Localization of the zinc binding site of aspartate transcarbamoylase in the regulatory subunit.

Aspartate transcarbamoylase (EC 2.1.3.2) from Escherichia coli contains six zinc ions per molecule. Renaturation studies of this allosteric enzyme and its isolated subunits show that the metal binding site is in the regulatory polypeptide chain. Ultraviolet difference spectra of the cadmium and zinc derivatives have been used to show that the sites in the isolated subunit and the full enzyme are similar. Results with an apo derivative of the regulatory subunit suggest that the metal is not required for the binding of the feedback inhibitor, but that it is involved in the structural stability of the protein. A close relationship between the reactivity of the cysteine residues in the regulatory subunit and the metal ion has been found.