Biology 301, Exercise #2
Enzyme Kinetics of
Enzymes as Catalysts
Enzymes are a subgroup of proteins which catalyze biochemical
Without enzymes most necessary chemical reactions in our bodies would be
unable to proceed.
An enzyme can increase the rate of a reaction
but it cannot change
the equilibrium of the reaction.
For example, if substrate S is converted to product P and
the equilibrium constant
, for this reaction is equal to 100, then the concentration of P
at equilibrium is 100 times greater than that of S.
However, it may take a long time to
reach this equilibrium in the absence of an enzyme.
An enzyme increases the rate of the reaction by decreasing the activation energy
Without an enzyme, a graph of free energy versus progress of the reaction is as shown in
Figure A (see below).
A large input of energy is necessary to make the reaction proceed.
However, when an enzyme is present, it lowers the energy of activation so that the
reaction can proceed with a much smaller amount of energy (Figure B).
The simplest reaction that can be catalyzed by an enzyme is one in
which a single substrate is converted to a single product.
In this reaction, the substrate
binds to the active site
of the enzyme, a relatively small three dimensional area that is
specific for the substrate, to form an enzyme-substrate complex
The "binding" is usually
by weak bonds such as hydrogen bonds, electrostatic or hydrophobic interactions, or van
der Waals forces.
The enzyme then converts the substrate to product, which is released
to give free enzyme again.
This reaction is described by the following equation in which
E, S and P represent enzyme, substrate and product, respectively, and the terms k
are the rate constants for the indicated reactions:
The rate of catalysis or velocity
(v), increases as the substrate
concentration is increased.
However, the velocity eventually reaches a maximum (V