Michaelis–Menten (M–M) theory is based on two important assumptions:

1. Assumption of Equilibrium (Leonor Michaelis and Maud Menten’s assumption) The reaction between the enzyme and its substrate (equation 1) remains in equilibrium, any effect of the equation 2 on its equilibrium being assumed to be negligible.These conditions will be achieved when the rate of breakdown of the ES complex is back to the free enzyme and substrate is much greater than the rate of its breakdown to free enzyme and products; that is, K₂ >>> K₃.It implies that the equilibrium between enzyme and substrate (equation 1) is attained so rapidly in comparison with the breakdown of ES (equation 2), that ES remains in equilibrium with E and S always while the enzyme action is proceeding. At equilibrium, the rate of forward reaction (V_f of equation 1) is equal to the rate of backward reaction (V_b). V_f = K₁[E] [S]
   V_b = K₂[ES] (by law of mass action)At equilibrium,K₁[E] [S] = K₂[ES]
   K₂ /K₁ = [E][S]/[ES] = K_sHere K_s is the dissociation constant of the first step in the enzymatic reaction. K_s = K_m only when K₂ >>> K₃ under equilibrium condition otherwise noted. K_m = K₂ + K₃/K₁
   K_m = K₂ /K₁ = K_s as K₂ is negligible.If we have used the equation assumption, we can say equivalent K_m = K_s in M–M equation.
2. Assumption of steady state (Briggs and Haldane’s assumption)The above assumption that the reaction between E and its S remains in equilibrium is not always correct. In view of the very high catalytic activities shown by many enzymes (the mixing of E and S is over in milliseconds), it is probable that this may not always be the case that the concentration of ES will differ from its equilibrium value.An alternate treatment, applicable to such cases, was put forward by Briggs and Haldane.This is based on the postulate that at any moment, the rates of formation and breakdown of the ES complex are essentially equal, so that its concentration [ES] can be regarded as a constant or in ‘steady state’ (until the S is entirely exhausted) over the short period of time necessary for a velocity measurement. (Over a longer period, of course, [ES] will change as the reaction proceeds.)
3. During the initial period, the concentration of free substrate remains unchanged so that in equations 1 and 2, the concentration of substrate [S] is equal to the total substrate concentration [S_T].These conditions are usually achieved when the total substrate concentration is much greater than the total enzyme concentration, as is usually the case in kinetics studies. [S] >>> [E]
4. By Michaelis–Menten equation, we measure only the initial velocity; at that condition, the amount of P is very less. So the formation of [ES] from E + P is negligible. Therefore, K₄ is ignored.