substrate
Summary
A substrate is the molecule an enzyme binds at its active site and converts to product. Raising substrate concentration speeds the reaction until the enzyme saturates at Vmax, and Km is the [S] at which the rate is half of Vmax.
Detail
Meaning: A substrate is the reactant an enzyme acts on. It binds at the active site to form an enzyme-substrate complex, the enzyme lowers the activation energy, and the substrate becomes product while the enzyme is regenerated. Binding is specific, and the MCAT favors the induced fit model over rigid lock-and-key. At low [S] the rate rises roughly in proportion to [S]; at high [S] the enzyme is saturated and the rate levels off at Vmax. (In organic chemistry, the substrate is the molecule being attacked, such as the alkyl halide in SN2.)
Equation: v = Vmax[S] / (Km + [S]), where v is the initial rate (M/s), Vmax is the maximum rate (M/s), [S] is substrate concentration (M), and Km is the Michaelis constant (M), the [S] at which v = Vmax/2.
On the MCAT: Passages give rate versus [S] curves or Lineweaver-Burk plots (1/v against 1/[S]; y-intercept 1/Vmax, x-intercept −1/Km). You predict how an inhibitor shifts them: a competitive inhibitor raises apparent Km with Vmax unchanged; a noncompetitive inhibitor lowers Vmax with Km unchanged.
Common confusion: Students expect more substrate to keep raising the rate without limit. Once the enzyme is saturated, only more enzyme raises Vmax. Likewise, extra substrate can overcome competitive inhibition but not noncompetitive inhibition.
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.