In Eq. ( ), Kd is the equilibrium dissociation constant of the ES into free substrate and free enzyme molecules, while ÑS is the number of substrate molecules released. Note that Eqs and ( ) serve the criteria that validate sQSSA or Michaelian zero-order kinetics. For the rQSSA case, the same structure of the equations applies, but the equations need to be restated to reflect prevailing reality as follows: