Binding of cytidine triphosphate to aspartate transcarbamylase

Abstract

Equilibrium dialysis studies reveal that cytidine triphosphate (CTP) binds to a total of six sites on native aspartate transcarbamylase (ATCase) from E. coli. Analysis of the binding curves suggests that there are two discrete classes of CTP binding sites on each native ATCase molecule. Two models are discussed in which a paired arrangement of regulatory chains in the native enzyme leads to heterogeneous binding of CTP. One model assumes that the folding of polypeptide chains in the regulatory dimer produces two CTP binding sites which are physically different and differ in their intrinsic affinities for CTP. Alternatively, interference between CTP molecules binding to identical CTP sites on each regulatory dimer may cause the two sites to bind the ligand with different affinities.