The pAR5 mutation and the allosteric mechanism of Escherichia coli aspartate carbamoyltransferase.

Mutation pAR5 replaces residues 145′‐153′ at the C terminus of the regulatory (r) chains of Escherichia coli ATCase by a new sequence of six residues. The mutated enzyme has been shown to lack substrate cooperativity and inhibition by CTP. Solution X‐ray scattering curves demonstrate that, in the absence of ligands, its structure is intermediate between the T form and the R form. In the presence of N‐phosphonacetyl‐L‐aspartate, the mutant is similar to the wild type. An examination of the crystal structure of unligated ATCase reveals that the mutated site is at an interface between r and catalytic (c) chains, which exists only in the T allosteric form. A computer simulation by energy minimization suggests that the pAR5 mutation destabilizes this interface and induces minor changes in the tertiary structure of r chains. The resulting lower stability of the T form explains the loss of substrate cooperativity. The lack of allosteric inhibition may be related to a new electrostatic interaction made in mutant r chains between the C‐terminal carboxylate and a lysine residue of the allosteric domain.