When a reaction is performed in zero-order kinetics

When a reaction is performed in zero-order kinetics

• A. The rate of the reaction is independent of the substrate concentration
• B. The substrate concentration is very low
• C. The rate of reaction is directly proportional to the substrate concentration
• D. The enzyme level is always high

Answer: (A)

Zero-order kinetics describes a situation where the reaction rate is set by the catalyst’s capacity, not by how much substrate is present. When substrate concentration is high enough that all enzyme active sites are occupied, every additional molecule of substrate cannot be processed faster—the enzyme is saturated. In this state the rate becomes constant and equals Vmax, determined by the enzyme concentration and its turnover number, and it no longer changes with [S]. That’s why the rate is independent of substrate concentration. If the substrate level is very low compared with the enzyme’s affinity (low [S] relative to Km), the enzyme isn’t saturated and the rate increases with more substrate—this is first-order behavior, not zero-order. Saying the rate is directly proportional to substrate would describe that first-order relationship rather than zero-order. The notion about the enzyme level being always high isn’t the defining factor for zero-order kinetics; saturation at high substrate is what drives the rate to be independent of [S].

Zero-order kinetics describes a situation where the reaction rate is set by the catalyst’s capacity, not by how much substrate is present. When substrate concentration is high enough that all enzyme active sites are occupied, every additional molecule of substrate cannot be processed faster—the enzyme is saturated. In this state the rate becomes constant and equals Vmax, determined by the enzyme concentration and its turnover number, and it no longer changes with [S]. That’s why the rate is independent of substrate concentration.

If the substrate level is very low compared with the enzyme’s affinity (low [S] relative to Km), the enzyme isn’t saturated and the rate increases with more substrate—this is first-order behavior, not zero-order. Saying the rate is directly proportional to substrate would describe that first-order relationship rather than zero-order. The notion about the enzyme level being always high isn’t the defining factor for zero-order kinetics; saturation at high substrate is what drives the rate to be independent of [S].