| "I think that
enzymes are molecules that are complementary in structure to the
activated complexes of the reactions that they catalyze, that is, to the
molecular configuration that is intermediate between the reacting
substances and products of reaction for these catalyzed processes.
The attraction of the enzyme molecule for the activated complex would
thus lead to a decrease in its energy and hence to a decrease in the
energy of activation of the reaction and to an increase in the rate of
reaction."
LINUS PAULING - 1948 |
Here is a description of enzyme catalysis by Linus Pauling.
The mechanisms that follow in enzyme catalysis are very complicated. It's rate is plotted as follows:
Rate of an enzyme catalyzed reaction plotted against enzyme concentration [E].
One simple model to explain this is:
i.e. 
Enzymes accelerate reactions by stabilizing the transition states, here are two energy profile diagrams showing the energy difference between a catalyzed and an uncatalyzed reaction:
Here is a plot of the reaction rate, as a function of substrate concentration [S]:
Rate of an enzyme catalyzed reaction plotted against substrate
concentration [S].
This plot explains why:
The rate of the enzyme catalyzed reaction is proportional to the enzyme concentration, as seen from the first graph.
This graph also explains why the reaction is proportional to the substrate concentration up to a certain point and then becomes independent of [S], i.e. a plateau is seen.
Therefore STEP 1 is the rate determining step at low concentrations of substrate [S], and at high [S] all the enzyme active sites have substrates bound to them, and are saturated and therefore the rate of product formation cannot be increased by increasing substrate concentration.
Continued on next page.