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Refer to chapters 8 &14, Stryer 5e and chapters 6 &13, Lehninger, 4e.

Bioenergetics & Metabolism II. Objectives 1. Metabolism is composed of many coupled interconnecting reactions. 2. The distinction between energetically favourable and energetically unfavourable reactions. 3. How reaction coupling is used to drive energetically unfavourable reactions.

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Refer to chapters 8 &14, Stryer 5e and chapters 6 &13, Lehninger, 4e.

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  1. Bioenergetics & Metabolism II Objectives 1. Metabolism is composed of many coupled interconnecting reactions. 2. The distinction between energetically favourable and energetically unfavourable reactions. 3. How reaction coupling is used to drive energetically unfavourable reactions. Refer to chapters 8 &14, Stryer 5e and chapters 6 &13, Lehninger, 4e. Lecture 14, Michael Schweizer

  2. DG = DH + (-TDS) (a) DH negative, -TDS negative: when glucose is fermented to EtOH, enthalphy decreases and entropy increases, so both enthalpy and entropy favour this reaction. (b) DH negative, -TDS positive: when EtOH burns enthalpy and entropy both decrease (see legend above). (c) DH positive, -TDS negative: when nitrogen pentoxide decomposes enthalphy and entropy both increase (see legend above).

  3. How the signs of DH and DS determine the effect of temperature on reactions.

  4. Energetically favourable reactions have a negative DG and energetically unfavourable reactions have a positive DG.

  5. Activated carriers can store and transfer energy needed for metabolism. By serving as energy shuttles activated carrier molecules perform their function as go-between that link the breakdown of foodstuff and the release of energy (catabolism) to the energy-requiring biosynthesis of small and large organic molecules (anabolism).

  6. A-R-P~P~P A-R-P~P + Pi A-R-P~P~P A-R-P + P~P Hydrolysis of ATP to ADP and inorganic phosphate

  7. Typical cellular conc of reactants and products in the reaction for ATP hydrolysis might be: [ATP] = 10 mM [ADP] = 1 mM [Pi] = 10 mM , DG = DG0’ + 2.303 RT DG = -7.3 kcal mol-1 + 1.4 kcal mol-1 = -7.3 kcal mol-1 + (1.4 kcal mol-1) (-3) DG = -11.5 kcal mol-1 or - 48.3 kJ mol-1

  8. Alternative route for the hydrolysis of ATP in which pyrophosphate is first formed and then hydrolysed. This route releases about twice as much energy as the hydrolysis of the terminal phosphate of ATP!

  9. A mechanical model illustrates the principle of coupled chemical reactions.

  10. B A The energetically infavourable reaction A to B is driven by the energetically favourable reaction C to D, because the free energy change for the pair of coupled reactions is less than zero.

  11. The standard free energy change of a reaction is related to the equilibrium constant = - 2.303RT log10Keq , , Keq = 10- DG°’/(2.303 RT) , Substitute R = 1.987 x 10-3 kcal mol-1 deg K-1 T = 2980 K (corresponding to 250C) Keq = 10- DG°’/1.36 ,

  12. Coupling of reactions A B Gº' = + 4.0 kcal mol-1 ATP + H2O ADP + Pi + H+ Gº' = -7.3 kcal mol-1 A + ATP + H2O B + ADP + Pi+ H+ Gº' = -3.3 kcal mol-1 A B K’eq= 10- Gº' /(2.303 RT) = 10-4.0/1.36 = 1.15 x 10-3 K’eq of the coupled reaction is: = 103.3/1.36 = 2.67 x 102

  13. Coupling of reactions, cont’d At equilibrium the ratio of [B] to [A] is given by The ATP-generating system of cells maintains the [ATP]/[ADP] [Pi] ratio in the order of 500 M-1; For the ratio [B]/[A]: K’eq = 2.67 x 102 x 500 = 1.34 x 105 This equilibrium ratio is strikingly different from the value of 1.15 x 10-3 for the reaction A to B in the absence of ATP hydrolysis. The calculation illustrates that by coupling the hydrolysis of ATP with A to B the K’eq of B to A has changed by a factor of 108!

  14. Note: It is important to stress that whether the DG for a reaction is larger, smaller or the same as DG0’ depends on the concentrationas of the reactants and products. The criterion of spontaneity for a reaction is DG not DG0’! This is important because reactions that are not spontaneous based on DG0’ can be made spontaneously by adjusting the concentrations of reactants and products. This principle is the basis of coupling of reactions to form metabolic pathways. (see p194, Stryer, 5e)

  15. For sequential reactions, DGo values are additive How an energetically unfavourable reaction can be driven by a second following reaction: DG0 = + 5 kcal/mol DG0 = -13 kcal/mol DG0 = -8 kcal/mol

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