part ii: energetics and enzymes
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PART II: Energetics and Enzymes. Free energy of formation (G 0 f ) for compounds. Energy needed for the formation of a compound Used to calculate ΔG 0 ’ of a reaction If the formation of a compound is exergonic, then Gº f is - If the formation of a compound is endergonic, then Gº f is +. - PowerPoint PPT PresentationTRANSCRIPT
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PART II: Energetics and Enzymes
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Free energy of formation (G0f) for
compounds• Energy needed for the formation of a
compound• Used to calculate ΔG0’ of a reaction• If the formation of a compound is exergonic,
then Gºf is -• If the formation of a compound is endergonic,
then Gºf is +
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Free energy of formation
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Predicting reactions
• ΔGº’ of A+BC+DGºf [products]- Gºf [reactants]
• ΔGº’ of the reaction H2 + ½ O2 H2OΔGº’= Gºf [H2O]- (Gºf [H2] + Gºf ½ O2)
(- 237.2) – (0 + ½ 0)= - 237.2 KJ/mol
• Is this endergonic or exergonic?
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What is the ΔGº’ of the following reactions?
• CH4 + ½ O2 CH3OH
• H2S + 4H20 + 8Fe+3 8Fe+2 + 10H+ + SO4-2
• (at pH7)
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Enzymes associate with other molecules
• Prosthetic groups– Bound very tightly to enzymes– Example: heme group in cytochromes
• Coenzymes– Bound loosely to the
enzyme– Examples: vitamins
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PART III Oxidation-Reduction and Energy-Rich Compounds
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Example• H2 + fumarate2-succinate2-
– Oxidation of Hydrogen: (electron donor)– H22 e- + 2 H+
– Reduction of fumarate: (electron acceptor) – fumarate2- + 2 e- + 2 H+ succinate2-
• 2 half reactions – are linked• 1 reaction = oxidation and electron donor• 1 reaction = reduction and electron acceptor
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Reduction potential, E0'• The tendency of a compound to accept or release electrons
• Vertical tower of reduction potentials• The most negative E0's on the top
– Greatest electron donor potential• Those with the most positive E0's on the bottom
– Greatest electron acceptor
• Donors can pass electrons to acceptors below them on tower only
• As electrons are passed from a donor, energy is released E0‘ = G0'
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Electron tower
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Questions?• Which compound on the
tower has the greatest ability to donate electrons?
• Which compound on the tower has the greatest ability to accept electrons?
• Can cytochrome B donate electrons to iron (Fe+3)?
• Can ubiquinone accept electrons from iron Fe+3?
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• Does anyone know in the electron transport chain, what compound has the greatest ability to accept electrons and what compound has the greatest ability to donate electrons?
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Reduction potentials
• ΔEo’= difference in potential– The larger the ΔEo’ during a redox reaction, the
greater the energy released – Or the farther the electrons fall down the tower the
more potential energy produced
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Redox reactions in metabolism
• NAD+/NADH: catabolic reactions• NADP+/NADPH: anabolic reactions
• NAD+/NADH, E0’ = -0.32
• Is NADH a good electron acceptor or electron donor?
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In metabolism there are different electron carriers
• NADH• FADH2
• NADPHThey carry hydride ions = 2 hydrogen + 2 electrons
Acetyl CoA – carries an activated sulfhydryl group
ATP carries an activated phosphate group
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Questions
• Is NAD+ in the oxidized or reduced form?• Is FADH2 in the oxidized or reduced form?• If a series of reactions is about to take place,
would you say that NADH is going to be oxidized or reduced? What about FAD+?
• Which of the following compounds would pick up electrons – NAD+, NADH, FAD+ or FADH2?
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Role of NAD+/NADH in metaobolism
• NADH is formed during glycolysis
• NAD+ is regenerated during fermentation or the electron transport
chain
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Carrier molecules involved in different biochemical reactions
Carrier molecule Group carried• ATP phosphoryl• NADH and NADPH electrons (and
hydrogen)• FADH2 electrons (and
hydrogen)• Coenzyme A acetyl
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High Energy Bonds