Km of an enzyme is

Km is the concentration of substrate which permits the enzyme to achieve half Vmax. An enzyme with a high Km has a low affinity for its substrate and requires a greater concentration of substrate to achieve Vmax. Increasing the substrate concentration indefinitely does not increase the rate of an enzyme-catalyzed reaction beyond a ceain point. This point is reached when there are enough substrate molecules to completely fill (saturate) the enzyme&;s active sites. The maximal velocity, or Vmax, is the rate of the reaction under these conditions. Vmax reflects how fast the enzyme can catalyze the reaction.