Substrate-activity curves for allosteric enzymes. (a) The sigmoid curve of a homotropic enzyme, in which the substrate also serves as a positive (stimulatory) modulator, or activator. (b) The effects of a positive modulator (+) and a negative modulator (−) on an allosteric enzyme in which K0.5 is altered without a change in Vmax. The central curve shows the substrate-activity relationship without a modulator. (c) A less common type of modulation, in which Vmax is altered and K0.5 is nearly constant.