPPhosphofructokinase- fructose-6-P + ATP Fructose-1,6-BP + ADP - ATPOxidative step:Triose phosphate dehydrogenase-2 3-glyceraldehyde-P + 2 NAD+ + 2 Pi 2 1,3-diphosphoglycerate + 2 NADH + 2 H+ + 2 NADHATP yielding:Phosphoglycerokinase-2 1,3-diphosphoglycerate + 2 ADP 2 3-phosphoglycerate + 2 ATP + 2 ATPPyuvate kinase-2 phosphoenolpyruvate + 2 ADP 2 pyruvate + 2 ATP + 2 ATP NET: 2 ATP + 2 NADHWhat about the NADH molecules? For glycolysis to function there must be continued supply of NAD+ for the oxidative step;under aerobic conditions the electrons from NADH are transferred to the mitochondrionwhere they eventually reduce oxygen to water (and also contribute to ATP production).We’ll talk about all this soon.What happens if there is no oxygen around? Anaerobic glycolysis takes place.Fig. 9.17In most animals the enzyme lactate dehydrogenase (pyruvate + NADH lactate +NAD+) produces lactate (which accumulates) and NAD+ which is then used in theoxidative step.In yeast the enzyme alcohol dehydrogenase ( acetaldehyde + NADH ethyl alcohol +NAD+) produces ethyl alcohol (which accumulates) and NAD+ which is then used in theoxidative step.Fig. 9.18 – In presence of oxygen pyruvate is transported into the mitochondrion3Refresh your memory with the structure of the mitochondrion.Krebs Cycle- the complete catabolism of glucose to CO2Under aerobic conditions pyruvate is transported into the mitochondrion (fig. 9.10); alarge protein complex called pyruvate dehydrogenase consisting of several
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