Km
Summary
Km is the Michaelis constant: the substrate concentration at which an enzyme works at half of Vmax. A lower Km means the enzyme reaches half-maximal rate at lower substrate, which usually indicates higher apparent affinity for the substrate.
Detail
Meaning: In Michaelis-Menten kinetics, the initial rate rises with substrate concentration and then levels off at Vmax as the enzyme becomes saturated. Km is the [S] at which v = ½Vmax. It is a property of a particular enzyme-substrate pair under set conditions (temperature, pH). If the dissociation step dominates, a small Km means tight binding, so a low Km is read as high affinity.
Equation: v = Vmax[S] / (Km + [S]), where v is the initial reaction rate (mol/(L·s)), Vmax is the maximum rate at saturating substrate (mol/(L·s)), [S] is substrate concentration (mol/L), and Km is the Michaelis constant (mol/L). In the Lineweaver-Burk plot, 1/v = (Km/Vmax)(1/[S]) + 1/Vmax, so the x-intercept is −1/Km and the y-intercept is 1/Vmax.
On the MCAT: You may be asked to read Km off a hyperbolic v versus [S] graph, or off a Lineweaver-Burk plot. The most common task is predicting inhibitor effects. A competitive inhibitor raises apparent Km and leaves Vmax unchanged. A noncompetitive inhibitor lowers Vmax and leaves Km unchanged. An uncompetitive inhibitor lowers both Vmax and Km.
Common confusion: Km is not the rate, and it is not the same as Vmax. Km is a concentration, so it has concentration units. Km does not change when you add more enzyme, but Vmax does, because Vmax depends on total enzyme concentration. Also, a high Km means low affinity, which is the opposite of what students often guess.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Campbell Biology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.