Bioenergetics. Energetic Considerations

Transcription

1 Bioenergetics Martin Könneke (10/2009) Energetic Considerations Introduction Definitions Calculation of free energy changes Examples of different biological processes Role of ATP Free energy and reduction potential 1

2 Why do microorganisms need energy? Maintain the highly defined cellular order Active Movement Detoxification Signaling/Communication Storage Growth / Reproduction Metabolism Chemotrophy Phototrophy Catabolic Reactions ATP Biosynthesis Anabolic Reactions Heterotrophy Autotrophy 2

3 Free Energy of Chemical Reactions A + B C + D C + D Free Energy A + B!G A + B!G C + D Progress of Reaction!G >0 (positive) endergonic reaction Progress of Reaction!G < 0 (negative) exergonic reaction Yield energy Catabolic reactions are in general exergonic reactions Free Energy A + B!G Conserved as ATP C + D Progress of Reaction ( or other high-energy bonds) = -32kJ/mol!G < 0 (negative) exergonic reaction 3

4 !G provides no information about the rate of a reaction Free Energy A + B!G C + D Progress of Reaction!G < 0 (negative) exergonic reaction 4

5 !G provides no information about the pathway of the reaction Free Energy A + B!G C + D Progress of Reaction!G < 0 (negative) exergonic reaction Definitions Free-energy change of a reaction aa + bb cc + dd!g =!G 0 + RT ln [C]c [D] d [A] a [B] b!g = Free-energy change under specific conditions (in KJ=kiloJoule)!G 0 = Standard free-energy change (25 C, unit activities; 1atm, 1M) R = Gas constant (8.314 J/mol/K) T = Absolute temperature (K; K= C ) [ A,B ] = Molar Concentration of reactants (Activity) [ C,D ] = Molar Concentration of products (Activity) a,b,c,d = Stoichiometric coefficients 5

6 !G of a reaction depends on a) the nature of the reactants!g =!G 0 + and b) on their concentrations!g =!G 0 + RT ln [C]c [D] d [A] a [B] b RT ln [C]c [D] d [A] a [B] b Standard free-energy changes A) Can be calculated from standard free energies of formation!g 0 = "!G f 0 (products) -!G f 0 (reactants) B) Can be calculated from equilibrium constant At equilibrium!g = 0 0 =!G 0 + RT ln [C]c [D] d [A] a [B] b!g 0 = - RT lnk 6

7 Enthalpies of formation!g '(f) of biologically relevant compounds Standard free-energy changes B) Can be calculated from equilibrium constant At equilibrium!g = 0 0 =!G 0 + RT ln [C]c [D] d [A] a [B] b!g 0 = - RT ln [C]c [D] d [A] a [B] b K = [C]c [D] d [A] a [B] b!g 0 = - RT lnk (K = equilibrium constant) K = e -!G0 / 2.3RT = 10 -!G 0 / 2.3RT 7

8 Conditional (biochemical) standard free-energy changes!g 0!G 0 = Free-energy change under biochemical standard condition at ph=7, unit activities, 25 C = 298 K!G 0 =!G 0 + m!g f (H + )!G 0 =!G 0 + mrt ln [H + ] =!G kJ m m = net number of protons in the reaction m < 0; when more protons are consumed than formed m > 0; when more protons are formed than consumed A + + ne - 2H + + 2e - 1/2 O 2 + 2e - H 2 + 1/2O 2 Redox potential E and free-energy change A H 2 O 2- H 2 O ; E = reduction potential! E 0 = Difference in potentials of half-reactions = E 0 electron-accepting - E 0 electron-donating n = Number of electrons E 0 = Standard potential for redox-half-reaction (in V,25 C, 1M) E 0 = E 0 at ph 7 8

9 The electron tower Couple CO 2 /glucose(-0.43) 24e - 2H + /H 2 (-0.42) 2e - NAD + /NADH (-0.32) 2e - CO 2 /acetate (-0.28) 8e - SO 4 2- /H 2 S (-0.28) 8e - E 0 (V) NO 3- /NO 2 - (+0.42) 2e - NO 3- /1/2N 2 (+0.74) 5e - Fe 3+ /Fe 2+ (+0.76) 1e -,(ph 2) 1/2O 2 /H 2 O (+0.82) 2e The standard free-energy change!g 0 is proportional to the redox-potential difference between e - -donor and e - -acceptor! E 0!G 0 = - nf! E 0!G 0 = - nf! E 0 n F = Number of electrons = Faraday s constant (96.48 kj/v) 9

10 The electron tower Couple E 0 (V) 2H + + 2e - 1/2 O 2 + 2e - H 2 + 1/2O 2 H 2 O 2- H 2 O CO 2 /glucose(-0.43) 24e - 2H + /H 2 (-0.42) 2e - NAD + /NADH (-0.32) 2e - CO 2 /acetate (-0.28) 8e - SO 4 2- /H 2 S (-.028) 8e !G 0 = -237 kj!g 0 = - nf! E 0 NO 3- /NO 2 - (+0.42) 2e - NO 3- /1/2N 2 (+0.74) 5e - Fe 3+ /Fe 2+ (+0.76) 1e -,(ph 2) 1/2O 2 /H 2 O (+0.82) 2e The substrate with lower E 0 provide the electrons (e- donor) Calculating free-energy changes for hypothetical reactions 10

11 Balancing of chemical reactions Oxidation-reduction (redox) balance All electrons removed from a substance on one side must be transferred to another substance on the other side Ionic balance Total ionic charge of all molecules must be equal on both sides In aqueous medium, ionic balance can be achieved by adding H + or OH -, and H 2 O (for elemental balance) Elemental balance Total number of each element must be equal on both sides of the equation Determining the oxidation state Oxidation state of elements in elementary substance or combined with itself is 0 (H 2, O 2, N 2, S (s)0 ) Except when combined with itself, H has the oxidation state +1 Except when combined with itself, oxygen has the oxidation state -2 Oxidation state of an ion of an element is equal to its charge (O 2-, Na +, Fe 3+ ) Sum of the oxidation states of all atoms in neutral molecule is 0 (H 2 O, 2 x +1, 1x -2) Sum of oxidation states of all atoms in an ion is equal to its charge (OH - = -1) The oxidation state of individual carbon atoms in organic compounds can vary (average ox-state can be calculated by assuming that: N is usually -3, S is usually -2) 11

12 Calculating free-energy yields Biological examples Aerobic respiration Fermentation Anaerobic respiration: e.g., Methanogenesis Syntrophic ethanol oxidation at anaerobic conditions Aerobic Respiration of Glucose: Glucose + Oxygen Carbon dioxide C 6 H 12 O 6 + O 2 CO 2 12

13 Aerobic Respiration of Glucose: Glucose + Oxygen Carbon dioxide C 6 H 12 O 6 + O 2 CO 2 Elemental balancing (6xC, 12xH, 18xO) (1xC; 2xO) C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O Aerobic Respiration of Glucose: Glucose + Oxygen Carbon dioxide C 6 H 12 O 6 + O 2 CO 2 Elemental balancing (6xC, 12xH, 18xO) (1xC; 2xH; 3xO) C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O Redox balancing C (0);H 12(+I);O 6(-II); O 6(0) C 6(+IV) O 12(-II); H 12(+I);O 6(-II) 13

14 Aerobic Respiration of Glucose: Glucose + Oxygen Carbon dioxide + Water C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O!G 0 = "!G f 0 (products) -!G f 0 (reactants) = 6(-394.4)+6( ) - ( ) = kj Aerobic Respiration of Glucose: Glucose + Oxygen Carbon dioxide + Water C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O C 6(0); H 12(+I); O 6(-II) C 6(+IV); O 18(-II); H 12(+I) C 6 H 12 O 6 6CO e V Glucose (e - donor); 6O e- 6 H 2 O +0.82V Oxygen (e - acceptor)!g 0 = - nf! E 0 = -24 (96.48 kj/v)(+0.82v -(-0.43V))= kj 14

15 Fermentation of Glucose: Glucose Ethanol + Carbon dioxide C 6 H 12 O 6 C 2 H 6 O + CO 2 Elemental balancing (6xC, 12xH, 6xO) (3xC, 6xH, 3xO) C 6 H 12 O 6 2C 2 H 6 O + 2CO 2 Redox balancing C (avg. 0) C 2(avg. -II); C 2(+II)!G 0 = (2(-394.4)+2( )) - ( ) = kj Anaerobic Respiration (i.g.: Methanogenesis) Hydrogen + Carbon dioxide Methane H 2 + CO 2 CH 4 Redox Balance C +IV; H 0 C -IV; H 4(+I) 8 e - 4H 2 + CO 2 CH 4 (e - donor) (e - acceptor) Elemental Balance 8xH, 1xC, 2xO 4H 2 + CO 2 4xH, 1xC CH 4 + 2H 2 O!G 0 = ( ) - (-394.4) = kj 15

16 Ethanol fermentation Ethanol Acetate + Hydrogen C 2 H 6 O C 2 H 3 O H 2 Ionic Balance C 2 H 6 O C 2 H 3 O H 2 + H + Elemental Balance C 2 H 6 O + H 2 O C 2 H 3 O H 2 + H + Redox Balance C 2(-II); H 6(+I); O (-II) C 2(0); H 3(+I); O 2(-II) + H (+I) Ethanol fermentation Ethanol Acetate + Hydrogen C 2 H 6 O C 2 H 3 O H 2 Ionic Balance C 2 H 6 O C 2 H 3 O H 2 + H + Elemental Balance C 2 H 6 O + H 2 O C 2 H 3 O H 2 + H + Redox Balance C 2(-II); H 6(+I); O (-II) C 2(0); H 3(+I); O 2(-II) + H (+I)!G 0 = (-39.83) - [( ) + ( )] = 9.68 kj 16

17 Effect of hydrogen partial pressure on free-energies Ethanol fermentation: ethanol + H 2 O acetate + 2H 2 + H +!G =!G 0 + RT ln [C]c [D] d [A] a [B] b!g =!G 0 + RT ln [H 2 ]2 [acetate] [H + ] [ethanol] [H 2 O] at 10-4 atm H 2!G =!G 0 + mrt ln [H 2 ]!G = RT ln [10-4 ] = kj/mol Syntrophic ethanol oxidation at anaerobic conditions Ethanol fermentation!g 0 (kj/reaction) 2 ethanol + 2H 2 O 2 acetate + 4H 2 + 2H Methanogenesis 4H 2 + CO 2 CH 4 + 2H 2 O Syntrophic coupled reaction 2 ethanol + CO 2 2 acetate + CH 4 + 2H

18 Syntrophic co-culture Methanobacillus omelianskii ethanol CO 2 Interspecies Hydrogen-transfer H 2 H 2 CH 4 acetate Strain S MoH Methanobacillus omelianskii Strain Hydrogen-producer Syntrophic co-cultures Interspecies hydrogen transfer Hydrogen-consumer Fermentation Anaerobic Respiration fatty-acids CO 2, SO -2 4, NO - 3 (e.g., butyrate, propionate) alcohols (e.g.,ethanol) H 2 H 2 acetate + CO 2 acetate, methane, HS -, N 2 O, NO, N 2 Syntrophomonas Syntrophobacter Methanogens Sulfate-reducing bacteria Homoacetogens Denitrifyers 18

19 Adenosintriphosphate (ATP) Free enthalpy of ATP Hydrolysis of ATP, AMP and pyrophosphate ATP + H 2 O! ADP + P i ATP + H 2 O! AMP + PP i + H + AMP + H 2 O! Adenosin + P i PP i + H 2 O! 2 P i "G ' = -32 kj/mol "G ' = -45 kj/mol "G ' = -13 kj/mol "G ' = -29 kj/mol ATP + AMP! 2 ADP "G ' = 0 kj/mol 19

20 How much ATP is in a cell? ATP Energy charge, EC of the cell EC = [ATP] [ADP] [ATP] + [ADP] + [AMP] > 0.8 e.g. [ATP] # 10 mm, ADP # 1 mm, AMP # 1 mm EC = 10.5/12 = The cell is energetically loaded. (During starvation?) What is the value of ATP in the cell? ATP Consideration of concentrations for energetical calculations: "G = "G + RT ln(c Product /c Reactant ) Textbook (standard conditions) ATP + H 2 O! ADP + P i "G ' = -32 kj/mol Multiply reactant concentrations, if there is more than 1 reactant: "G = "G + RT ln(c P1. C P2 / C R1. C R2 ) In the cell: [ATP]#0.01 M, [H 2 O]='1', ADP#0.001 M, [P i ] #0.01 M Product-reactant ratio is (0.001*0.01)/(0.01 * 1) = "G biol. = "G 0 ' + RT ln "G biol. = 32 kj/mol + (8,315 J/K mol) (298 K)(ln 0.001) "G biol. = "G 0 ' + RT ln = "G 0 ' -17 = -49 kj/mol For Regeneration of ATP spent: mostly about 75 kj/mol ATP 20

21 What is the value of ATP in the cell? ATP Consideration of concentrations for energetical calculations: "G = "G + RT ln(c Product /c Reactant ) Textbook (standard conditions) ATP + H 2 O! ADP + P i "G ' = -32 kj/mol In the cell: [ATP]#0.01 M, [H 2 O]='1', ADP#0.001 M, [P i ] #0.01 M Product-reactant ratio is (0.001*0.01)/(0.01 * 1) = "G biol. = "G 0 ' + RT ln = "G 0 ' -17 = -49 kj/mol "G biol = -50 kj/mol Multiply reactant concentrations, if there is more than 1 reactant: "G = "G + RT ln(c P1. C P2 / C R1. C R2 ) For Regeneration of ATP spent: mostly about 75 kj/mol ATP In the cell ATP has a higher value than under standard conditions, and requires even more energy to be regenerated. Mechanisms of ATP regeneration There are only two possibilities. Substrate level Phosphorylation b + a (last slide) backwards, coupled to an exergonic reaction, e.g. redox reaction Ion transport Phosphorylation (H + or Na + ) (membrane bound, driven by electrical membrane potential + chemical gradient) Terms: Energy conservation Substrate level phosphorylation, dt. Substratketten-Phosphorylierung? There is no oxidative phosphorylation, neither electron transportdriven phosphorylation, nor photophosphorylation Do not get stupefied by obsolete terms! 21

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24 Gibbs free energy and reduction potential!g 0 = - nf! E 0 24

25 Gibbs free energy and reduction potential of NAD NAD + + 2H + + 2e -! NADH + H + E 0 = V 0.5O 2 + 2H + + 2e -! H 2 O E 0 = 0.82 V NADH + 0.5O 2 + H +! NAD + + H 2 O!E 0 = E 0 O2 - E 0 NADH = 0.82 V - (-0.32 V) = 1.14 V!G 0 = - nf! E 0!G 0 = -(2) (96.48kJ/V mol) (1.14V) = -220kJ/mol 25

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