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
[{'content': 'Km of an enzyme is \nSelect and explain. \nA. 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', 'role': 'user'}, {'content': 'Answer: C \nExplanation: 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.', 'role': 'assistant'}]