SOLVENT PERTURBATION OF THE ALLOSTERIC REGULATION OF ASPARTATE-TRANSCARBAMYLASE

Escherichia coli aspartate transcarbamylase (ATCase) catalyzes the fir st committed step in pyrimidine biosynthesis, the condensation of aspa rtate and carbamyl phosphate. ATCase is positively allosterically regu lated by ATP and negatively regulated by CTP. We have used mild solven t perturbation to gain global molecular information about the mechanis m of heterotropic allostery. The [NaCl], temperature, and osmotic pres sure dependence of the enzymatic activity of ATCase has been examined in the presence and absence of allosteric effecters. The results indic ate that: 1) Regulation of aspartate binding by CTP appears to involve a unique set of electrostatic interactions not involved in enzyme fun ction in the presence of ATP or in the absence of effecters. 2) Aspart ate binding is enthalpically driven in the presence and absence of all osteric effecters. 3) The apparent enthalpy and entropy of aspartate b inding (Delta H, Delta S), and activation energy of catalysis (E-a) ar e substantially altered in the presence of CTP but not ATP. 4) The cha nge in hydration of ATCase upon substrate binding is the same in the p resence and absence of allosteric effecters. 5) The linkage between he terotropic and homotropic allostery is different for ATP and CTP. (C) 1998 Elsevier Science B.V, All rights reserved.