Biochemist______________________________
2-4 Enzymes
Date _____________
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.
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.