Biochemist_________________________ Date _____________
2-4 Enzymes
The chemical reactions that occur in the cells of living things
take place in part because of the actions of biological
catalysts called enzymes. Each type of enzyme catalyzes only
one type of reaction, and since thousands of different kinds
of reactions occur in cells, thousands of different enzymes
exist. All enzymes are proteins, so they are created though
the mechanism of protein synthesis.
Energy must be supplied in order for most cellular chemical
reactions to proceed. The initial input of energy required to
start a reaction is called activation energy. As is shown in the
diagram called Enzyme Function, chemical reactants (A –
color code) can be converted into products (B – color code)
only after a substantial input of energy. Enzymes act to lower
the activation energy of chemical reactions; therefore
reactants (C – color code) react more quickly to form
products (D – color code) because the activation energy of
the reaction is far less.
An enzyme (E – color code) is a biological catalyst that speeds
up a chemical reaction without itself being changed.
Enzymes’ names can be easily recognized because they usually
end in –ase. For example, the enzyme protease acts on
protein, and lactase acts on lactose. Some enzymes are named
specifically according to the type of reaction they catalyze.
One example of this is synthetase, which catalyzes synthetic
reactions.
A key portion of the enzyme is the active site (E1 – color
code). The active site is the region in which reactants bind
and, essentially, the region in which the chemical reaction
takes place. To operate enzymes first need to have a
reactant, called a substrate (F – color code) that binds loosely
to the active site. In reaction 1 (arrow 1 – color code) we see
the enzyme and substrate combine at the active site. This
reaction forms what is known as an enzyme-substrate
complex. In reaction 2 (arrow – color code), a chemical
change takes place at the active site and the result is the
formation of two products (G – color code). In reaction 3
(arrow – color code), the products are released from the
active site and the enzyme is recycled for use in another
reaction.
You will read and take notes about enzymes.
Follow the directions provided by your teacher.
There are numerous ways in which enzymes can be inhibited,
or changed. For example, heat can denature, or change its
structure, making it unable to bind to a substrate. Enzymes
can also be denatured by acidic or basic environments.
Many chemical substances interfere with the activity of
enzymes by binding to them, and these are referred to as
inhibitors. Inhibitors are classified by the way in which they
bind to an enzyme. Notice that the enzyme (E) can react with
the substrate (F) as well as with an inhibitor (H – color code).
In these cases, there is competition for the enzymes’ active
site. When large mounds of the inhibitor are present, it is
more likely that the inhibitor, and not the substrate, will bind
at the active site, as shown in chemical reaction 2. Once the
inhibitor is locking the active site, the substrate molecule
cannot bind, as you can see in reaction 3, and no chemical
reaction will occur.
Color the diagrams on the next page using the codes below.
ENZYMES
Reaction Without Enzyme
Reaction With Enzyme
Enzyme………………………...E
(pink)
Reactants …………….A
(DARK BLUE/NAVY)
Reactants …………….C
(LIGHT BLUE/SKY)
Products ……………...B
(DARK GREEN/HUNTER)
Products ……………...D
(LIGHT GREEN/LIME)
Active Site…………………..E1
(yellow)
Substrate…………………..….F
(black)
Products……..………….……G
(purple)
Inhibitor…………………..….H
(red)