Enzyme kinetics describes the speed at which an enzyme-catalyzed chemical reaction proceeds. First and foremost, the speed of the reaction depends on the amount of the enzyme used as well as on the amount of substrate. Whereas speed increases proportionally to the concentration of the enzyme, should substrate be added, speed will at first increase in a linear fashion until it eventually approaches a maximum speed. In addition, the activity of the enzyme and, as a result, the course of the reaction, is influenced by additional environmental factors such as pH, temperature or salt concentration.
Enzyme assays are mainly carried out for the purpose of characterizing the enzyme through its activity. The concentration of the substrate, too, can be determined using this technique. Measurement methods are based on determining the decrease in substrate (or the increase in product, respectively) over a defined time interval. The required value can be calculated from the measured parameters. Since assays can only be compared with one another if all reaction conditions are the same, parameters such as a suitable pH, the buffer and the temperature must be defined and kept constant throughout the duration of the reaction. The physiological conditions present inside the organism from which the enzyme originates may serve as a guideline for the reaction conditions employed in the laboratory. For the purpose of analysis, the linear range of the reaction curve is critical and is quite often observed in the initial phase of the reaction, which also displays the highest reaction speed. At this time, the reaction is not yet impeded by either diminishing substrate concentrations or by possible inhibitory effects of the product (Figure 2).