The symbols in square brackets represent the concentrations of total enzyme [E]t, free substrate [S] and free modifier [X], and Km is the Michaelis constant. The equation is valid under the equilibrium assumption, whose terms of validity have been discussed [Baici (2015), Chapter 2]. Even if the equilibrium or the quasi-equilibrium assumptions are not valid, the expected deviations from the complete model based on the steady-state rate equation are masked by experimental errors, with the exception of extreme cases.
Baici A (2015) Kinetics of Enzyme-Modifier Interactions – Selected Topics in the Theory and Diagnosis of Inhibition and Activation Mechanisms. Springer, Vienna