Thus, catalytic and regulatory functions were separated, and are distinct. The enzyme was concluded to be a complex made of two proteins, one designed for catalysis and the other for regulation.15,16,18 This is in principle similar to adding a thermostat to regulative a furnace’s heat production. Regulatory sites and catalytic activities of ATCase were physically separated by heating, or by treatments with urea or heavy metals. Properties of its regulatory and catalytic subunits were investigated in detail by enzymology and physical chemistry.19 The complete structure determined by X-ray diffraction shows that there are six catalytic and six regulatory subunits.20 In agreement with these findings, Jean-Pierre Changeux in Jacques Monod’s laboratory investigated the mechanism of the feedback inhibition of L-threonine deaminase discovered by Edward Adelberg and Umbarger , and from kinetic studies with extracts proposed that this enzyme has inhibitory sites in addition to catalytic ones.21 These several findings revealed a mechanism for the classical non-competitive inhibitions, as involving interactions at regulatory sites.