ATP and Energetics of
Metabolism
C483 Spring 2013
1. What is false of a near-equilibrium reaction?
A) Concentrations of reactants and products are near equilibrium concentrations.
B) The enzyme reaction is most likely not regulated
C) The standard free energy of the reaction is near zero.
D) The free energy of the reaction is near zero.
2. Which thermodynamic quantity is used to determine if a reaction in a cell is
spontaneous?
A) △G°'
B) △G
C) △H
D) △S
3. The hydrolysis of ATP to ADP has a △G°' of -30 kJ/mol. If in an E. coli cell the
concentrations of ATP, ADP and inorganic phosphate are 7.90 mM, 1.04 mM and 7.9
mM, respectively, which statement is true about the hydrolysis of ATP in the cell?
A) Hydrolysis cannot proceed spontaneously.
B) The hydrolysis is at equilibrium.
C) The free energy of the reaction is less negative than the standard free energy.
D) The free energy of the reaction is more negative than the standard free energy.
E. More than one of the above are true.
4. Which of the following is not a factor contributing to the large free energy of
hydrolysis of ATP?
A) Electrostatic repulsion of oxygen atoms.
B) Better solvation of products than ATP itself.
C) Complexes with Mg2+ or Mn2+ ions.
D) Better stability of products than ATP itself.
E) Electrical shielding of products of hydrolysis.
5. ATP is thermodynamically suited as a carrier of phosphoryl groups in animal
cells because
A) it is stable under cell conditions.
B) it is not hydrolyzed in cells without enzyme action.
C) it is intermediate in group-transfer potential.
D) All of the above.
Catabolism
Anabolism
ATP Hydrolysis
• High energy
bonds—inherent
chemistry
– Electrostatic
repulsion
– Solvation of
products
– Resonance
• Rxn goes to
“completion”
Qualitative Predictions
• Inherently favorable, unfavorable, or near
equilibrium?
Energetics: A metaphor
Standard Free Energy vs. Free Energy
DGo’ is negative
DG is negative
DGo’ is negative
DG is zero
Standard Free Energy vs. Free Energy
ATP
ATP
AD
ADP + P
ADP
P
DGo’ is -32 kJ
DG is -32 kJ
DGo’ is -32 kJ
DG is zero
Quantitative
• Inherent component
• Concentration component
Equilibrium
• Equilibrium = DEAD!
• What is [product]/[rxt] equilibrium of ATP
hydrolysis to ADP?
• [ADP][Pi]/[ATP] = 4.1 x 105
Free Energy of ATP hydrolysis
• Actual cellular concentrations don’t vary much
from [Pi]=[ATP] = 5 mmol and [ADP]= 1 mmol
• What is the actual free energy of ATP
hydrolysis in the cell? More or less than -32
kJ?
Two Types of Reactions
• Near-equilibrium reactions
– [pdt] and [rxt] near equilibrium concentrations
– DG close to zero (regardless of DGo’ )
– Not regulated—part of overall flux of metabolism
• Metabolically irreversible reactions
– DG far from zero
– Can only be overcome by energy input
– regulated
ATP in Metabolism
• Overcoming a barrier
– Can’t change concentrations (ammonia is toxic!)
– Couple the reaction to a spontaneous reaction!
– Write an equation to couple to ATP hydrolysis:
Mechanism of Coupling
Phosphoryl Transfer in
Energetic Intermediates
Phosphoryl group transfer
potential
Answers
1.
2.
3.
4.
5.
C
B
D
C
D