Glycolysis a'k'a' EmdenMeyerhof Pathway

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Glycolysis (a.k.a. Emden-Meyerhof Pathway)

• Occurs in cytoplasm
• Begins w/ oxidation of glucose, 10
  enzyme-catalyzed rxns
• initial S input (ATP) required to phosphorylate
  Glu into Glu-6-P
• Glu-6-P rearranged to Fru-6-P
• another phosphorylation thus Fru-1,6-bisP
• Fru-1,6-bisP split into 2 3C cmpds

Glyceraldehyde phosphate
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Glycolysis

• Next set of rxns yields S pay-off
• glyceraldehyde P converted to 1,3
  bisphosphoglycerate
• 2 NAD molecules reduced to 2 NADH molecules
• 1,3 bisphosphoglycerate used to phosphorylate ADP
  to ATP (substrate-level phosphorylation)
• 2-step conversion to PEP
• another substrate-level phosphorylation ATP and
  pyruvate

Phosphoenolpyruvate (PEP)
1,3 bisphosphoglycerate
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Glycolysis Yields

• Total yield per glucose
• initial investment of 2 ATP
• 2 NADH molecules produced via oxidation of
  glyceraldehyde P
• 4 ATP molecules produced via substrate level
  phosphorylation
• 2 pyruvate molecules produced
• Net yield of 2 ATP, 2 NADH, 2 pyruvate per glucose

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Regulation of Glycolysis

• Phosphofructokinase catalyzes syn. of
  Fru-1,6-bisP from Fru-6-P
• uses ATP phosphorylate Fru-6-P
• 2 binding sites for ATP
• active site where ATP is
  
  reactant
• regulatory site where ATP
  inhibits
  enzyme

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Regulation of Glycolysis

• ? ATP inhibit phosphofructokinase by feedback
  inhibition or product inhibition
• allows cell to conserve Glu when ATP plentiful
• committed step b/c Fru-1,6-bisP only used in
  glycolytic rxns
• before this step, process can be stopped and
  intermediates can be used elsewhere

Fructose 1,6--bisphosphate
Regulatory site
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Krebs Cycle

• Krebs cycle completes oxidation of glucose
• a.k.a. citric acid cycle,
  tricarboxylic acid cycle
• 8 small carboxylic acids
  involved in redox rxns
  that produce CO2
• pyruvate reacts w/ end product
  of the pathway (oxaloacetate) to
  produce citrate

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Location of Krebs Cycle

• In eukaryotes, Krebs cycle occurs in
  mitochondrial matrix
• mitochondria double-membrane organelles w/
  folded inner
• invaginations called cristae
• space between membranes inter-membrane space
• space contained w/i inner membrane matrix

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Krebs Cycle

• Pyruvate reacts with coenzyme A (CoA) to produce
  acetyl CoA
• CoA enzyme cofactor that transfers acetyl
  groups (COCH3) groups to substrates
• catalyzed by pyruvate dehydrogenase
• 1 CO2 lost, NADH produced
• Acetyl CoA reacts w/ oxaloacetate (OAA) to form
  citrate
• 2 remaining carbons oxidized to CO2

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Krebs Cycle Yields

• Complete oxidation of 1 pyruvate (1 glucose 2
  pyruvate)
• 3 CO2, 4 NADH (1 from CoA, 3 from Krebs), 1
  FADH2, 1 guanosine triphosphate (GTP) (converted
  to ATP)
• Thus complete oxidation of 1 glucose via Krebs
• 6 CO2, 8 NADH, 2 FADH2, 2 ATP
• plus 2 NADH and 2 ATP per glucose via glycolysis
• thus 10 NADH, 2 FADH2 and 4 ATP via glycolysis
  and Krebs
• C6H12O6 10 NAD 2 FAD ? 6 CO2 10 NADH 2
  FADH2 4 ATP

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Regulation of Krebs Cycle

• Pyruvate dehydrogenase subject to feedback
  inhibition by ATP, acetyl CoA, and NADH
• activated by ? NAD, CoA, AMP
• thus, Krebs slows w/ ? ATP, NADH, acetyl
  coA ?
• speeds up w/ ? ATP and NADH
• ? ATP regulate enzyme that catalyzes formation
  of citrate

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What Happens to All the NADH and FADH2?

• NADH and FADH2 donate e- to electron transport
  chain (ETC)
• ATP is eventually generated
• NADH FADH2 O2 ? NAD FAD H2O ATP
• NADH 2.5 ATP, FADH2 1.5 ATP
• ETC series of increasingly oxidized molecules
  in inner mitochondrial membrane that accept e-
  (oxidation) from NADH and FADH2 and pass them to
  O2

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Electron Transport Chain

• ETC components organized from least oxidized to
  most oxidized
• e- proceeding down ETC release small amnt S, thus
  have less potential S as they are passed
• NADH donates e- to a flavin-containing protein at
  the top of the chain
• FADH2 donates e- to Fe-S complex that donates
  them to ubiquinone (coenzyme Q, Q)
• O2 final e- acceptor, highly oxidized

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Electron Transport Chain

• ETC pumps H from matrix into intermembrane space
• generates electrochemical gradient (proton-motive
  force)
• used to make ATP
• ETC proteins organized into 4 complexes anchored
  to inner membrane and 1 lipid-soluble coenzyme
  ubiquinone (Q)
• shuttles e- btwn protein complexes, carries H

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Electron Transport Chain

• Complex I (NADH dehydroxenase)
• NADH oxidized
• contains FMN (flavin mononucleotide), Fe-S
  centers
• txf to Q which carries H and e-
• 4 H pumped for every e- pair
• H pass directly through complex into
  intermembrane space
• Complex II (succinate dehydroxenase)
• FADH2 oxidized
• no H pumping

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Electron Transport Chain

• Complex III (cytochrome bc1 complex)
• oxidizes Q
• txf e- via Fe-S centers, cytochromes b, c1 and c
• cytochrome c mobile e- carrier btwn complex III
  and IV
• 4 H pumped for every e- pair
• Complex IV (cytochrome c oxidase)
• contains CuA and CuB (copper centers),
  cytochromes a and a3
• accepts e- from cytochrome c
• terminal oxidase (4 e- 4 H O2 ? H2O)
• H pass directly through complex

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ATP Synthase

• ATP synthase uses PMF to generate ATP
• electrochemical gradient due to complexes I, III,
  IV
• pH 8 in matrix, acidic in intermembrane space
• base region (F0) anchored in inner membrane
• knob region (F1) suspended in matrix
• stalk connects base and knob
• when protons diffuse through stalk
• potential S converted to kinetic S
• kinetic S causes knob to spin thus changes
  conformation so ADP is phosphorylated to ATP
  (converted to chemical bond)
• 2nd law of thermodynamics any nonuniform
  distribution of matter or S represents a source
  of S

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Oxidative Phosphorylation

• Oxidative phosphorylation formation of ATP by
  ATP synthase that is fueled by PMF generated by
  ETC
• ATP synthase produces 26 of 30 ATP produced per
  glucose
• aerobic respiration occurs when O2 is final e-
  acceptor
• all eukaryotes and many prokaryotes
• anaerobic respiration occurs when nitrate or
  sulphate are final e- acceptors in environments
  w/o O2
• some prokaryotes

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Making ATP Without Oxygen

• Without O2, OP not possible
• ETC stops moving e- and pumping H
• only source of ATP is glycolysis
• eventually all components of ETC become reduced
• thus, e- from NADH cannot be transferred
• glycolysis and Krebs cycle continue, eventually
  all NAD converted to NADH
• NADH builds up and NAD becomes depleted
• glycolysis cannot proceed without NAD to receive
  e-
• Fermentation allows glycolysis to continue

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Fermentation

• Fermentation generates ATP w/o O2 as final e-
  acceptor
• pyruvate or derivative of pyruvate accepts e-
  from NADH
• fermentation allows glycolysis to continue by
  recycling NAD
• thus glycolysis continues producing some ATP via
  substrate-level phosphorylation
• only 2 ATP produced per glucose

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Examples When Cells Use Fermentation

• Human muscle cells - during vigorous exercise
  muscle cells use and make ATP very rapidly
• O2 in muscle gets used for OP faster than our
  blood can deliver so O2 runs out
• muscle cells have enzymes for lactic acid
  fermentation thus generate ATP w/o O2 for short
  periods of time
• Human brain cells - lack enzymes for fermentation
• no means to recycle NAD, thus brain cells die
  more rapidly when deprived of O2 than muscle
  cells
• Yeast Cells - grow quickly and use up available
  O2 in high nutrient environment (i.e., juice)
• O2 used up thus switch to fermentation
• cells convert pyruvate to acetylaldehyde and give
  off CO2, creating bubbles in fermenting juice
• acetylaldehyde accepts e- from NADH and becomes
  ethanol, thus alcohol
• Bacteria and Archaea - many bacteria and archaea
  live in environments devoid of O2
• E. Coli in digestive tracts

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Catabolism of Biomolecules

• Catabolic pathways breakdown molecules, produce
  ATP
• break down carbohydrates/fats/proteins
• many catabolic pathways precede cell respiration
• for ATP production, cells 1st use carbohydrates,
  then fats, and lastly proteins

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Anabolism of Biomolecules

• Anabolic pathways syn. of larger molecules from
  smaller
• when ATP is abundant, anabolic pathways use
  intermediates from glycolysis and Krebs cycle for
  macromolecule synthesis
• pyruvate and lactate converted to glucose which
  is stored as starch/glycogen
• acetyl CoA is building block for FA syn., AA syn.
  from Krebs cycle
• Glu-6-phosphate starting pt for DNA/RNA production

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Interaction of Metabolic Pathways

• Carbohydrates stored as starch or glycogen
• glycogen/starch/other sugars broken down into
  glucose or fructose
• glucose and fructose used in glycolysis
• Fats are highly reduced, S rich molecules
• glycerol bound to FA chains
• glycerol converted to glyceraldehyde 3-phosphate
  and can enter glycolysis
• FA converted to acetyl CoA that enter Krebs
• Proteins can be used for S
• broken down into AA
• AA can be disassembled and ammonia groups
  excreted as waste
• remaining C cmpds converted to pyruvate/acetyl
  CoA/other intermediates that enter glycolysis or
  Krebs