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 +

Free energy of formation

Predicting reactions ΔGº’ of A+B  C+D Gº f [products]- Gº f [reactants] ΔGº’ of the reaction H 2 + ½ O 2  H 2 O ΔGº’= Gº f [H 2 O]- (Gº f [H 2 ] + Gº f ½ O 2 ) ( ) – (0 + ½ 0) = KJ/mol Is this endergonic or exergonic?

What is the ΔGº’ of the following reactions? CH 4 + ½ O 2  CH 3 OH H 2 S + 4H Fe +3  8Fe H + + 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 H 2 + fumarate 2-  succinate 2- –Oxidation of Hydrogen: (electron donor) –H 2  2 e H + –Reduction of fumarate: (electron acceptor) –fumarate e H +  succinate 2- 2 half reactions – are linked 1 reaction = oxidation and electron donor 1 reaction = reduction and electron acceptor

Reduction potential, E 0 ' The tendency of a compound to accept or release electrons Vertical tower of reduction potentials The most negative E 0 's on the top – Greatest electron donor potential Those with the most positive E 0 '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  E 0 ‘ =  G 0 '

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 ΔE o ’ 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, E 0 ’ = Is NADH a good electron acceptor or electron donor?

In metabolism there are different electron carriers NADH FADH 2 NADPH They 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 FADH 2 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 FADH 2 ?

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 moleculeGroup carried ATPphosphoryl NADH and NADPHelectrons (and hydrogen) FADH 2 electrons (and hydrogen) Coenzyme Aacetyl

High Energy Bonds