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Glycolysis

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Energy Transfer

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Oxidation/Reduction Reactions (REDOX)

  • oxidation - removal of electrons

    • decrease in potential energy​

    • dehydrogenation - removal of H

    • liberated H transferred by coenzymes

      • NAD and FAD​​

  • reduction - addition of electrons

    • increase in potential energy​

Adenosine Triphosphate (ATP)

  • links anabolic and catabolic reactions

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ATP

  • a nucleotide consisting of the nitrogenous base adenosine, the sugar ribose, and three phosphate groups​

  • the energy from the hydrolysis of ATP is directly coupled to endergonic processes by the transfer of the phosphate group to another molecule

    • the recipient molecule is now phosphorylated​

Two Mechanisms of ATP generation

  • substrate level phosphorylation

    • transferring high-energy phosphate group from an intermediate directly to ADP​

  • oxidative phosphorylation

    • remove electrons and pass them through electron transport chain to oxygen​

  • ATP is continually regenerated by adding a phosphate group to ADP

    • energy support renewal comes from catabolic reactions in the cell​

    • regeneration (endergonic) requires an investment of energy

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Glucose Catabolism

  1. glycolysis (does not require oxygen - anaerobic)

  2. ​formation of acetyl-CoA

  3. krebs cycle

  4. electron transport chain (requires oxygen - aerobic)

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Glycolysis

  • first step in making ATP

  • occurs in cytosol

  • splits 6C sugar glucose into 2-3C molecules of pyruvic acid

  • consumes 2 ATP but generates 4

  • 10 reactions

  • fate of pyruvic acid depends on O2 availability

    • when O2 absent -> reduced to lactic acid (small ATP production)​

    • when O2 present -> converted to acetyl-CoA and proceeds to TCA cycle (large ATP production)

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Energy Investment (step 1-5)

Step 1: Glucose -> G6P

  • energy investment

    • "activation" step​

  • G6P is trapped in cell

    • irreversible​

  • A phosphate group is added to carbon 6 of glucose

    • first use of ATP​

    • energy from ATP transferred to glucose to make G6P, now trapped in the cell

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Step 2: G6P -> F6P

  • isomerization

  • converts glucose to fructose

    • Aldehyde to ketone​

  • necessary for later steps

  • reversible

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Step 3: F6P -> F1,6BP

  • energy investment

    • "activation" step​

  • Enzyme - phosphofructokinase (PFK) 

    • modulated allosterically; ATP inhibits when energy stores high​

    • adds another P to C1

    • now have P at both ends

  • irreversible

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Step 4: F1,6BP -> G3P + DHAP

  • splitting up

Step 5: DHAP -> G3P (2 G3Ps)

  • isomerization

  • cleavage or split of C6 into two C3

    • both C3 are sugars​

    • the enzyme is adolase

    • C3s are not identical, but are interconvertible via isomerase (step 5)

  • SO FAR: used 2 ATP and split C6 into C3

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Energy Payoff (step 6-10)

Step 6: G3P -> 1,3BPG

  • VERY IMPORTANT STEP

  • reversible

  • oxidation (dehydrogenation) of G3P

  • REDOX reaction

    • NAD+ is reduced to NADH + H​

      • primary e- and H+ acceptor​

  • removal of H and 2e- from C1

  • the second H comes from the enzyme

  • insertion of Pi added to the C3 sugar

  • conserves the energy released by the oxidation of G3P

    • transferring H+ and e- is one way we move energy around​

    • energy released when G3P is oxidized

  • results

    • 1,3 BPG and NADH + H​

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Step 7: 1,3BPG -> 3PG

  • ATP production

  • substrate level phosphorylation

    • Pi from 1,3 BPG + ADP -> ATP​

  • reversible

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Step 8: 3PG -> 2PG

  • rearrange location of P

    • P becomes higher energy P by moving it from carbon 3 to 2​

    • setting up for another substrate-level phosphorylation

  • reversible

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Step 9: 2PG -> PEP

  • removal of water

  • rearrangement of atoms makes the P even more high energy

  • PEP has a very high energy bond

    • it can be used to donate P to ADP to make ATP​

    • reversible step

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Step 10: PEP -> Pyruvate

  • produce pyruvate (pyruvic acid)

  • this is also the second ATP formed

    • via substrate level phosphorylation​

  • irreversible step

  • when there is O2, glycolysis stops here

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Energy Yield

Per molecule of glucose (G6)

  • 2 substrate level phosphorylation (2 ATP/glucose)

    • step 7 and 10​

    • but these are really C3 molecules x2

  • really 4/molecule of glucose, but 2 ATP are used

    • step 1 and 3​

  • NET ATP OF 2

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Glycolysis Step 11: Anaerobic

  • ONLY UNDER ANAEROBIC CONDITIONS

  • pyruvate -> lactate

    • pyruvate is reduced to lactate​

    • NADH + H donates the H+ and e-

    • coenzyme now becomes oxidized (NAD+)

    • this is how we regenerate under anaerobic conditions​

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Fate of Lactic Acid

  • lactate will leave tissue and go into blood

  • ends up in liver

  • liver will convert lactate -> pyruvate

  • liver can use pyruvate in aerobic metabolism

  • this recycling is called the cori cycle

    • pyruvate -> lactate under anaerobic conditions​

    • lactate -> pyruvate in liver

      • will become very important when we challenge metabolism by fasting​

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©2023 by Syracuse University Dr.Margaret Voss

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