A reciprocal allosteric mechanism for efficient transfer of labile intermediates between active sites in CAD, the mammalian pyrimidine-biosynthetic multienzyme polypeptide.

Carbamoyl phosphate is the product of carbamoyl phosphate synthetase (CPS II) activity and the substrate of the aspartate transcarbamoylase (ATCase) activity, each of which is found in CAD, a large 240-kDa multienzyme polypeptide in mammals that catalyses the first three steps in pyrimidine biosynthesis. In our study of the transfer of the labile intermediate between the two active sites, we have used assays that differentiate the synthesis of carbamoyl phosphate from the overall reaction of CPS II and ATCase that produces carbamoyl aspartate. We provided excess exogenous carbamoyl phosphate and monitored its access to the respective active sites through the production of carbamoyl phosphate and carbamoyl aspartate from radiolabelled bicarbonate. Three features indicate interactions between the folded CPS II and ATCase domains causing reciprocal conformational changes. First, even in the presence of approximately 1 mM unlabelled carbamoyl phosphate, when the aspartate concentration is high ATCase uses endogenous carbamoyl phosphate for the synthesis of radiolabelled carbamoyl aspartate. In contrast, the isolated CPS II forward reaction is inhibited by excess unlabelled carbamoyl phosphate. Secondly, the affinity of the ATCase for carbamoyl phosphate and aspartate is modulated when substrates bind to CPS II. Thirdly, the transition-state analogue phosphonacetyl-L-aspartate is a less efficient inhibitor of the ATCase when the substrates for CPS II are present. All these effects operate when CPS II is in the more active P state, which is induced by high concentrations of ATP and magnesium ions and when 5'-phosphoribosyl diphosphate (the allosteric activator) is present with low concentrations of ATP; these are conditions that would be met during active biosynthesis in the cell. We propose a phenomenon of reciprocal allostery that encourages the efficient transfer of the labile intermediate within the multienzyme polypeptide CAD. In this model, binding of aspartate to the active site of ATCase causes a conformational change at the active site of the liganded form of CPS II, which protects it from inhibition by its product, carbamoyl phosphate; reciprocally, the substrates for CPS II affect the active site of ATCase by increasing the affinity for its substrates, endogenous carbamoyl phosphate and aspartate, and thus impede access of exogenous carbamoyl phosphate or the transition-state analogue. Reciprocal allostery justifies the close association of the enzyme activities within the polypeptide and ensures that carbamoyl phosphate is efficiently synthesised and is dedicated to the second step of pyrimidine biosynthesis. These conditions fulfill those required for metabolic channeling in the cell.