Bioenergetics

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Bioenergetics

• Nov. 13, 2007

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Outline

• Quick review of Week 6
• Bioenergetics
• Thermodynamics
• Show me the money! ATP
• Biological phosphate compounds
• Coupled rxns
• Biological redox rxns

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Week 6 Quick Review

• Enzyme regulation
• Why it is necessary
• Inhibitors
• Allosteric Enzymes
• Metabolic pathways
• Metabolites, characteristics, anabolic,
  catabolic, localization of pathways

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I am so Intrigued by metabolism. Please tell
me more.
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Bioenergetics

• The study of how organisms manage their energy
  resources

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Cells require sources of free energy

• Heat flow is not an energy source for cells
• Cells use free energy (G)
• Gibbs free-energy
• Helps us predict direction of rxn
• Helps us predict equilibrium position

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Recall

• Rxn will proceed A--gtB only if Gibbs Free Energy
  of B is lower than that of A
• Catalysis does not influence thermodynamics
• ?G? GB? - GA ?

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Recall

• GA GA? RT ln A
• ?G(A--gtB) ?G?(A--gtB) RT ln B/A
• R 8.31 x 10-3 kJ/(K mol)
• Standard T 298 K
• ln B/A 2.3 log B/A
• At equilibrium, ?G 0 and B/A Keq
• 0 ?Gº RT ln Keq
• ?Gº(A--gtB) -RT ln Keq

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?Gº(A--gtB) -RT ln Keq

• Biochemical standard free energy change (?Gº) of
  a chemical rxn is simply an alternative way of
  expressing an equilibrium constant (Keq)
• Actual free-energy changes (?G) depend on
  reactant and product

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Note the following

• ?G(A--gtB) ?G?(A--gtB) RT ln B/A
• ?Gº(A--gtB) -RT ln Keq
• The sign of ?Gº or ?G indicates direction of rxn
• Magnitude of ?Gº or ?G is proportional to how
  far rxn can proceed before reaching equilibrium
• ?Gº and ?G are independent of rxn pathway
• ?Gº and ?G of coupled rxns are additive
• ?GA--gtB -?GB--gtA

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Table 1. Thermodynamic Relationships
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Cellular reactions need to be thermodynamically
favorable

• A ----gt B
• Lets consider three different cellular
  situations
• ?GºA--gtB ltltlt 0
• ?GºA--gtB gt 0 or ?GºA--gtB lt 0
• ?GºA--gtB gtgtgt 0

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?GºA--gtB ltltlt 0

• ATP hydrolysis
• ATP H2O --gt ADP Pi ?Gº -30.5 kJ/mol
• ?Gº -RT lnKeq
• ln Keq -30.5 kJ/mol (-K mol/0.0083 kJ) (298
  K)-1
• ln Keq 12.3, Keq 2.2 x 105
• What does this mean?
• ADPPi/ATP 220,000 at eq.

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?GºA--gtB gt 0, or ?GºA--gtB lt 0

• Glucose-6-P lt-----gt Glucose-1-P
• Keq 1/19
• ?Gº -RT lnKeq
• ?Gº (-0.00831 kJ/K mol)(298 K) x ln(1/19)
• ?Gº 7.3 kJ/mol
• What does this mean?
• What if the actual cellular B and/or A are
  different?
• ?G(A--gtB) ?G?(A--gtB) RT ln B/A

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What if the actual cellular Glucose-6-P and
Glucose-1-P are different?

• Glucose-6-P 0.199 M
• Glucose-1-P 0.001 M
• ?G(A--gtB) ?G?(A--gtB) RT ln B/A
• ?G 7.3 kJ/mol (0.00831 kJ/K mol)(298 K) x
  ln(0.001/0.199)
• ?G -5.8 kJ/mol

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Standard free-energy change

• What does it tell us?
• Tells us
• The direction for a given rxn
• How far the rxn must go to reach equil.
• Applies when initial concentration of each
  component is 1.0 M, pH 7.0, 298 K, 1 atm
• It is constant

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?G?(A--gtB) gtgtgt 0 Figure 16-21a
Page 567
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exergonic vs exothermic endergonic vs endothermic

• -gonic is a broader term and refers to free
  energy
• -thermic refers to heat energy

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In other words

• A ----gt B ?Gº1
• B ----gtC ?Gº2
• Sum A----gtC ?Gº1 ?Gº2

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Show me the money!
Thats nice and all, but Show me the money!
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Figure 16-20 ATP is energy currency
Page 566
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The free-energy change for ATP hydrolysis is
large and negative

• chemical energy is donated to endergonic rxns
• Synthesis
• Transport
• Mechanical motion
• Dont confuse bond energy for high-energy bonds

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Why are the phosphoryl transfer rxns of ATP so
exergonic?

• ATP ---gt ADP Pi
• Hydrolysis
• Separates negatively charged phosphates
• Relieves electrostatic repulsion
• Pi (HPO42-) orthophosphate is resonance
  stabilized
• ADP2- ionizes

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ATP Hydrolysis
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Actual free-energy change for ATP Hydrolysis
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Keeping it real The real cost of doing metabolic
business

• Phosphporylation potential (?Gp) is the cellular
  ?G for ATP hydrolysis
• Calculate the the ?Gp in human erythrocytes.
  Assume ATP, ADP, and Pi are 2.25, 0.25, and 1.65
  mM, respectively. The pH is 7.0 and the
  temperature is 25 ºC.

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• Calculate the the ?Gp in human erythrocytes.
  Assume ATP, ADP, and Pi are 2.25, 0.25, and 1.65
  mM, respectively. The pH is 7.0 and the
  temperature is 25 ºC.
• ?Gp ?Gº RT ln ADPPi/ATP
• ?Gp -30,500 J/mol (8.315 J/ mol K)(298 K) x
  ln (2.50 x 10-4)(1.65 x 10-3)/(2.25 x 10-3)
• ?Gp -51.8 kJ/mol

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ATP Provides Energy by Group Transfers, not by
Simple Hydrolysis

• ATP ---gt ADP Pi
• ATP hydrolysis generates heat
• In reality, ATP participates covalently in an
  enzyme catalyzed rxn

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Figure 16-25 modified
Page 569