Rateversussubstrate

Rate-versus-substrate describes how the rate of a chemical reaction depends on the concentration of substrate. In many systems, the rate rises with increasing substrate concentration, but not indefinitely. At low substrate levels the rate may be approximately proportional to [S], while at higher levels the rate levels off as catalytic sites become saturated or other limiting steps dominate.

In enzyme-catalyzed reactions, the common model is Michaelis-Menten kinetics. The initial rate v relates to substrate concentration [S] by v = (Vmax[S])/(Km + [S]). Vmax represents the maximum rate when the enzyme is saturated with substrate, and Km is the substrate concentration at which the rate is half of Vmax. Km provides a measure of apparent affinity between enzyme and substrate: a lower Km indicates higher affinity. Catalytic efficiency is often expressed as kcat/Km, combining turnover number with substrate affinity.

Not all systems follow Michaelis-Menten behavior. Deviations can occur due to substrate inhibition at high [S], allosteric regulation producing sigmoidal curves (often described by the Hill equation), product inhibition, diffusion limitations, or environmental factors such as temperature and pH.

Applications of rate-versus-substrate analysis include determining kinetic parameters for comparing enzymes, characterizing inhibitors (which alter apparent Km or Vmax), and guiding experimental design. Data are commonly fitted to Michaelis-Menten models, with linear transformations such as Lineweaver-Burk, Eadie-Hofstee, or Hanes-Woolf used to estimate parameters from rate versus substrate data.