FREE ENERGY IN LIVING

SYSTEM

BCH 202

GENERAL BIOCHEMISTRY II

ODUGBEMI A. I.

Textbooks

• BIOCHEMISTRY, Berg, Tymoczko & Stryer 7th Ed.

• HARPER’S ILLUSTRATED BIOCHEMISTRY 26th Ed.

• BIOCHEMISTRY, Donald Voet & Judith Voet 4th Ed.

Learning Objectives

To understand the law mass action

To understand what Gibbs free energy is

To understand endergonic and exergonic reactions

To understand the importance of coupled reactions in

biological system

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The rate of a chemical reaction is directly proportional to the

product of the activities or concentrations of the reactants.

Mass action law is usually introduced by using a general

chemical reaction equation in which reactants A and B react to

give product C and D.

aA + bB cC + dD

where a, b, c, d are the coefficients for a balanced chemical

equation.

Law of Mass Action

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The mass action law states that if the system is at equilibrium at

a given temperature, then the following ratio is a constant.

The law of mass action essentially proposed that the rate of a

chemical reaction is proportional to the concentrations of

reactants.

The square brackets "[ ]" around the chemical species represent

their concentrations. The units for K depend upon the units used

for concentrations. If M is used for all concentrations, K has

units – M(c+d)-(a+b)

Law of Mass Action

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Gibbs Free Energy

Gibbs change in free energy (∆G) is that portion of the total

energy change in a system that is available for doing work—

i.e., the useful energy, also known as the chemical potential.

“G” is a function of Enthalpy and Entropy

Enthalpy, H, a measure of the energy (heat content) of the

system at constant pressure, and

Entropy, S, a measure of the randomness (disorder) of the

system. It is the energy in a system that is unavailable to do

work.

G = H – TS

G, H and S are state functions

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For any process,

If ΔH is negative, then heat is released (a favorable enthalpy

change)

ΔH < 0, favorable (exothermic reaction)

ΔH > 0, unfavorable (endothermic reaction)

If ΔS is positive, then the randomness of the system

increases (a favorable entropy change).

Increased disorder: ΔS > 0, favorable

Increased order: ΔS < 0, unfavorable

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Change in free energy for any process:

ΔG = ΔH – TΔS

T = absolute temperature in units of K (T = oC + 273)

If ΔG is negative (ΔG < 0) (exergonic reaction): process goes in

direction written (left to right)

If ΔG = 0: process is at equilibrium (no net reaction in either

direction)

If ΔG is positive (ΔG > 0) (reaction in direction written would be

endergonic; process goes in reverse (right to left))

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The change in free energy for going from Standard

Conditions to Equilibrium (ΔG°) can be written as:

Gibbs Free Energy in reversible reactions

ΔG° = - RTlnKeq

OR

ΔG° = - 2.303RTlogKeq

ΔG° = standard free energy Keq = equilibrium constant

R = gas constant (8.314 J/K/mol) T = absolute temperature (Kelvin)

ΔG° is the free energy change where all reactants and products

are in their standard states: 1.0 M concentration and pH = 0

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Free energy change for any reaction to go to equilibrium

from any starting conditions (ΔG) can be written as:

Recall that

ΔG = ΔG° + RTln[�]�[�]�

[�][]�

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Gibbs Free Energy in biochemical reactions

ΔG°ʹ is the free energy change where all reactants and products

are in their biochemical standard states: 1.0 M concentration

(except [H+] = 10-7 M) and pH = 7

ΔG = ΔG°ʹ + RTln[�]�[�]�

[�][]�

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0 = ΔG°ʹ + RTln[�]�[�]�

[�][]�

For a system already at equilibrium, ΔG = 0

Therefore,

ΔG°ʹ = − RTln[�]�[�]�

[�][]�

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For ATP hydrolysis reaction,

�� → �� + �

The concentration of reactants and products are as

follows:

�� = �. � × �����

�� = �. � × ����

� = �. � × ����

ΔG°ʹ = − ��. � ��/��

Using the formula,

ΔG = ΔG°ʹ + RTln[�]�[�]�

[�][]�

ΔG = ΔG°ʹ + RTln�� [ �]

[�� ]

ΔG = −30.5 KJ/mol + RTln[�. � × ����] � × ����

[� × ����]

ΔG = − !". # ��/��

R = �. ��! J/K/mol

R = �. �����! KJ/K/mol

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ΔG°ʹ of a reaction may also be calculated from reference

values of standard free energy of formation ΔG°ʹf of

individual product and reactant

For example

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Coupled Reactions

The terms exergonic and endergonic rather than the normal

chemical terms “exothermic” and “endothermic” are used to

indicate that a process is accompanied by loss or gain,

respectively, of free energy in any form, not necessarily as heat.

In practice, an endergonic process cannot exist independently

but must be a component of a coupled exergonic-endergonic

system where the overall net change is exergonic.

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Coupled Reactions

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Coupled Reactions

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Coupled Reactions

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It is the first reaction in metabolism of glucose that enters a

cell from the blood

�� + $%� → �� + �

& '��() + � → & '��() # *+�(*+,) + $%�

& '��() + �� → & '��() # *+�(*+,) + ��

(ΔGo' = + 13.8 kJ/mol, endergonic)

(ΔGo' = - 30.5 kJ/mol, exergonic)

(ΔGo' = - 16.7 kJ/mol, exergonic)

Rxn 1:

Rxn 2:

Coupled

rxn

Coupled Reactions

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High-Energy Compounds (High-Energy Phosphates)

High-energy compounds play a central role in

energy capture and transfer for sustenance of

living system.

ATP plays a central role in the transference of

free energy from the exergonic to the

endergonic processes.

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High-Energy Compounds (High-Energy Phosphates)

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http://www.life.illinois.edu/crofts/bioph354/dg_biochem.html

https://www.youtube.com/watch?v=aNEDU6EL8jc

https://www.youtube.com/watch?v=1GiZzCzmO5Q

Additional Materials

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