Background on Chemical
Reactions
Section 2.4 of the Textbook
Chemical Reactions...
• change substances into different substances by
breaking and forming bonds
• are sometimes easily detected (ex: if there is a
change in temperature, color, or state of matter)
Chemical Equations Show What is
Happening During a Reaction
• The equation shows the reactants or products
– Reactants: substances changed during a chemical
reaction
– Products: substances made during a chemical
reaction (found on the side the arrow points to)
reactants
products
products
reactants
Energy is Needed for a Chemical
Reaction to Occur
• Activation energy is the energy that must be
absorbed for a reaction to start
ENZYMES
How the confectioners make the runny
yolk-like inside to chocolate eggs?
The answer is : use of an enzyme
What are enzymes?
• Enzymes are proteins that increase the rate of
chemical reactions without being changed in
the reaction; also known as organic or
biochemical catalysts.
• Catalyst: speeds up a chemical reaction
without itself being changed by the reaction.
Enzyme Structure
• Enzymes are globular proteins. They have a
complex tertiary and quaternary structure in
which polypeptides are folded around each other
to form a roughly spherical or globular shape.
• The overall 3D shape of an enzyme molecule is
very important: if it is altered, the enzyme cannot
bind to its substrate and so cannot function.
Denature
• Enzyme shape is maintained by hydrogen bonds
and ionic forces.
Enzyme Function
 Many of the complex chemicals that living
organisms need cannot be made in a single
reaction. Instead a series of simpler reactions
occur, one after another, forming a metabolic
pathway. A single pathway may have many steps
in which each chemical is converted to the next.
A specific enzyme controls each reaction.
 Enzymes control cell metabolism by regulating
how and when reactions occur.
Enzyme Substrate Complex
The chemical which an enzyme works on is called
its substrate.
 An enzyme combines with its substrate to form a
short-lived enzyme/substrate complex. Once a
reaction has occurred, the complex breaks up into
products and enzyme.
E+S
ES
EP
E+P
 The enzyme remains unchanged at the end of
reaction and is can interact again with more
substrate and be reused.
Vocab Check
1. Substrate: Substance that the enzyme acts
upon.
2. Product: Formed from the substrate at the
end of the reaction.
3. Enzyme: Biological catalyst. Speeds up the
chemical reaction without being changed by
the reaction.
Enzyme Substrate Complex
How Do Enzymes Work?
• Lower the activation energy, allowing
reactions to occur more quickly.
Enzyme Specificity
• Two models that may explain how enzymes
work are:
– 1) The lock and key hypothesis
– 2) The induced fit hypothesis
Lock and Key Hypothesis
• Enzyme has a particular
shape into which the
substrate or substrates fit
exactly. This is often referred
to as the ‘lock and key’
hypothesis where the
substrate is imagined being
like a key whose shape is
complementary to the
enzyme or lock. The site
where the substrate bonds in
the enzyme is known as the
active site and it has a
specific shape.
Vocabulary Check
1. Substrate: substance that the enzyme acts
upon.
2. Product: formed from the substrate at the
end of the reaction.
3. Enzyme: biological catalyst. Increase the
rate of the reaction without being changed
by the reaction.
4. Active site: area on the enzyme that binds it
to the substrate.
Lock and Key Fit
Induced Fit Hypothesis
– The active site in many enzymes is not exactly the same
shape as the substrate, but molds itself around the
substrate as the enzyme substrate complex is formed.
– Only when the substrate binds to the enzyme is the active
site the correct shape to catalyze the reaction. As the
reaction’s products form they fit the active site less well and
fall away from it. Without the substrate, the enzyme reverts
to its ‘relaxed’ state, until the next substrate comes along.
Naming Enzymes
• Is it an enzyme?
• Look for the ending. Many, but not all,
enzymes end in –ase.
– EXAMPLES:
•
•
•
•
•
•
Lipase
Amylase
Protease
Polymerases
Ligases
Transferases
What factors affect enyzme activity?
• The factors that affect enzyme activity also
affect cell functions and ultimately the
organism. Enzymes are proteins and their
function is therefore affected by:
 Temperature
 pH
 Substrate concentration
 Enzyme concentration
 Cofactors
 Inhibitors
Temperature
• For a non-enzymatic chemical
reaction, the general rule is: the
higher the temperature, the faster
the reaction. This same rule holds
true for a reaction catalyzed by an
enzyme, but only up to about 40450C.
(104-113 deg.F)
• Above this temperature, enzyme
molecules begin to vibrate so
violently that the delicate bonds
that maintain tertiary and
quaternary structure are broken,
irreversibly changing the shape of
the molecule. When this happens,
the enzyme can no longer function
and it is said to be denatured.
pH
 Like other proteins, enzymes are
stable over a limited range of pH.
Outside this range, at the extremes
of pH, enzymes are denatured. Free
hydrogen ions (H+) or hydroxyl ions
(OH-) affect the changes on amino
acid residues, distorting the 3D
shape and causing an irreversible
change in the proteins tertiary
structure.
 Enzymes are particularly sensitive to
changes in pH because of the great
sensitivity of their active site. A
slight change in pH can denature the
enzyme and make it unusable.
•
Substrate Concentration
• The rate of an enzyme-controlled
reaction increases as the substrate
concentration increases, until the
enzyme is working at full capacity
(turnover rate). At this point, the
enzyme molecules reach their
turnover number and assuming
that all other conditions such as
temperature are ideal, the only
way to increase the speed of the
reaction even more is to add more
enzyme.
Enzyme Concentration
• In any reaction catalyzed by
an enzyme, the number of
enzyme molecules present is
very much smaller than the
number of substrate
molecules. When an
abundant supply of substrate
is available, the rate of
reaction is limited by the
number of enzyme
molecules present. In this
situation, increasing the
enzyme concentration
increases the rate of
reaction.
Cofactors
• Some enzymes cannot work on their own, they
need a molecule called a cofactor in order to
work properly. Cofactors modify the enzyme
complex so that it has the chemical properties
necessary to catalyze a reaction.
• One common type is known as coenzymes:
Relatively small organic molecules are not
permanently attached to the enzyme molecule.
• Also known as Coenzymes. Vitamins are
examples of Coenzymes.
Inhibitors
Inhibitors slow down or stop enzyme reaction.
Usually, enzyme inhibition is a natural process,
a means of switching enzymes on or off when
necessary.
 Inhibition can be reversible and the enzyme
returns to full activity once the inhibitor is
removed. Drugs and poisons can inhibit
particular enzymes, this type of inhibition is
often non-reversible.
Competitive Inhibitors
• Compete with normal substrate molecules to
occupy the active site. A competitive inhibitor
fits into the active site of the enzyme
preventing the real substrate from gaining
access. The inhibitor cannot be converted to
the products of the reaction and so the overall
rate of reaction is slowed down.
• Type of reversible inhibitor
• Fig: Competitive inhibitors bind reversibly to the
enzyme, preventing the binding of substrate. On the
other hand, binding of substrate prevents binding of
the inhibitor. Substrate and inhibitor compete for
the enzyme.
Non-Competitive Inhibitors
• Non-competitive inhibitors bind to the
enzyme away from the active site but change
the overall shape of the molecule(enzyme),
modifying the active site so that it can no
longer turn substrate molecules into product.
Non-competitive inhibition has this name
because there is no competition for the active
site.
Non-competitive Inhibitor
Irreversible Inhibitors
• Irreversible inhibitors bind permanently to
the enzyme, rendering it useless. For example,
cyanide is an irreversible inhibitor.
Review of Important Enzyme Properties
• Enzymes have several important properties:
– Enzymes are specific: each enzyme usually catalyses only
one reaction.
– Enzymes combine with their substrates to form temporary
enzyme-substrate complex.
– Enzymes are not altered or used up by the reactions they
catalyze, so can be used again and again.
– Enzymes are sensitive to temperature and pH.
– Many enzymes need cofactors in order to function.
– Enzyme function may be slowed down or stopped by
inhibitors.