PART II: Energetics and Enzymes
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Transcript of PART II: Energetics and Enzymes
PART II: Energetics and Enzymes
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 +
Free energy of formation
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?
What is the ΔGº’ of the following reactions?
• CH4 + ½ O2 CH3OH
• H2S + 4H20 + 8Fe+3 8Fe+2 + 10H+ + SO4-2
• (at pH7)
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
PART III Oxidation-Reduction and Energy-Rich Compounds
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
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'
Electron tower
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?
• 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?
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
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?
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
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?
Role of NAD+/NADH in metaobolism
• NADH is formed during glycolysis
• NAD+ is regenerated during fermentation or the electron transport
chain
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
High Energy Bonds