What does the Km value of an enzyme represent?

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The enzyme-substrate dissociation constantThe typical substrate level found in physiological conditionsThe substrate concentration at which the reaction rate reaches half of its maximumA value that is the same for all isozymes performing the same reaction

Explanation

Km is defined as the substrate concentration at which the enzyme-catalyzed reaction proceeds at half of its maximum velocity. It is not necessarily the dissociation constant, nor is it always equal for all isozymes. It also does not represent the usual substrate concentration in physiological conditions.