BioenergeticsINTER 111: Graduate Biochemistry

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.

Coupling an energetically favorable process with and an unfavorable one

Reactions are coupled through a common intermediate

X Y + ZD +

B C + DA + D

D

Exergonic hydrolysis of ATP is often coupled to endergonic processes

g b a

ATP ADP + Pi

ATP AMP + PPi