Enzyme Notes and Model Activity
Everything that happens in an organism is based on chemical reactions. Chemical
reactions break bonds in reactants and form new bonds in the products. The products
are completely different from the beginning substances. Reactions either produce
energy or use energy. Many reactions that produce energy require some energy in the
beginning to get them started.
Enzymes are biological catalysts that cells use to speed up chemical reactions within
the cell. They speed up reactions by lowering the activation energy. Activation
energy is the amount of energy required to start the chemical reaction. Enzymes are
very substrate specific. They are usually named after the reaction that they
catalyze. Enzymes generally catalyze only one type of chemical reaction. They are not
used up in the reaction, but only provide a place for the reaction to occur. Once the
reaction is over, the products are released and the enzyme is free to act as a site for
another reaction of the same type to take place. Most enzymes work best at certain pH
and temperature values. Enzymes play essential roles in regulating chemical pathways,
making materials that cells need, releasing energy and transferring information.
An enzyme is both chemically and physically substrate specific. Only certain reactants
will “fit” the enzyme. These reactants are called substrates. The substrate will
temporarily bind to the enzymes at a particular place on the enzyme called the active
site. The way that the substrate fits into the enzyme at the active site is like a key
fitting into a lock or puzzle pieces fitting together. While the enzyme and the substrate
are bound together, they form an enzyme-substrate complex. At the active site, the
activation energy required to start the reaction is lowered, the reaction takes place and
the products are released. The enzyme is unchanged and is then free to repeat the
process again.
Label the following diagram:
or
Word Bank: Substrate, active site, enzyme-substrate complex,
enzyme, reactants, products
What can interfere with this process? Several things can interfere with the ability of
the substrate to bind at the active site or diminish the ability of the enzyme to function
properly. If the enzyme and substrate bind, but the temperature or pH is not in the
optimum range for the enzyme to function, the reaction may be slowed down or may
not be able to take place at all. In addition to pH and temperature, there are two other
inhibitors that may affect the ability of the enzyme and substrate to bind. One type of
inhibitor is called a competitive inhibitor. It is the right size to fit into the active site
of the enzyme because it resembles the substrate, but it cannot react chemically. It just
blocks the substrate from binding because it fills up the active site where the substrate
should fit. Another type of inhibitor is called a non-competitive inhibitor. It changes
the shape of the enzyme making the active site unreceptive to the substrate. If the
substrate cannot fit in the active site, the reaction will not take place.
Enzymes speed up chemical reactions that take place in cells.