So why does the Km remained constant? So the Km, in the presence of a non competitive inhibitor, remains constant. How can this remain constant, and yet the VMAX is lowered? Well, the reason that the Km remains constant is because the inhibitor, by binding onto that enzyme, even though it changes the shape of the active side of the enzyme, and so it changes the efficiency, the Kcat value of that enzyme, it does not change the likelihood that that particular substrate is going to bind onto the active side. So it doesn't matter if the inhibitor is bound to that enzyme or not. In either case, the substrate will have no problem actually binding onto that active side.