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 H2O

ΔGº’= 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