Enzyme Activity

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 influenced by:

Spacing (steric hindrance)
pH
Temperature
Substrate Concentration (Michaelis-Menten Kinetics)

Spacing

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 are enough.

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 for all.)

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.

_{k1 k2}
Enzyme + Substrate <----> Complex ----> Products + Enzyme
^k-1

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]