**In this article, I clarify these terms' definitions, differences and biochemical significance**. Though related to each other, Km and Kd are not the same. To clarify this, I review two ways the Michaelis-Menten equation can be derived: the rapid equilibrium assumption and the steady-state approach.

**The dissociation constant (Kd)**

**The Michaelis constant (Km) and the Michaelis-Menten equation**

**The rapid equilibrium derivation**

## The steady-state derivation

**The difference between Km and Kd**

Because Km is related to Kd, people often take this parameter to be a measure of binding affinity. It does provide information about this, but assuming they are equivalent is a common and problematic fallacy. It is impossible to know whether the rapid equilibrium assumption is obeyed from simple Michaelis-Menten kinetics alone. In extreme cases, taking Km to approximate Kd might underestimate the substrate’s binding affinity by orders of magnitude. To demonstrate this, we can evaluate the difference between Km and Kd by calculating their ratio.