So Km is known as the Michaela's constant. The question is what is the physiological meaning of this Km value? Well, to answer this question we're going to begin by making a simplification. And the reason we want to make the simplification is to basically figure out the meaning behind Km. Now, because Km appears in the denominator we can see that the units of Km are the same as that for the concentration of the substrate. And so what we're going to assume initially is we're going to set the Km value equal to the substrate concentration and we'll see why we do that in just a moment.