This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Enzymes, Subject name: Biochemistry
Km of an enzyme is 
Select and explain. 
A. Dissociation constant, B. The normal physiological substrate concentration, C. The substrate concentration at half maximum velocity, D. Numerically identical for all isozymes that catalyze a given reaction
C: The substrate concentration at half maximum velocity
Answer: C: The substrate concentration at half maximum velocity 
Explanation: 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.