substrate concentration

Meaning: Substrate concentration, written [S], is the molar amount of substrate available to an enzyme. At low [S], most active sites are empty, so rate rises nearly in proportion to [S]. Near saturation almost every active site holds substrate, so the rate levels off at Vmax and extra substrate barely helps; only more enzyme raises it.

Equation: v = Vmax[S] / (Km + [S]), where v is the initial reaction rate (M/s), Vmax is the rate at enzyme saturation (M/s), [S] is substrate concentration (M), and Km is the Michaelis constant (M), the [S] at which v = ½Vmax.

On the MCAT: Expect a v versus [S] hyperbola or a Lineweaver-Burk plot (1/v against 1/[S]; y-intercept 1/Vmax, x-intercept −1/Km). If [S] doubles, rate roughly doubles when [S] is well below Km but barely changes when [S] is well above it. Competitive inhibitors raise apparent Km with Vmax unchanged, and high [S] can overcome them. Allosteric enzymes give sigmoidal curves instead.

Common confusion: Students assume more substrate keeps speeding up the reaction. At saturation the enzyme limits the rate. Km is also not the [S] you chose; it is a property of the enzyme and substrate under set conditions.