JBC, Vol. 250, Issue 2, 661-667, Jan, 1975
Subunit interactions in aspartate transcarbamylase. The interaction between catalytic and regulatory subunits and the effect of ligands
W. W. Chan
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.
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