Here's a basic presentation on what endergonic and exergonic reactions mean and their relevance in the field on enzyme engineering

1. Energy Releasing and Energy
Consuming Reactions
Lakshmi Rajan Menon (21BPT402)
Course: Enzyme Technology
ICT, Mumbai
24.03.2022

2. Energy in reactions
• Biochemical reactions involve changes in the chemical bonds that join atoms in
compounds
• Some reactions release energy & often occur on
their own, or spontaneously.
• Chemical reactions that absorb energy will not occur
without a source of energy.
• Energy changes are one of the most important factors
in determining whether a chemical reaction will occur.
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3. Exothermic and Endothermic Reactions
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4. Thermodynamics
ΔG=ΔH−TΔS
• ∆H is the enthalpy change. Enthalpy in biology refers to energy stored in bonds, and the
change in enthalpy is the difference in bond energies between the products and the
reactants
• ∆S is the entropy change of the system during the reaction. If ∆S is positive, the system
becomes more disordered during the reaction
• Temperature (T) determines the relative impacts of the ∆S and ∆H terms on the overall
free energy change of the reaction
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5. ΔG : Gibbs free energy change
• ΔG=ΔH−TΔS
• Reactions with a negative ∆G release
energy, which means that they can
proceed without an energy input
(are spontaneous). In contrast, reactions
with a positive ∆G need an input of
energy in order to take place (are non-
spontaneous)
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6. Exergonic and Endergonic
Reactions
• Reactions that have a negative ∆G, release free energy and are called exergonic reactions
ΔG = G final – G initial​
• A negative ∆G means that the reactants, or initial state, have more free energy than the
products, or final state.
• Reactions with a positive ∆G, require an input of energy and are called endergonic
reactions.
• In this case, the products, or final state, have more free energy than the reactants, or initial
state. Endergonic reactions are non-spontaneous, meaning that energy must be added before
they can proceed
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7. Endergonic and Exergonic
Reactions : An Example
• If a reaction is endergonic in one direction (e.g., converting products to reactants), then it must
be exergonic in the other, and vice versa.
• In biochemical systems, endergonic & exergonic reactions often are coupled, so the energy from
one reaction can power another reaction.
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8. Metabolic pathways
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9. Other examples :
• Examples of exergonic reactions include exothermic reactions, such as mixing sodium and
chlorine to make table salt, combustion, and chemiluminescence (light is the energy that is
released).
• Examples of endergonic reactions include endothermic reactions, such as photosynthesis
and the melting of ice into liquid water.
• Please note: A spontaneous (exergonic), endothermic reaction can occur when the changes
in enthalpy and entropy yield a negative Gibbs free energy (ΔG will be negative only if TΔS is
bigger than ΔH. Hence these reactions are spontaneous only if the entropy increase is sufficient
to overpower the unfavorable enthalpy change)
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11. Role of enzyme
• Chemical reactions of metabolic pathways do not take place on their own. Each reaction step is
facilitated, or catalyzed, by an enzyme.
• Enzymes are important for catalyzing all types of biological reactions—those that require
energy as well as those that release energy.
• In order for the reaction to take place, some or all of the chemical bonds in the reactants must be
broken so that new bonds, those of the products, can form.
• To get the bonds into a state that allows them to break, the molecule must be contorted
(deformed, or bent) into an unstable state called the transition state.
• The transition state is a high-energy state, and some amount of energy must be added in order
for the molecule reach it.
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13. Enzymes and Activation Energy
• The energy that is needed to get a
reaction started is called the
activation energy.
• Enzyme speeds up the rate of a
chemical reaction.
• They work by lowering a reaction’s
activation energy.
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15. Endergonic and Exergonic
Reactions Enzymes : An Example
ATP Synthase ATP Hydrolase
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16. It is important to remember that enzymes do not change whether a
reaction is exergonic (spontaneous) or endergonic. This is because
they do not change the free energy of the reactants or products. They
only reduce the activation energy required for the reaction to go forward
Please note!
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17. Summary
• Energy releasing reaction  spontaneous  Exergonic reactions
• Energy absorbing reaction  non-spontaneous  Endergonic reactions
• Spontaneous ≠ rate of reaction
• Enzyme speeds up the rate of a chemical reaction by
lowering a reaction’s activation energy
• They catalyze both spontaneous and non-spontaneous
reactions
• They cannot change exergonic to endergonic and vice
versa
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18. References
• https://www.masterorganicchemistry.com/2011/09/28/hammonds-postulate/
• https://opentextbc.ca/biology/chapter/4-1-energy-and-metabolism/
• https://rwu.pressbooks.pub/bio103/chapter/introduction-to-metabolism-enzymes-and-energy/
• https://www.khanacademy.org/science/biology/energy-and-enzymes/introduction-to-enzymes/a/activation-
energy
• https://openstax.org/books/microbiology/pages/8-1-energy-matter-and-enzymes
• https://www.ncbi.nlm.nih.gov/books/NBK9921/
• https://www.khanacademy.org/science/biology/energy-and-enzymes/free-energy-tutorial/a/gibbs-free-
energy
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19. Thank you!
Any questions?