The turnover number (kcat) is defined as the number of substrate molecules converted into product per enzyme molecule per unit time when the enzyme is fully saturated with substrate.
It is a measure of catalytic efficiency and reflects how fast an enzyme can operate under optimal conditions.
The Formula
The turnover number is calculated using the maximum reaction velocity ($V_{max}$) and the total enzyme concentration ($[E]_t$). It is mathematically expressed as:
$$k_{cat} = \frac{V_{max}}{[E]_t}$$
Where:
- $k_{cat}$ = Turnover number (typically expressed in units of reciprocal time, e.g., $\text{s}^{-1}$ or $\text{min}^{-1}$)
- $V_{max}$ = Maximum rate of the reaction when the enzyme is saturated
- $[E]_t$ = Total concentration of enzyme active sites
Units: s-1 (per second)
Key Characteristics
- Efficiency Indicator: A higher kcat value indicates that the enzyme can process more substrate molecules in a given time, reflecting greater efficiency.
- Substrate Independent: Because it is measured under saturating substrate concentrations, $k_{cat}$ reveals the ultimate speed limit of the enzyme regardless of how much substrate is floating around.
- Catalytic Efficiency ($k_{cat}/K_m$): While $k_{cat}$ measures pure speed, the ratio of $k_{cat}$ to the Michaelis constant ($K_m$) is used to evaluate an enzyme's overall efficiency under physiological (low substrate) conditions.
To see how these kinetic parameters are derived, check out our detailed breakdown of the Michaelis-Menten Equation
Examples of Turnover Numbers
Enzymes vary wildly in their speeds. Below are some typical turnover numbers showing the biological spectrum from relatively slow to blindingly fast:
| Enzyme | Typical Substrate | Turnover Number (kcat in s-1) |
|---|---|---|
| Lysozyme | Bacterial cell wall polysaccharides | 0.5 |
| Chymotrypsin | Proteins | 100 |
| Acetylcholinesterase | Acetylcholine | 14,000 |
| Carbonic Anhydrase | CO2 and H2O | 1,000,000 |
| Catalase | H2O2 | 400,000 |
Note: Carbonic anhydrase is one of the fastest known enzymes, approaching the theoretical limit of diffusion-controlled catalysis.