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BSC 300, Week 6

by: Ashley Bartolomeo

BSC 300, Week 6 BSC 300

Ashley Bartolomeo
GPA 3.9

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Notes on glycolysis part 2
Cell Biology
John yoder
Class Notes
Cell, Biology
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This 4 page Class Notes was uploaded by Ashley Bartolomeo on Sunday September 25, 2016. The Class Notes belongs to BSC 300 at University of Alabama - Tuscaloosa taught by John yoder in Fall 2016. Since its upload, it has received 6 views. For similar materials see Cell Biology in Biology at University of Alabama - Tuscaloosa.


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Date Created: 09/25/16
Glycolysis: Energy Payoff Stage Steps 6-10: energy is extracted from G3P and its metabolites in the form of 2 NADH and 4 ATP molecules What makes this energy capture possible? Two sequential enzymatically catalyzed reactions couple carbon oxidation to generate these energy carriers 1. Oxidation of G3P to 1,3-bisphosphoglycerate is energetically favorable and is coupled to reduction of NAD+ to NADH and phosphorylation of G3P 2. Hydrolysis of 1,3-bisphosphoglycerate is energetically favorable and is coupled to phosphorylation of ADP 6. In a two-step reaction NAD+ is reduced to NADH when glyceraldehyde 3- phosphate is oxidized and phosphorylated to 1,3-bisphosphoglycerate  Dehydrogenase enzymes oxidize and reduce cofactors or coenzymes  NAD+ is a glyceraldehyde phosphate dehydrogenase coenzyme  NAD+ reduced to NADH in numerous cellular reactions  NADH donates electrons to various molecules. For example, to the electron transport chain in the mitochondria  The aldehyde is oxidized to a carboxylic acid through reduction of NAD+ to NADH o G= -12 kcal/mol  Both G3P and NAD+ are substrate and the enzyme forms a transient covalent bond with the sugar substrate – a high energy thioester bond; recall high energy bonds are unstable  The high energy thioester bond is highly unstable and is immediately replaced by a free molecule of inorganic phosphate forming 1,3- bisphosphoglycerate  This new bond is also high energy, but less so than the thioester – making its formation favorable NAD+ = Nicotinamide adenine dinucleotide  Receives 2 electrons and 1 proton (here from glyceraldehyde 3- phosphate)  The other proton is released  ATP formation from NADH is indirect o Electrons are passed through electron transport chain: oxidative phosphorylation 7. ATP is generated when 1,3-bisphosphoglycerate is converted to 3- phosphoglycerate by phosphoglycerate kinase  Substrate-level phosphorylation occurs when ATP is formed by a kinase enzyme  Notice the generation of ATP is driven by the reduction of the coenzyme NAD+  Without NAD+, glycolysis cannot generate ATP 8-10) 3-phosphoglycerate is converted to pyruvate via three sequential reactions. In the final irreversible step pyruvate kinase phosphorylates ADP generating 2 additional molecules of ATP per glucose molecule  Glycolysis can generate a net of 2 ATPs for each glucose  Glycolysis occurs in the absence of oxygen, it is an anaerobic pathway  The end product, pyruvate, can enter aerobic or anaerobic catabolic pathways Energy Coupling: Transfer Potential  ATP formation is only moderately endergonic  G’ of many phosphomolecules is more negative than that of ATP  this higher transfer potential means such molecules can easily phosphorylate ADP Metabolism Anaerobic Oxidation of Pyruvate: The Fermentation Process Fermentation restores NAD+ from NADH  Under anaerobic conditions, glycolysis depletes the supply of NAD+ by reducing it to NADH  If NAD+ is absent glycolysis cannot continue  Certain cell types can operate in the absence of oxygen by maintaining a supply of NAD+ through the process of fermentation  Lactate is quickly released and absorbed by liver cells where a pathway called the Cori Cycle oxidizes it back to pyruvate which is then reduced to glucose  In yeast and other microbes, pyruvate is reduced and converted to ethanol  In muscle and tumor cells pyruvate is reduced to lactate  Fermentation is inefficient with only about 8% of the energy of glucose captured as ATP Cancer Metabolism The Warburg Effect: Hijacking Glycolysis  Most cancer cells exhibit up to 200x the glycolytic rate of normal cells. They produce the bulk of their ATP via glycolytic substrate level phosphorylation and NAD+ recycling (lactic acid fermentation)  Reason is unclear, but likely related to: o Mitochondrial damage in cancer cells o Low oxygen levels in tumors o Many proliferating cells preferentially perform aerobic glycolysis:  ATP production is inefficient, but rapid  Glucose metabolites are a readily available source for synthesis of other building blocks (nucleic and amino acids)  2008 Nature paper showed most cancers express the fetal form of pyruvate kinase (PKM2)  pyruvate kinase catalyzes the third irreversible step of glycolysis  PKM2 has a higher Km than adult PK, therefore limits production of pyruvate and expands the glycolytic metabolic intermediates  PKM2 has roles beyond PEP conversion to pyruvate: o Enhances cell division by stimulating a critical proliferation genetic pathway o Promotes dedifferentiation by activating a gene involved in stem cell fate o Enhances glucose uptake by stimulating expression of the glucose transporter GLUT4 Metabolism Reducing Power  ATP is the principle energy source for anabolic processes  But complex biomolecules (proteins, fats, nucleic acids) are highly reduced compared to the metabolites from which they are built. Therefore, the synthesis of macromolecules requires the reduction of metabolites  The molecule NADPH donates electrons to build large biomolecules o NADPH is a non-protein coenzyme similar to NADH o The supply of NADPH represents the cell’s reducing power o NADP+ is formed by phosphate transfer from ATP to NAD+ o It is then reduced to NADPH  NADPH and NADH have different metabolic roles  They are coenzymes for different enzymes  Anabolic pathways (like fatty acid synthesis) use NADPH  Enzymes that function in catabolic processes use NAD+  The enzymes transhydrogenase catalyzes the transfer of hydrogen atoms from one cofactor to the other o NADPH is favored when energy is abundant o NADH is used to make ATP when energy is scarce The Pentose Phosphate Pathway  Parallel anabolic pathway to glycolysis  Also produces NADPH by oxidation of glucose-6-phosphate and its metabolites  And yields the 5 carbon ribose-5-phosphate used to make nucleic acids  As well as a 4 carbon sugar necessary for some amino acids Metabolism Metabolic Regulation  Cellular activity is regulated as needed  Regulation may involve controlling key enzymes of metabolic pathways  Enzymes are controlled by alteration in active sites o Covalent modification of enzymes regulated by protein kinases and other enzymes o Allosteric modulation of enzymes regulated by compounds binding to allosteric sites  Feedback inhibition, the product of the pathway allosterically inhibits one of the first enzymes of the pathway. E.g., ATP and phosphofructokinase Separating Catabolic and Anabolic Pathways  Metabolites of many catabolic pathways can be used to regenerate stores of the starting material  Example, when blood sugar levels drop, the pancreas stops making insulin and releases glucagon; stimulating liver cells to 1) convert glycogen to glucose (glycogenolysis) and generate glucose from various metabolites including pyruvate – gluconeogenesis  Several of the same enzymes used for glycolysis also drive gluconeogenesis  Some enzymatic steps are irreversible due to large G’ (e.g., hexokinase, phosphofructokinase and pyruvate kinase)  Reversing such steps of catabolic pathways requires catalysis by different enzymes  Glucagon signaling regulates many enzymes both positively and negatively to shut down glycolysis and stimulate gluconeogenesis  Example: glucagon leads to phosphorylation and inactivation of phosphofructokinase  And the activating phosphorylation of fructose bisphophatase – stimulating gluconeogenesis  At the same time, glucagon inhibits production of the phosphofructokinase activator fructose 2,6 bisphosphate  Together, this eliminates activity of phosphofructokinase  And since its product, fructose 1,6-bisphosphate, is an allosteric activator of pyruvate kinase, this terminal enzyme is also down- regulated


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