Km represents which of the following in enzyme kinetics?

• A. The substrate concentration at which the reaction rate is half of Vmax, reflecting affinity.
• B. The maximum rate achieved by an enzyme at saturating substrate concentration.
• C. The amount of enzyme present in the reaction.
• D. The rate constant for product formation.

Answer: (A)

Km is the substrate concentration that gives half of the maximum reaction velocity. This makes it a practical indicator of how tightly the enzyme binds its substrate: a smaller Km means the enzyme reaches half-maximal speed at a lower substrate level, indicating higher affinity; a larger Km means lower affinity.

In the Michaelis-Menten framework, v = (Vmax [S])/(Km + [S]). When [S] equals Km, the velocity is half of Vmax. Vmax is the highest rate achieved at saturating substrate and depends on how much enzyme is present, while Km itself does not change with enzyme concentration. Km isn’t a rate constant for product formation (that role belongs to kcat or k2); it’s a composite constant derived from the underlying rate constants that describes the substrate-binding and catalysis balance.