B is a straight line representing a zero order reaction which permits accurate determination of enzyme activity for part or all of the reaction time. A represents the type of reaction that was shown in Figure 6. This reaction is zero order initially and then slows, presumably due to substrate exhaustion or product inhibition. This type of reaction is sometimes referred to as a "leading" reaction. True "potential" activity is represented by the dotted line. Curve C represents a reaction with an initial "lag" phase. Again the dotted line represents the potentially measurable activity. Multiple determinations of product concentration enable each curve to be plotted and true activity determined. A single end point determination at E would lead to the false conclusion that all three samples had identical enzyme concentration.
The rate of a reaction involving enzymes also increases as the substrate concentration increases. However, the number of enzyme active sites available is limited. At low enzyme concentrations or high substrate concentrations, all of the available enzyme active sites could be occupied with substrates. Therefore, increasing the substrate concentration further will not change the rate of diffusion. In other words, there is some maximum reaction rate (V max ) when all enzyme active sites are occupied. The reaction rate will increase with increasing substrate concentration, but must asymptotically approach the saturation rate, V max . V max is directly proportional to the total enzyme concentration, E, and the cata lytic constant of the enzyme, k cat, which describes the frequency at which the enzyme-substrate complex is converted to product. How quickly enzyme active sites become saturated can be described by the variable K, the substrate concentration at which the reaction rate is V max . K is called the Michaelis-Menten constant after the scientists who originally derived it. The reaction rate can be described by the equation