In enzyme assays, zero-order kinetics are best described by which statement?

In enzyme assays, zero-order kinetics are best described by which statement?

• A. Enzyme present in excess; rate of reaction is variable with time and depends only on enzyme concentration
• B. Substrate present in excess; rate of reaction is constant with time and dependent only on the concentration of enzyme in the system
• C. Substrate present in excess; rate of reaction is constant with enzyme concentration and dependent only on the time in which the reaction is run
• D. Enzyme present in excess; rate of reaction is independent of both time and concentration of the enzyme in the system

Answer: (B)

Zero-order kinetics in enzyme assays happens when the substrate is in large excess compared to the enzyme, so every enzyme molecule is saturated with substrate. In this situation the reaction runs at its maximum rate (Vmax), which does not change as you add more substrate. That rate is determined by how much enzyme is present, because Vmax is proportional to enzyme concentration. Since the substrate no longer limits the reaction, the rate stays essentially constant over time—until substrate begins to run low enough that saturation and the constant rate no longer apply. So the best description is: substrate present in excess; rate is constant with time and dependent only on the concentration of enzyme in the system.

Zero-order kinetics in enzyme assays happens when the substrate is in large excess compared to the enzyme, so every enzyme molecule is saturated with substrate. In this situation the reaction runs at its maximum rate (Vmax), which does not change as you add more substrate. That rate is determined by how much enzyme is present, because Vmax is proportional to enzyme concentration. Since the substrate no longer limits the reaction, the rate stays essentially constant over time—until substrate begins to run low enough that saturation and the constant rate no longer apply.

So the best description is: substrate present in excess; rate is constant with time and dependent only on the concentration of enzyme in the system.