An enzymatic reaction is the
conversion of one molecule into another; a chemical reaction catalyzed
at the reactive sites on the enzyme. Considering the complex nature of
the enzyme itself, it is not unreasonable to expect that many parameters
will affect the rate of this catalytic activity. Enzyme activity can be
Spacing (steric hindrance)
Substrate Concentration (Michaelis-Menten Kinetics)
Any groups that separate the
enzyme from the support (or backbone) are referred to as spacing groups.
For an enzyme only one spacing group away, it would be very difficult for
a substrate to find the active site. The backbone interferes sterically.
But with more than one CH2 (or other spacing groups), the enzyme
can whip around and twist so that the active site is much more accessible.
Usually, spacers that provide as much distance as six CH2 groups
Effect of pH Change
Since enzymes are proteins,
they are very sensitive to changes in pH. Each enzyme has its own
optimum range for pH where it will be most active. This is the result
of the effect of pH on a combination of factors: (1) the binding of the
enzyme to substrate, (2) the catalytic activity of the enzyme, (3) the
ionization of the substrate, and (4) the variation of protein structure.
The initial rates for many enzymatic reactions exhibit bell-shaped curves
as a function of pH as shown in the example below. (Note that this
particular enzyme is most active at a pH of zero, but this is not the case
Effect of Temperature Change
As temperature increases, the
rate of reaction also increases, as is observed in many chemical reactions.
However, the stability of the protein also decreases due to thermal degradation.
Holding the enzyme at a high enough temperature for a long period of time
may cook the enzyme.
Effect of Substrate Concentration
Enzymes are not passive surfaces
on which reactions take place but rather, are complex molecular machines
that operate through a great diversity of chemical mechanisms. According
to Michaelis-Menten kinetics, enzyme-substrate reactions are actually comprised
of two elementary reactions. The first is the when the substrate
forms a complex with the enzyme and then in the second, the complex decomposes
to product and enzyme.
Enzyme + Substrate <----> Complex ----> Products +
According to this model,
when the substrate concentration becomes high enough to entirely convert
all of the enzyme to the complex form, the second step of the reaction
becomes the rate-limiting step. Therefore, the overall conversion
to product becomes insensitive to further increases in substrate concentration.
The general expression for the rate of this reaction (velocity) becomes:
v = d[P]/dt = k2*[complex]
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The above graphs were produced by running the program EnzymeAct.m