Bioenergetics INTER 111: Graduate Biochemistry
Dec 19, 2015
Bioenergetics: Learning objectives
The change in free energy for a reaction predicts the direction in which it will spontaneously proceed.
What do positive, negative, and zero values for DG signify for a chemical reaction?
You should know the relationship of DG between the forward and back reactions and understand the coupling of standard free energy changes (DGo) for multiple reactions.
There is a thermodynamic relationship between free energy, enthalpy, and entropy.
Bioenergetics
Studies how energy is utilized and transferred in cells.
Is concerned with energy involved in making/breaking of molecular bonds within biological organisms.
includes the study of different cellular processes (cellular respiration, metabolism, growth, and development) that lead to production and utilization of energy in forms such as ATP molecules.
Bioenergetics
It is primarily concerned with the initial and final energy states of a chemical reaction.
The change in free energy between the initial and final states of a reaction allows prediction if the reaction is possible.
A B
Relationship between changes in free energy, enthalpy, and entropy
FREE ENERGYCHANGE
Energy available to do work
Heat released or absorbed during a reaction
ENTHALPY CHANGE
DG = DH – TDS
ENTROPY CHANGE
Measure of randomness
DHDG DS
Changes in free energy of a reaction are defined in two different forms
For any specified [A] and [B]
Standard free energy change[A] = 1 M, [B] = 1 M, pH = 7
DGrxn
= Gproduct – Greactant
A B
DG
DGo
A B
A negative DG indicates the reaction is spontaneous as written
DGforward rxn
= Gproduct – Greactant
= GB – GA
A B
A positive DG indicates the reaction is not spontaneous as written
A BDGback rxn
= Gproduct – Greactant
= GA – GB
The magnitude of DG for the forward and back reactions is equivalent
DGforward rxn = - DGback rxn
A B
At constant temperature and pressure, DG and DGo are related.
DG = DGo + RT ln [B][A]
A B
glucose6-phosphate
fructose6-phosphate
A B
DGo is predictive only standard conditions
DG = DGo + RT ln [B][A]
A B
glucose6-phosphate
fructose6-phosphate
A B
standard conditions
A
B
= 1 M
= 1 M
DG = DGo
DGo = standard free energy change
DG depends on the concentration of reactants and products
DG = DGo + RT ln [B][A]
A B
glucose6-phosphate
fructose6-phosphate
A B
DG = DGo = + 0.4 kcal/mol
A
B
= 0.90 M
= 0.09 M
Can the reaction proceed in the forward direction, i.e. produce fructose 6-phosphate?
nonequilibrium conditions
DG of a reaction is zero at equilibrium
nonequilibrium conditions
standard conditions
A
B
= 0.90 M
= 0.09 M
A
B
= 1 M
= 1 M
DG = - 0.96 kcal/mol DG = DGo = + 0.4 kcal/mol
equilibrium conditions
A
B
= 0.66 M
= 0.33 M
DG = 0 kcal/mol
A BA B
DG of a reaction is zero at equilibrium
equilibrium conditions
A
B
= 0.66 M
= 0.33 M
DG = 0 kcal/mol
A BA B
= Keq = equilibrium constant[B]eq
[A]eq
DGo = - RT ln Keq
DGo values of two consecutive reactions are additive
DGo = -4.0 kcal/mol
glucose6-phosphate
fructose6-phosphateA B DGo = +0.4 kcal/mol
DGo = -3.6 kcal/mol
G A
G B
ATP + glucose
ADP + glucose6-phosphate
glucose fructose6-phosphate
DG values of consecutive reactions are also additive
Additive property of free energy changes is central to biochemical pathways.
If sum of individual reaction DGs is negative, pathway can proceed as written.