Only a small minority of enzyme-catalysed reactions have just one substrate, and even the number is increased by treating two-substrate reactions in which one substrate is water as one-substrate reactions the number is still small. One might accordingly suppose that the Michaelis–Menten equation, normally written with just one substrate, is of limited usefulness. This supposition is misleading, however. One of the common equations for a two-substrate reaction can be written as follows to express  in terms of two substrate concentrations  and :
the other symbols represent kinetic constants. Suppose now that  is varied with  held constant. Then it is convenient to reorganize the equation as follows:
This has exactly the form of the Michaelis–Menten equation  
with apparent values  and  defined as follows: