AAMC FL Practice Exam

What experiment step should be performed first when testing enzyme activity?

1. Vary the enzyme concentration

2. Vary the substrate concentration

3. Include the inhibitor

4. Measure the initial rate

The correct answer is: Vary the substrate concentration

Testing enzyme activity typically begins with varying the substrate concentration. This step is essential because substrate concentration is a crucial factor that influences the rate of enzyme-catalyzed reactions. By starting with substrate concentration, you can determine how an increase in substrate availability affects reaction rates, which is fundamental for identifying the enzyme's kinetics. When substrate concentration is varied, the initial rate of reaction can be measured at different concentrations. This allows for the determination of parameters such as the Michaelis-Menten constant (Km) and maximum velocity (Vmax), which are critical for understanding enzyme efficiency and activity. Addressing the other options, altering enzyme concentration can be important but usually follows substrate concentration adjustments since enzyme kinetics often consider the substrate's effect first. Including an inhibitor can be essential in understanding enzyme regulation, but this step is generally performed after establishing the baseline activity with varying substrate concentrations. Measuring the initial rate is indeed key, but it can only be accurately done once you have established the proper substrate concentrations to assess the reaction kinetics.