Michaelis-Menten kinetics
Summary
Michaelis-Menten kinetics describes how the initial rate of many single-substrate enzymes rises hyperbolically with substrate concentration and levels off at Vmax. Km is the [S] at which the rate is ½Vmax, and a lower Km usually means tighter substrate binding.
Detail
Meaning: The model describes enzymes following E + S ⇌ ES → E + P, measured as initial velocity while product is still negligible. At low [S] the rate rises almost linearly with [S]. At high [S] the enzyme is saturated and the rate levels off near Vmax, which equals kcat[E]total. Km is the substrate concentration giving half of Vmax. When catalysis is slow compared with dissociation, Km approximates the ES dissociation constant, so a lower Km usually means tighter binding.
Equation: v = Vmax[S] / (Km + [S]), where v is initial velocity (M/s), Vmax is maximum velocity (M/s), [S] is substrate concentration (M), and Km is the Michaelis constant (M).
On the MCAT: Expect a hyperbolic curve or a Lineweaver-Burk plot, 1/v = (Km/Vmax)(1/[S]) + 1/Vmax, with y-intercept 1/Vmax and x-intercept −1/Km. Predict inhibitor effects: competitive inhibition raises apparent Km with Vmax unchanged; pure noncompetitive inhibition lowers Vmax with Km unchanged; uncompetitive inhibition lowers both. Adding more enzyme raises Vmax but leaves Km unchanged.
Common confusion: Km is not the [S] needed to reach Vmax. It is the [S] at ½Vmax, and Vmax is only approached asymptotically. Allosteric, cooperative enzymes give sigmoidal curves and do not follow this model.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Voet's Biochemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.