kcat
Summary
kcat is the turnover number: the number of substrate molecules one enzyme active site converts to product per second when the enzyme is saturated with substrate. It equals Vmax divided by total enzyme concentration, so unlike Vmax it does not change when you add more enzyme.
Detail
Meaning: kcat is the first-order rate constant for the catalytic step at saturating substrate. In the simple scheme E + S ⇌ ES → E + P, kcat is the rate constant of ES → E + P. In multistep mechanisms it reflects the slow step or steps. Its unit is s⁻¹, so 1/kcat is the average time for one turnover.
Equation: Vmax = kcat[E]t, where Vmax is the maximum velocity (M/s), kcat is the turnover number (s⁻¹), and [E]t is the total enzyme concentration (M). Catalytic efficiency is kcat/Km, where Km is the Michaelis constant (M), giving units of M⁻¹s⁻¹.
On the MCAT: A passage gives Vmax and the enzyme concentration, and you calculate kcat. Or it gives a table of enzymes or mutants and asks which is the most efficient catalyst, which you judge with kcat/Km. You may also be asked to predict that doubling [E]t doubles Vmax while kcat stays the same. A noncompetitive inhibitor lowers Vmax, which lowers the apparent kcat, while Km is unchanged.
Common confusion: Students treat kcat and Vmax as the same thing. Vmax depends on how much enzyme is present, and kcat is a per-enzyme (per-active-site) property that does not. Also, kcat alone is not efficiency, because it ignores how well the enzyme binds substrate at low concentration. That is why kcat/Km is used.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Fundamentals of Biochemistry (Voet, Voet, Pratt)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.