Enzymes
An enzyme is a biological catalyst.
 A catalyst is a substance that speeds up
a chemical reaction without being
consumed in the process.
 It can increase the rate of biochemical
reactions more than a billion times.

Enzymes are proteins (amino acids polypeptides).
 They are present in all living cells, where
they perform a vital function by controlling
the metabolic processes whereby nutrients
are converted into energy and fresh cell
material.

Enzyme Structure
A substrate is the reactant that an
enzyme acts on when it catalyses a
chemical reaction.
 The active site is the location where the
substrate binds to an enzyme.
 Enzyme-substrate complex is an
enzyme with its substrate attached to the
active site.

During a chemical reaction, reactants need
to gain energy to break bonds, so they
can form new bonds. This energy is called
activation energy.
 Enzymes work by reducing the activation
energy of a reaction and thus, increase
the rate of the reaction.

How do enzymes decrease activation
energy?
 When a substrate or two, comes into the
enzyme, the enzyme can put strain on the
substrate (helps break bonds easier or it
can help break/make bonds).

Two Main Theories as to how
Enzymes work:
1.
Lock and Key Theory
Substrate fits into a rigid active site on
the enzyme like a key into a lock. Forms
enzyme-substrate complex. Enzyme can
help change the substrate, either by
splitting it apart or linking pieces
together.
2. Induced Fit Theory
Active site is able to change to enfold a
substrate. The enzyme takes up its most
effective catalytic shape after binding with
the substrate. The attachment of the
substrate to the enzymes’ active site
creates the enzyme-substrate complex.
Factors that affect the rate of
enzyme-catalyzed reactions:
1.
2.
Rate increases in direct proportion to
substrate concentration until the reaction
reaches maximum rate (the more
substrate the faster the reaction)
Rate is directly proportional to the
concentration of the enzyme present
(the more enzyme the faster the
reaction)
3. Incubation time (length of time over
which a reaction has taken place)
increases and the reaction rate decreases.
4. The rate increases with temperature up
to a maximum (optimum temperature).
Most human enzymes work best at 37 C.
5. Most enzymes are effective in a narrow
pH range.
Substances that Reduce Enzyme
Activity
There are substances that reduce the
activity of enzymes or prevent it
completely.
 Some enzyme inhibitors are competitive
and some are noncompetitive.

Competitive Inhibition
See hand-out
 Competitive inhibitors are substances
that compete with the substrate for an
enzyme’s active site. They enter the
enzyme’s active site and block the normal
substrate from binding.

Noncompetitive Inhibition
Noncompetitive inhibitors are
substances that attach to a binding site on
an enzyme other than the active site,
causing a change in the enzyme’s shape
and a loss of affinity for its substrate.
 This type of inhibition lowers the reaction
the most.

Controlling Metabolic Pathways
Metabolic pathways are many chemical
reactions carried out in a particular
sequence
 An enzyme catalyses each reaction
 Reactions take place inside cells
 Some anabolic/catabolic

Glycolysis consists of chain reactions
 Kreb’s Cycle consists of cycles of reactions

In many pathways, the product of the last
reaction, attaches to the enzyme that
catalyses the first reaction and inhibits
that enzyme
 This is called end-product inhibition
 The enzyme that is inhibited is an example
of an allosteric enzyme
 This is a type of noncompetitive inhibition
that controls reactions in the body

Inorganic Catalysts



What are inorganic catalysts?
Many metals are inorganic catalysts
because they provide a surface for
molecules to hang out (Think of a dance)
When molecules are hanging out on the
surface of a metal, they are more likely
to react than if they are floating around
Inorganic catalysts vs. Enzymes
Enzymes are specific, inorganic catalysts
are not.
2. Enzymes function effectively only in a
limited range of temperature, inorganic
catalysts have a wide range of
temperature, pressure and pH that they
can function in.
1.