UNIT 2: Metabolic Processes
Chapter 3: An Introduction to Metabolism
3.3: Enzymes and Activation Energy
pg. 124 - 165
pg. 146 - 150
Enzymes enable chemical reactions to proceed more readily by reducing the
amount of activation energy that the reactants must overcome, enzymes
lower the energy required for the reaction to occur.
Just because a reaction can proceed on it own, does not mean that it will
proceed.
Enzymes as Catalysts
Enzymes are biological catalysts that are mostly proteins. Some RNA
molecules can also act as enzymes.
Maud Menten, was one of the first scientists to determine the fundamental
role of enzymes, she determined a mathematical equation to measure the
rates of enzymes and explain enzyme kinetics.
An enzyme binds with reactant molecules; speed up the rate of the reaction,
and then is released unchanged to support the reaction again. The enzyme
lowers the potential energy of the transition state for the reaction, causing
the reaction to occur faster, and more often.
An enzyme has one or more binding sites that bind to specific type reactant
substrate, than the chemical reaction can occur. These catalyzed reactions
are reversible.
You can recognize an enzyme and its substrate by its name. The root of its
name is often based on the substrate or type of chemical reaction and usually
ends in the suffix –ase.
The enzyme maltase catalyzes a reaction involving maltose. Other names
indicate the type of reaction that the enzymes catalyze, such as; carboxylase.
maltase
Maltose + water
↔
glucose + glucose
Activation Energy as a Barrier to Chemical Reactions
In a typical reaction the bonds of the reactants must break before the
products can form. The reactant possesses activation energy (Ea) barrier that
must be overcome for the reaction to occur. Once the activation energy
barrier has been reached, reactant bonds are broken, the reaction may move
forward. The reactants must meet at an appropriate geometrical position and
collide with other reactants for the chemical reaction to produce its products
and form new bonds. Transition state must be reached before new bonds and
new products can form.
Figure 2: Catalyzed reactions, molecules must overcome the activation
energy barrier for the reaction to proceed. pg. 147
What provides this activation energy? The motion of the molecules
themselves, an increase in temperature, or enzymes will allow reactions to
occur.
An increase in temperature is a problem for biological reaction, because it is
harmful to the cell, destroying DNA and proteins.
Lowering Activation Energy with Enzymes
Enzymes increase the rate of reactions by lowering the activation energy
required to start the reaction. When the enzyme lowers the activation energy
barrier, more reactant molecules can reach the transition state at a faster rate.
Figure 4: Enzymes lower the Activation Energy of both, a) exergonic
reactions and b) endergonic reactions. The enzymes do not affect the ∆G,
pg. 148
The enzyme reduces the activation energy require for a reaction to occur by
attaching to substrates, and bring these molecules closer together.
The enzymes also expose some substrates to altered charged environments.
The active site of some enzymes may contain ionic groups (positive/negative)
that attract or repel parts of the substrate. This can stress bonds of the
substrates.
Enzymes can also change the shape of the substrate which can weaken the
bonds, reducing the amount of energy required for the reaction to occur.
This mechanism of enzyme-substrate interaction is called the Induced-Fit
model.
Figure 5: Induced-Fit Model, a) Enzymes can bring both substrates into contact by
binding them to specific sites. This allows them to collide more easily so that bonds can
be broken more quickly and new bonds can form.
b) Enzymes can distort or bend the shape of a substrate, which weakens the chemical
bond. pg. 149
Enzyme can become non-functioning if they are heated. An increase in
temperature will denature the protein molecule, changing its shape, making
it unable to interact with the reactants.