The three-dimensional structure of the multisubunit allosteric enzyme, aspartate transcarbamylase, has been determined t o 5.5 a resolution. An unusual feature of the molecule is a large central aqueous cavity 50 a X 50 a X 25 a, into which the active sites face. Access t o the central cavity and the active site region is provided by six equivalent channels of 1 5 a diameter.
A complex C6R4, composed of catalytic tnmers C3 and of regulatory dimers R,, has been isolated upon treatment o f aspartate transcarbamylase (ATCase, C6 R6) by mercurials. The specific catalytic activity of c 6 R4 is essentially the same as that of ATCase, about 70% of that of the catalytic trimers at 30 mM aspartate and saturating carbamyl phosphate. Allosteric interactions are reduced in c6 R4 as compared with those in ATCase. In the homotropic interactions the Hill coefficient is reduced from approximately 3.3 to 2.1 a t pH 8.3, while the heterotropic interactions of both cytidine triphosphate (CTP) and adenosine triphosphate (ATP) are reduced substantially but not abolished at pH 8.3. Thus, the allosteric transitions involved in the regulatory mechanisms do not require the intact structure C6R6. Also, this regulation is not simply the control of access of substrates or products t o or from the large central aqueous cavity in the ATCase molecule.
Comparison of electron density maps at 5.5 a resolution for ATCase and for the complex of ATCase with CTP shows substantial similarities throughout the three-dimensional electron density maps. Significant differences are seen, however, in the region of the regulatory dimers R, where CTP adds, and near the active sites in the catalytic trimers C3.
Allosteric enzymes participate in one of the major control mechanisms which have evolved in living organisms for coordination of the myriad chemical reactions within cells.
Many of these enzymes, like aspartate transcarbamylase (EC 2.1.3.2) of E. coli, catalyze