MBIO 2700 / Topic 10: Bioenergetics

> What’s the 1st law of thermodynamics?

> What’s the 2nd law of thermodynamics?

> Energy is neither created nor destroyed.

> Entropy of the universe is always increasing.

Why do living systems make a problem for the 2nd law of thermodynamics? What’s the solution to this problem?

> Cells are highly ordered, non-random things. ↓ S when a cell is made.

> Energy must be supplied. Disordering an ordered state is something a cell works against.

What is the rs btwn S and E?

∆G = ∆H - T∆S.

> Why is G termed free energy?

> Differentiate exergonic vs. endergonic.

> Measures of max amount of energy potentially available for useful work @ constant T and P.

> Exergonic = negative ∆G ; Endergonic = positive ∆G.

For our purposes, fwd rxn is spontaneous, while rev rxn is non-spontaneous.

Does a large negative ∆G ensure a rxn to go thru?

Depends if there is an enzyme to bring a mechanism and catalyze a reaction for a large negative ∆G.

> What would happen if an exergonic rxn occured by itself?

> What would happen if an exergonic rxn occured w/ an endergonic reaction?

> Energy released mainly as heat.

> Energy released to drive endergonic reaction, i.e. energy coupling.

Why can’t ∆G be directly related to K_eq, and what fixes this?

Because ∆G varies with reactant/product concentrations, while Keq is constant → only ∆G°′ relates reliably to Keq.

Define ∆Gº.

∆G under 1atm, 298K, every solute at 1M, and every other component in state most stable in these conditions.

Why is ΔG°′ used instead of ΔG° in biology?

> Any rxn involving H⁺ is defined for pH=0 (b/c [H⁺] = 1M), which is not physiological.

> ∆Gº’ is used for biological systems, wherein pH=7 while keeping all other solutes at 1M except H⁺.

What’s the rs btwn ∆Gº’ and K_eq?

∆Gº’ = -RTlnK_eq’

where R = 8.3 joules/mol•K and T = temp in K

K = °C + 273.15

> What’s the standard reduction potential (Eº’)?

> What does a higher Eº’ mean?

> What does a lower Eº’ mean?

> Voltage needed to pull e^- away from e^- donor of redox couple under standard conditions.

> Higher Eº’ → harder to to pull e^- away from donor or substance has higher aff for e^-.

> Lower Eº’ → easier to to pull e^- away from donor or substance has lower aff for e^-.

Why does e^- flow do work?

> e^- move from higher energy state (lower E°) to lower energy state (higher E°) in an electrochemical cell.

> Energy released to do work.

What are the four most common biological electron carriers?

> Nicotinamide adenine dinucleotide (NAD⁺).

> Nicotinamide adenine dinucleotide phosphate (NADP⁺).

> Flavin adenine dinucleotide (FAD).

> Flavin mononucleotide (FMN).

What is the main role of NAD⁺ and NADPH in metabolism?

> NAD⁺ usually accepts e^- from catabolism and NADH delivers e^- to resp. chain.

> NADPH supplies e^- for synthetic pathways.

How does a voltmeter work in measuring voltage difference btwn two half-cells?

> One electrode = e^- donor (oxidation).

> Other electrode = e^- acceptor (reduction).

> Voltmeter connects them to detect the flow of electrons.

> It shows how much “push” (potential) exists for electrons to move.

> What’s the rs btwn Eº’ and possibility of being an oxidizing agent?

> What’s the formula for ∆Eº’?

> What’s the formula relating ∆Eº’ and ∆Gº’?

> ↑ Eº’ ↑ e^- affinity ↑ wants to get reduced ↑ is an oxidizing agent.

> ∆Eº’ = Eº’ (e^- acceptor) - Eº’ (e^- donor)

> ∆Gº’ = -nF∆Eº’, where n = # of e^-’s and F = 96.1 kJ/volt•mol

What are the steps to calculate ∆Eº’?

> Write the rxns.

> Determine which Eº' is more +ive.

> Rewrite the rxns by reversing the sign of the more -ive.

> Sum them.

Why does biological oxidation occur in multiple steps?

To break down energy release into small ∆G steps, allowing efficient energy conservation from glucose oxidation.

Energy released by glucose oxidation is preserved by coupled rxns.

What are the two ways that a reaction can be coupled?

> PO₄³⁻ from ATP is transferred to an E or to a S which raises energy E or S (activated) to make the rxn a -ive ∆G from a +∆G.

> Displacement of P_i which releases energy.

Formation of ATP is coupled to exergonic processes.

If ATP has so much free energy, why does it not spontaneously decompose?

> E_a high.

> Spontaneous w/ ∆Gº’ = -30.5kJ/mol… but slow.

> No mechanism for hydrolysis w/o enzymes.

When coupled w/ enzymes for hydrolysis, why does hydrolysis release this much energy?

> ↓ Repulsion: One molecule w/ 4 negative charges into two molecules w/ 2 negative charges each.

> ↑ Entropy: More resonance structures of ADP + P_i than ATP.

> ↓ ∆G: Upon another hydrolysis, ADP spontaneously dissociates one more proton.

> Solvation of ADP + P_i stabilizes P relative to R.

What are three other compounds that have large ∆G hydrolysis values for reasons similar to ATP?

> 1,3 bis-phosphoglycerate (-49kJ/mol).

> Phosphoenolpyruvate (-62kJ/mol).

> Phosphocreatin (-43kJ/mol).

Differentiate the three types of phosphorylation that generate ATP.

> Substrate level phosphorylation: Uses high-∆G compounds to make ATP directly.

> Oxidative phosphorylation: In mitochondria, couples ATP to energy-releasing redox rxns.

> Photophosphorylation: In chloroplasts, couples ATP formation to energy derived from absorption of visible light.

What are some highly exergonic phosphate bond hydrolysis reactions apart from ATP + H₂O → ADP + P_i?

> ADP + H₂O → AMP + P_i (-33kJ/mol)

> ATP + H₂O → AMP + PP_i (-45.6kJ/mol).

> PP_i + H₂O → 2P_i (-20kJ/mol).

Why is ATP considered an intermediate-energy compound?

> Produced by high-energy compounds (ΔGº’ ≈ -40 to -60 kJ/mol).

> Used to phosphorylate lower-energy compounds (ΔGº’ ≈ -14 kJ/mol), with its own ΔGº’ ≈ -35 kJ/mol.