Enzyme Turnover Number

The turnover number, or kcat, tells you how many substrate molecules one enzyme active site can convert into product each second when the enzyme is working at full capacity. It comes from dividing the maximum reaction rate, Vmax, by the total concentration of active enzyme, [E], because at saturation each enzyme molecule is assumed to be cycling as fast as it can. This makes kcat an intrinsic catalytic property of the enzyme under defined conditions, not a measure of how much enzyme is present.

Use this when comparing how fast different enzymes work, or when converting a measured Vmax into a per-enzyme catalytic rate. It is especially useful in enzyme kinetics, biochemical assay analysis, and evaluating enzyme engineering or mutation effects.

• using Vmax without converting enzyme amount and rate units to match
• confusing kcat with Km or kcat/Km, which describe different aspects of enzyme behavior
• dividing by total protein concentration instead of the concentration of active enzyme molecules