Subunit interactions in aspartate transcarbamylase. The interaction between catalytic and regulatory subunits and the effect of ligands.

The interaction between the catalytic subunit (c3) and the regulatory subunit (r2) of aspartate transcarbamylase from Escherichia coli was studied by measuring the reversible formation of the c3r6 complex as a function of r2 concentration. Conversion to the native enzyme was prevented by using a very low concentration of c2 (40 ng per ml) in the presence of bovine serum albumin. A simple hyperbolic r2 saturation curve was obtained suggesting the presence of only one kind of c:r domain. From the association constant for the formation of c3r6, the free energy of c:r interaction can be estimated to be about -10 Cal per mole. Neither CTP nor ATP appears to affect the strength of c:r interaction in this complex. Succinate in the presence of carbamyl phosphate promotes tighter binding. At higher concentration of c3 and nonsaturating levels of r2, conversion to the native enzyme (c3r6) takes place. This renaturation process is second order with respect to the concentration of c3 and is virtually irreversible. Renaturation is inhibited by saturating levels of r2 and to some extent by both CTP and ATP. The effect of ligands on c:r interactions reported here may have significance in the allosteric mechanism of the native enzyme.